Documentation/powerpc/pci_iov_resource_on_powernv.txt - third_party/linux - Git at Google

 Wei Yang <weiyang@linux.vnet.ibm.com>
 Benjamin Herrenschmidt <benh@au1.ibm.com>
 Bjorn Helgaas <bhelgaas@google.com>
 26 Aug 2014

 This document describes the requirement from hardware for PCI MMIO resource
 sizing and assignment on PowerKVM and how generic PCI code handles this
 requirement. The first two sections describe the concepts of Partitionable
 Endpoints and the implementation on P8 (IODA2). The next two sections talks
 about considerations on enabling SRIOV on IODA2.

 1. Introduction to Partitionable Endpoints

 A Partitionable Endpoint (PE) is a way to group the various resources
 associated with a device or a set of devices to provide isolation between
 partitions (i.e., filtering of DMA, MSIs etc.) and to provide a mechanism
 to freeze a device that is causing errors in order to limit the possibility
 of propagation of bad data.

 There is thus, in HW, a table of PE states that contains a pair of "frozen"
 state bits (one for MMIO and one for DMA, they get set together but can be
 cleared independently) for each PE.

 When a PE is frozen, all stores in any direction are dropped and all loads
 return all 1's value. MSIs are also blocked. There's a bit more state that
 captures things like the details of the error that caused the freeze etc., but
 that's not critical.

 The interesting part is how the various PCIe transactions (MMIO, DMA, ...)
 are matched to their corresponding PEs.

 The following section provides a rough description of what we have on P8
 (IODA2).  Keep in mind that this is all per PHB (PCI host bridge).  Each PHB
 is a completely separate HW entity that replicates the entire logic, so has
 its own set of PEs, etc.

 2. Implementation of Partitionable Endpoints on P8 (IODA2)

 P8 supports up to 256 Partitionable Endpoints per PHB.

   * Inbound

     For DMA, MSIs and inbound PCIe error messages, we have a table (in
     memory but accessed in HW by the chip) that provides a direct
     correspondence between a PCIe RID (bus/dev/fn) with a PE number.
     We call this the RTT.

     - For DMA we then provide an entire address space for each PE that can
       contain two "windows", depending on the value of PCI address bit 59.
       Each window can be configured to be remapped via a "TCE table" (IOMMU
       translation table), which has various configurable characteristics
       not described here.

     - For MSIs, we have two windows in the address space (one at the top of
       the 32-bit space and one much higher) which, via a combination of the
       address and MSI value, will result in one of the 2048 interrupts per
       bridge being triggered.  There's a PE# in the interrupt controller
       descriptor table as well which is compared with the PE# obtained from
       the RTT to "authorize" the device to emit that specific interrupt.

     - Error messages just use the RTT.

   * Outbound.  That's where the tricky part is.

     Like other PCI host bridges, the Power8 IODA2 PHB supports "windows"
     from the CPU address space to the PCI address space.  There is one M32
     window and sixteen M64 windows.  They have different characteristics.
     First what they have in common: they forward a configurable portion of
     the CPU address space to the PCIe bus and must be naturally aligned
     power of two in size.  The rest is different:

     - The M32 window:

       * Is limited to 4GB in size.

       * Drops the top bits of the address (above the size) and replaces
 	them with a configurable value.  This is typically used to generate
 	32-bit PCIe accesses.  We configure that window at boot from FW and
 	don't touch it from Linux; it's usually set to forward a 2GB
 	portion of address space from the CPU to PCIe
 	0x8000_0000..0xffff_ffff.  (Note: The top 64KB are actually
 	reserved for MSIs but this is not a problem at this point; we just
 	need to ensure Linux doesn't assign anything there, the M32 logic
 	ignores that however and will forward in that space if we try).

       * It is divided into 256 segments of equal size.  A table in the chip
 	maps each segment to a PE#.  That allows portions of the MMIO space
 	to be assigned to PEs on a segment granularity.  For a 2GB window,
 	the segment granularity is 2GB/256 = 8MB.

     Now, this is the "main" window we use in Linux today (excluding
     SR-IOV).  We basically use the trick of forcing the bridge MMIO windows
     onto a segment alignment/granularity so that the space behind a bridge
     can be assigned to a PE.

     Ideally we would like to be able to have individual functions in PEs
     but that would mean using a completely different address allocation
     scheme where individual function BARs can be "grouped" to fit in one or
     more segments.

     - The M64 windows:

       * Must be at least 256MB in size.

       * Do not translate addresses (the address on PCIe is the same as the
 	address on the PowerBus).  There is a way to also set the top 14
 	bits which are not conveyed by PowerBus but we don't use this.

       * Can be configured to be segmented.  When not segmented, we can
 	specify the PE# for the entire window.  When segmented, a window
 	has 256 segments; however, there is no table for mapping a segment
 	to a PE#.  The segment number *is* the PE#.

       * Support overlaps.  If an address is covered by multiple windows,
 	there's a defined ordering for which window applies.

     We have code (fairly new compared to the M32 stuff) that exploits that
     for large BARs in 64-bit space:

     We configure an M64 window to cover the entire region of address space
     that has been assigned by FW for the PHB (about 64GB, ignore the space
     for the M32, it comes out of a different "reserve").  We configure it
     as segmented.

     Then we do the same thing as with M32, using the bridge alignment
     trick, to match to those giant segments.

     Since we cannot remap, we have two additional constraints:

     - We do the PE# allocation *after* the 64-bit space has been assigned
       because the addresses we use directly determine the PE#.  We then
       update the M32 PE# for the devices that use both 32-bit and 64-bit
       spaces or assign the remaining PE# to 32-bit only devices.

     - We cannot "group" segments in HW, so if a device ends up using more
       than one segment, we end up with more than one PE#.  There is a HW
       mechanism to make the freeze state cascade to "companion" PEs but
       that only works for PCIe error messages (typically used so that if
       you freeze a switch, it freezes all its children).  So we do it in
       SW.  We lose a bit of effectiveness of EEH in that case, but that's
       the best we found.  So when any of the PEs freezes, we freeze the
       other ones for that "domain".  We thus introduce the concept of
       "master PE" which is the one used for DMA, MSIs, etc., and "secondary
       PEs" that are used for the remaining M64 segments.

     We would like to investigate using additional M64 windows in "single
     PE" mode to overlay over specific BARs to work around some of that, for
     example for devices with very large BARs, e.g., GPUs.  It would make
     sense, but we haven't done it yet.

 3. Considerations for SR-IOV on PowerKVM

   * SR-IOV Background

     The PCIe SR-IOV feature allows a single Physical Function (PF) to
     support several Virtual Functions (VFs).  Registers in the PF's SR-IOV
     Capability control the number of VFs and whether they are enabled.

     When VFs are enabled, they appear in Configuration Space like normal
     PCI devices, but the BARs in VF config space headers are unusual.  For
     a non-VF device, software uses BARs in the config space header to
     discover the BAR sizes and assign addresses for them.  For VF devices,
     software uses VF BAR registers in the *PF* SR-IOV Capability to
     discover sizes and assign addresses.  The BARs in the VF's config space
     header are read-only zeros.

     When a VF BAR in the PF SR-IOV Capability is programmed, it sets the
     base address for all the corresponding VF(n) BARs.  For example, if the
     PF SR-IOV Capability is programmed to enable eight VFs, and it has a
     1MB VF BAR0, the address in that VF BAR sets the base of an 8MB region.
     This region is divided into eight contiguous 1MB regions, each of which
     is a BAR0 for one of the VFs.  Note that even though the VF BAR
     describes an 8MB region, the alignment requirement is for a single VF,
     i.e., 1MB in this example.

   There are several strategies for isolating VFs in PEs:

   - M32 window: There's one M32 window, and it is split into 256
     equally-sized segments.  The finest granularity possible is a 256MB
     window with 1MB segments.  VF BARs that are 1MB or larger could be
     mapped to separate PEs in this window.  Each segment can be
     individually mapped to a PE via the lookup table, so this is quite
     flexible, but it works best when all the VF BARs are the same size.  If
     they are different sizes, the entire window has to be small enough that
     the segment size matches the smallest VF BAR, which means larger VF
     BARs span several segments.

   - Non-segmented M64 window: A non-segmented M64 window is mapped entirely
     to a single PE, so it could only isolate one VF.

   - Single segmented M64 windows: A segmented M64 window could be used just
     like the M32 window, but the segments can't be individually mapped to
     PEs (the segment number is the PE#), so there isn't as much
     flexibility.  A VF with multiple BARs would have to be in a "domain" of
     multiple PEs, which is not as well isolated as a single PE.

   - Multiple segmented M64 windows: As usual, each window is split into 256
     equally-sized segments, and the segment number is the PE#.  But if we
     use several M64 windows, they can be set to different base addresses
     and different segment sizes.  If we have VFs that each have a 1MB BAR
     and a 32MB BAR, we could use one M64 window to assign 1MB segments and
     another M64 window to assign 32MB segments.

   Finally, the plan to use M64 windows for SR-IOV, which will be described
   more in the next two sections.  For a given VF BAR, we need to
   effectively reserve the entire 256 segments (256 * VF BAR size) and
   position the VF BAR to start at the beginning of a free range of
   segments/PEs inside that M64 window.

   The goal is of course to be able to give a separate PE for each VF.

   The IODA2 platform has 16 M64 windows, which are used to map MMIO
   range to PE#.  Each M64 window defines one MMIO range and this range is
   divided into 256 segments, with each segment corresponding to one PE.

   We decide to leverage this M64 window to map VFs to individual PEs, since
   SR-IOV VF BARs are all the same size.

   But doing so introduces another problem: total_VFs is usually smaller
   than the number of M64 window segments, so if we map one VF BAR directly
   to one M64 window, some part of the M64 window will map to another
   device's MMIO range.

   IODA supports 256 PEs, so segmented windows contain 256 segments, so if
   total_VFs is less than 256, we have the situation in Figure 1.0, where
   segments [total_VFs, 255] of the M64 window may map to some MMIO range on
   other devices:

      0      1                     total_VFs - 1
      +------+------+-     -+------+------+
      |      |      |  ...  |      |      |
      +------+------+-     -+------+------+

                            VF(n) BAR space

      0      1                     total_VFs - 1                255
      +------+------+-     -+------+------+-      -+------+------+
      |      |      |  ...  |      |      |   ...  |      |      |
      +------+------+-     -+------+------+-      -+------+------+

                            M64 window

 		Figure 1.0 Direct map VF(n) BAR space

   Our current solution is to allocate 256 segments even if the VF(n) BAR
   space doesn't need that much, as shown in Figure 1.1:

      0      1                     total_VFs - 1                255
      +------+------+-     -+------+------+-      -+------+------+
      |      |      |  ...  |      |      |   ...  |      |      |
      +------+------+-     -+------+------+-      -+------+------+

                            VF(n) BAR space + extra

      0      1                     total_VFs - 1                255
      +------+------+-     -+------+------+-      -+------+------+
      |      |      |  ...  |      |      |   ...  |      |      |
      +------+------+-     -+------+------+-      -+------+------+

 			   M64 window

 		Figure 1.1 Map VF(n) BAR space + extra

   Allocating the extra space ensures that the entire M64 window will be
   assigned to this one SR-IOV device and none of the space will be
   available for other devices.  Note that this only expands the space
   reserved in software; there are still only total_VFs VFs, and they only
   respond to segments [0, total_VFs - 1].  There's nothing in hardware that
   responds to segments [total_VFs, 255].

 4. Implications for the Generic PCI Code

 The PCIe SR-IOV spec requires that the base of the VF(n) BAR space be
 aligned to the size of an individual VF BAR.

 In IODA2, the MMIO address determines the PE#.  If the address is in an M32
 window, we can set the PE# by updating the table that translates segments
 to PE#s.  Similarly, if the address is in an unsegmented M64 window, we can
 set the PE# for the window.  But if it's in a segmented M64 window, the
 segment number is the PE#.

 Therefore, the only way to control the PE# for a VF is to change the base
 of the VF(n) BAR space in the VF BAR.  If the PCI core allocates the exact
 amount of space required for the VF(n) BAR space, the VF BAR value is fixed
 and cannot be changed.

 On the other hand, if the PCI core allocates additional space, the VF BAR
 value can be changed as long as the entire VF(n) BAR space remains inside
 the space allocated by the core.

 Ideally the segment size will be the same as an individual VF BAR size.
 Then each VF will be in its own PE.  The VF BARs (and therefore the PE#s)
 are contiguous.  If VF0 is in PE(x), then VF(n) is in PE(x+n).  If we
 allocate 256 segments, there are (256 - numVFs) choices for the PE# of VF0.

 If the segment size is smaller than the VF BAR size, it will take several
 segments to cover a VF BAR, and a VF will be in several PEs.  This is
 possible, but the isolation isn't as good, and it reduces the number of PE#
 choices because instead of consuming only numVFs segments, the VF(n) BAR
 space will consume (numVFs * n) segments.  That means there aren't as many
 available segments for adjusting base of the VF(n) BAR space.
	Wei Yang <weiyang@linux.vnet.ibm.com>
	Benjamin Herrenschmidt <benh@au1.ibm.com>
	Bjorn Helgaas <bhelgaas@google.com>
	26 Aug 2014

	This document describes the requirement from hardware for PCI MMIO resource
	sizing and assignment on PowerKVM and how generic PCI code handles this
	requirement. The first two sections describe the concepts of Partitionable
	Endpoints and the implementation on P8 (IODA2). The next two sections talks
	about considerations on enabling SRIOV on IODA2.

	1. Introduction to Partitionable Endpoints

	A Partitionable Endpoint (PE) is a way to group the various resources
	associated with a device or a set of devices to provide isolation between
	partitions (i.e., filtering of DMA, MSIs etc.) and to provide a mechanism
	to freeze a device that is causing errors in order to limit the possibility
	of propagation of bad data.

	There is thus, in HW, a table of PE states that contains a pair of "frozen"
	state bits (one for MMIO and one for DMA, they get set together but can be
	cleared independently) for each PE.

	When a PE is frozen, all stores in any direction are dropped and all loads
	return all 1's value. MSIs are also blocked. There's a bit more state that
	captures things like the details of the error that caused the freeze etc., but
	that's not critical.

	The interesting part is how the various PCIe transactions (MMIO, DMA, ...)
	are matched to their corresponding PEs.

	The following section provides a rough description of what we have on P8
	(IODA2). Keep in mind that this is all per PHB (PCI host bridge). Each PHB
	is a completely separate HW entity that replicates the entire logic, so has
	its own set of PEs, etc.

	2. Implementation of Partitionable Endpoints on P8 (IODA2)

	P8 supports up to 256 Partitionable Endpoints per PHB.

	* Inbound

	For DMA, MSIs and inbound PCIe error messages, we have a table (in
	memory but accessed in HW by the chip) that provides a direct
	correspondence between a PCIe RID (bus/dev/fn) with a PE number.
	We call this the RTT.

	- For DMA we then provide an entire address space for each PE that can
	contain two "windows", depending on the value of PCI address bit 59.
	Each window can be configured to be remapped via a "TCE table" (IOMMU
	translation table), which has various configurable characteristics
	not described here.

	- For MSIs, we have two windows in the address space (one at the top of
	the 32-bit space and one much higher) which, via a combination of the
	address and MSI value, will result in one of the 2048 interrupts per
	bridge being triggered. There's a PE# in the interrupt controller
	descriptor table as well which is compared with the PE# obtained from
	the RTT to "authorize" the device to emit that specific interrupt.

	- Error messages just use the RTT.

	* Outbound. That's where the tricky part is.

	Like other PCI host bridges, the Power8 IODA2 PHB supports "windows"
	from the CPU address space to the PCI address space. There is one M32
	window and sixteen M64 windows. They have different characteristics.
	First what they have in common: they forward a configurable portion of
	the CPU address space to the PCIe bus and must be naturally aligned
	power of two in size. The rest is different:

	- The M32 window:

	* Is limited to 4GB in size.

	* Drops the top bits of the address (above the size) and replaces
	them with a configurable value. This is typically used to generate
	32-bit PCIe accesses. We configure that window at boot from FW and
	don't touch it from Linux; it's usually set to forward a 2GB
	portion of address space from the CPU to PCIe
	0x8000_0000..0xffff_ffff. (Note: The top 64KB are actually
	reserved for MSIs but this is not a problem at this point; we just
	need to ensure Linux doesn't assign anything there, the M32 logic
	ignores that however and will forward in that space if we try).

	* It is divided into 256 segments of equal size. A table in the chip
	maps each segment to a PE#. That allows portions of the MMIO space
	to be assigned to PEs on a segment granularity. For a 2GB window,
	the segment granularity is 2GB/256 = 8MB.

	Now, this is the "main" window we use in Linux today (excluding
	SR-IOV). We basically use the trick of forcing the bridge MMIO windows
	onto a segment alignment/granularity so that the space behind a bridge
	can be assigned to a PE.

	Ideally we would like to be able to have individual functions in PEs
	but that would mean using a completely different address allocation
	scheme where individual function BARs can be "grouped" to fit in one or
	more segments.

	- The M64 windows:

	* Must be at least 256MB in size.

	* Do not translate addresses (the address on PCIe is the same as the
	address on the PowerBus). There is a way to also set the top 14
	bits which are not conveyed by PowerBus but we don't use this.

	* Can be configured to be segmented. When not segmented, we can
	specify the PE# for the entire window. When segmented, a window
	has 256 segments; however, there is no table for mapping a segment
	to a PE#. The segment number is the PE#.

	* Support overlaps. If an address is covered by multiple windows,
	there's a defined ordering for which window applies.

	We have code (fairly new compared to the M32 stuff) that exploits that
	for large BARs in 64-bit space:

	We configure an M64 window to cover the entire region of address space
	that has been assigned by FW for the PHB (about 64GB, ignore the space
	for the M32, it comes out of a different "reserve"). We configure it
	as segmented.

	Then we do the same thing as with M32, using the bridge alignment
	trick, to match to those giant segments.

	Since we cannot remap, we have two additional constraints:

	- We do the PE# allocation after the 64-bit space has been assigned
	because the addresses we use directly determine the PE#. We then
	update the M32 PE# for the devices that use both 32-bit and 64-bit
	spaces or assign the remaining PE# to 32-bit only devices.

	- We cannot "group" segments in HW, so if a device ends up using more
	than one segment, we end up with more than one PE#. There is a HW
	mechanism to make the freeze state cascade to "companion" PEs but
	that only works for PCIe error messages (typically used so that if
	you freeze a switch, it freezes all its children). So we do it in
	SW. We lose a bit of effectiveness of EEH in that case, but that's
	the best we found. So when any of the PEs freezes, we freeze the
	other ones for that "domain". We thus introduce the concept of
	"master PE" which is the one used for DMA, MSIs, etc., and "secondary
	PEs" that are used for the remaining M64 segments.

	We would like to investigate using additional M64 windows in "single
	PE" mode to overlay over specific BARs to work around some of that, for
	example for devices with very large BARs, e.g., GPUs. It would make
	sense, but we haven't done it yet.

	3. Considerations for SR-IOV on PowerKVM

	* SR-IOV Background

	The PCIe SR-IOV feature allows a single Physical Function (PF) to
	support several Virtual Functions (VFs). Registers in the PF's SR-IOV
	Capability control the number of VFs and whether they are enabled.

	When VFs are enabled, they appear in Configuration Space like normal
	PCI devices, but the BARs in VF config space headers are unusual. For
	a non-VF device, software uses BARs in the config space header to
	discover the BAR sizes and assign addresses for them. For VF devices,
	software uses VF BAR registers in the PF SR-IOV Capability to
	discover sizes and assign addresses. The BARs in the VF's config space
	header are read-only zeros.

	When a VF BAR in the PF SR-IOV Capability is programmed, it sets the
	base address for all the corresponding VF(n) BARs. For example, if the
	PF SR-IOV Capability is programmed to enable eight VFs, and it has a
	1MB VF BAR0, the address in that VF BAR sets the base of an 8MB region.
	This region is divided into eight contiguous 1MB regions, each of which
	is a BAR0 for one of the VFs. Note that even though the VF BAR
	describes an 8MB region, the alignment requirement is for a single VF,
	i.e., 1MB in this example.

	There are several strategies for isolating VFs in PEs:

	- M32 window: There's one M32 window, and it is split into 256
	equally-sized segments. The finest granularity possible is a 256MB
	window with 1MB segments. VF BARs that are 1MB or larger could be
	mapped to separate PEs in this window. Each segment can be
	individually mapped to a PE via the lookup table, so this is quite
	flexible, but it works best when all the VF BARs are the same size. If
	they are different sizes, the entire window has to be small enough that
	the segment size matches the smallest VF BAR, which means larger VF
	BARs span several segments.

	- Non-segmented M64 window: A non-segmented M64 window is mapped entirely
	to a single PE, so it could only isolate one VF.

	- Single segmented M64 windows: A segmented M64 window could be used just
	like the M32 window, but the segments can't be individually mapped to
	PEs (the segment number is the PE#), so there isn't as much
	flexibility. A VF with multiple BARs would have to be in a "domain" of
	multiple PEs, which is not as well isolated as a single PE.

	- Multiple segmented M64 windows: As usual, each window is split into 256
	equally-sized segments, and the segment number is the PE#. But if we
	use several M64 windows, they can be set to different base addresses
	and different segment sizes. If we have VFs that each have a 1MB BAR
	and a 32MB BAR, we could use one M64 window to assign 1MB segments and
	another M64 window to assign 32MB segments.

	Finally, the plan to use M64 windows for SR-IOV, which will be described
	more in the next two sections. For a given VF BAR, we need to
	effectively reserve the entire 256 segments (256 * VF BAR size) and
	position the VF BAR to start at the beginning of a free range of
	segments/PEs inside that M64 window.

	The goal is of course to be able to give a separate PE for each VF.

	The IODA2 platform has 16 M64 windows, which are used to map MMIO
	range to PE#. Each M64 window defines one MMIO range and this range is
	divided into 256 segments, with each segment corresponding to one PE.

	We decide to leverage this M64 window to map VFs to individual PEs, since
	SR-IOV VF BARs are all the same size.

	But doing so introduces another problem: total_VFs is usually smaller
	than the number of M64 window segments, so if we map one VF BAR directly
	to one M64 window, some part of the M64 window will map to another
	device's MMIO range.

	IODA supports 256 PEs, so segmented windows contain 256 segments, so if
	total_VFs is less than 256, we have the situation in Figure 1.0, where
	segments [total_VFs, 255] of the M64 window may map to some MMIO range on
	other devices:

	0 1 total_VFs - 1
	+------+------+- -+------+------+
	\| \| \| ... \| \| \|
	+------+------+- -+------+------+

	VF(n) BAR space

	0 1 total_VFs - 1 255
	+------+------+- -+------+------+- -+------+------+
	\| \| \| ... \| \| \| ... \| \| \|
	+------+------+- -+------+------+- -+------+------+

	M64 window

	Figure 1.0 Direct map VF(n) BAR space

	Our current solution is to allocate 256 segments even if the VF(n) BAR
	space doesn't need that much, as shown in Figure 1.1:

	0 1 total_VFs - 1 255
	+------+------+- -+------+------+- -+------+------+
	\| \| \| ... \| \| \| ... \| \| \|
	+------+------+- -+------+------+- -+------+------+

	VF(n) BAR space + extra

	0 1 total_VFs - 1 255
	+------+------+- -+------+------+- -+------+------+
	\| \| \| ... \| \| \| ... \| \| \|
	+------+------+- -+------+------+- -+------+------+

	M64 window

	Figure 1.1 Map VF(n) BAR space + extra

	Allocating the extra space ensures that the entire M64 window will be
	assigned to this one SR-IOV device and none of the space will be
	available for other devices. Note that this only expands the space
	reserved in software; there are still only total_VFs VFs, and they only
	respond to segments [0, total_VFs - 1]. There's nothing in hardware that
	responds to segments [total_VFs, 255].

	4. Implications for the Generic PCI Code

	The PCIe SR-IOV spec requires that the base of the VF(n) BAR space be
	aligned to the size of an individual VF BAR.

	In IODA2, the MMIO address determines the PE#. If the address is in an M32
	window, we can set the PE# by updating the table that translates segments
	to PE#s. Similarly, if the address is in an unsegmented M64 window, we can
	set the PE# for the window. But if it's in a segmented M64 window, the
	segment number is the PE#.

	Therefore, the only way to control the PE# for a VF is to change the base
	of the VF(n) BAR space in the VF BAR. If the PCI core allocates the exact
	amount of space required for the VF(n) BAR space, the VF BAR value is fixed
	and cannot be changed.

	On the other hand, if the PCI core allocates additional space, the VF BAR
	value can be changed as long as the entire VF(n) BAR space remains inside
	the space allocated by the core.

	Ideally the segment size will be the same as an individual VF BAR size.
	Then each VF will be in its own PE. The VF BARs (and therefore the PE#s)
	are contiguous. If VF0 is in PE(x), then VF(n) is in PE(x+n). If we
	allocate 256 segments, there are (256 - numVFs) choices for the PE# of VF0.

	If the segment size is smaller than the VF BAR size, it will take several
	segments to cover a VF BAR, and a VF will be in several PEs. This is
	possible, but the isolation isn't as good, and it reduces the number of PE#
	choices because instead of consuming only numVFs segments, the VF(n) BAR
	space will consume (numVFs * n) segments. That means there aren't as many
	available segments for adjusting base of the VF(n) BAR space.