powerpc/pci: Add PCI resource alignment documentation

In order to enable SRIOV on PowerNV platform, the PF's IOV BAR needs to be
adjusted:

1. size expanded
2. aligned to M64BT size

This patch documents this change on the reason and how.

[bhelgaas: reformat, clarify, expand]
Signed-off-by: Wei Yang <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

Documentation/powerpc/pci_iov_resource_on_powernv.txt

···
+Wei Yang <[e-mail]>
+Benjamin Herrenschmidt <[e-mail]>
+Bjorn Helgaas <[e-mail]>
+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.