pci_iov_resource_on_powernv.rst (14926B)
1=================================================== 2PCI Express I/O Virtualization Resource on Powerenv 3=================================================== 4 5Wei Yang <weiyang@linux.vnet.ibm.com> 6 7Benjamin Herrenschmidt <benh@au1.ibm.com> 8 9Bjorn Helgaas <bhelgaas@google.com> 10 1126 Aug 2014 12 13This document describes the requirement from hardware for PCI MMIO resource 14sizing and assignment on PowerKVM and how generic PCI code handles this 15requirement. The first two sections describe the concepts of Partitionable 16Endpoints and the implementation on P8 (IODA2). The next two sections talks 17about considerations on enabling SRIOV on IODA2. 18 191. Introduction to Partitionable Endpoints 20========================================== 21 22A Partitionable Endpoint (PE) is a way to group the various resources 23associated with a device or a set of devices to provide isolation between 24partitions (i.e., filtering of DMA, MSIs etc.) and to provide a mechanism 25to freeze a device that is causing errors in order to limit the possibility 26of propagation of bad data. 27 28There is thus, in HW, a table of PE states that contains a pair of "frozen" 29state bits (one for MMIO and one for DMA, they get set together but can be 30cleared independently) for each PE. 31 32When a PE is frozen, all stores in any direction are dropped and all loads 33return all 1's value. MSIs are also blocked. There's a bit more state that 34captures things like the details of the error that caused the freeze etc., but 35that's not critical. 36 37The interesting part is how the various PCIe transactions (MMIO, DMA, ...) 38are matched to their corresponding PEs. 39 40The following section provides a rough description of what we have on P8 41(IODA2). Keep in mind that this is all per PHB (PCI host bridge). Each PHB 42is a completely separate HW entity that replicates the entire logic, so has 43its own set of PEs, etc. 44 452. Implementation of Partitionable Endpoints on P8 (IODA2) 46========================================================== 47 48P8 supports up to 256 Partitionable Endpoints per PHB. 49 50 * Inbound 51 52 For DMA, MSIs and inbound PCIe error messages, we have a table (in 53 memory but accessed in HW by the chip) that provides a direct 54 correspondence between a PCIe RID (bus/dev/fn) with a PE number. 55 We call this the RTT. 56 57 - For DMA we then provide an entire address space for each PE that can 58 contain two "windows", depending on the value of PCI address bit 59. 59 Each window can be configured to be remapped via a "TCE table" (IOMMU 60 translation table), which has various configurable characteristics 61 not described here. 62 63 - For MSIs, we have two windows in the address space (one at the top of 64 the 32-bit space and one much higher) which, via a combination of the 65 address and MSI value, will result in one of the 2048 interrupts per 66 bridge being triggered. There's a PE# in the interrupt controller 67 descriptor table as well which is compared with the PE# obtained from 68 the RTT to "authorize" the device to emit that specific interrupt. 69 70 - Error messages just use the RTT. 71 72 * Outbound. That's where the tricky part is. 73 74 Like other PCI host bridges, the Power8 IODA2 PHB supports "windows" 75 from the CPU address space to the PCI address space. There is one M32 76 window and sixteen M64 windows. They have different characteristics. 77 First what they have in common: they forward a configurable portion of 78 the CPU address space to the PCIe bus and must be naturally aligned 79 power of two in size. The rest is different: 80 81 - The M32 window: 82 83 * Is limited to 4GB in size. 84 85 * Drops the top bits of the address (above the size) and replaces 86 them with a configurable value. This is typically used to generate 87 32-bit PCIe accesses. We configure that window at boot from FW and 88 don't touch it from Linux; it's usually set to forward a 2GB 89 portion of address space from the CPU to PCIe 90 0x8000_0000..0xffff_ffff. (Note: The top 64KB are actually 91 reserved for MSIs but this is not a problem at this point; we just 92 need to ensure Linux doesn't assign anything there, the M32 logic 93 ignores that however and will forward in that space if we try). 94 95 * It is divided into 256 segments of equal size. A table in the chip 96 maps each segment to a PE#. That allows portions of the MMIO space 97 to be assigned to PEs on a segment granularity. For a 2GB window, 98 the segment granularity is 2GB/256 = 8MB. 99 100 Now, this is the "main" window we use in Linux today (excluding 101 SR-IOV). We basically use the trick of forcing the bridge MMIO windows 102 onto a segment alignment/granularity so that the space behind a bridge 103 can be assigned to a PE. 104 105 Ideally we would like to be able to have individual functions in PEs 106 but that would mean using a completely different address allocation 107 scheme where individual function BARs can be "grouped" to fit in one or 108 more segments. 109 110 - The M64 windows: 111 112 * Must be at least 256MB in size. 113 114 * Do not translate addresses (the address on PCIe is the same as the 115 address on the PowerBus). There is a way to also set the top 14 116 bits which are not conveyed by PowerBus but we don't use this. 117 118 * Can be configured to be segmented. When not segmented, we can 119 specify the PE# for the entire window. When segmented, a window 120 has 256 segments; however, there is no table for mapping a segment 121 to a PE#. The segment number *is* the PE#. 122 123 * Support overlaps. If an address is covered by multiple windows, 124 there's a defined ordering for which window applies. 125 126 We have code (fairly new compared to the M32 stuff) that exploits that 127 for large BARs in 64-bit space: 128 129 We configure an M64 window to cover the entire region of address space 130 that has been assigned by FW for the PHB (about 64GB, ignore the space 131 for the M32, it comes out of a different "reserve"). We configure it 132 as segmented. 133 134 Then we do the same thing as with M32, using the bridge alignment 135 trick, to match to those giant segments. 136 137 Since we cannot remap, we have two additional constraints: 138 139 - We do the PE# allocation *after* the 64-bit space has been assigned 140 because the addresses we use directly determine the PE#. We then 141 update the M32 PE# for the devices that use both 32-bit and 64-bit 142 spaces or assign the remaining PE# to 32-bit only devices. 143 144 - We cannot "group" segments in HW, so if a device ends up using more 145 than one segment, we end up with more than one PE#. There is a HW 146 mechanism to make the freeze state cascade to "companion" PEs but 147 that only works for PCIe error messages (typically used so that if 148 you freeze a switch, it freezes all its children). So we do it in 149 SW. We lose a bit of effectiveness of EEH in that case, but that's 150 the best we found. So when any of the PEs freezes, we freeze the 151 other ones for that "domain". We thus introduce the concept of 152 "master PE" which is the one used for DMA, MSIs, etc., and "secondary 153 PEs" that are used for the remaining M64 segments. 154 155 We would like to investigate using additional M64 windows in "single 156 PE" mode to overlay over specific BARs to work around some of that, for 157 example for devices with very large BARs, e.g., GPUs. It would make 158 sense, but we haven't done it yet. 159 1603. Considerations for SR-IOV on PowerKVM 161======================================== 162 163 * SR-IOV Background 164 165 The PCIe SR-IOV feature allows a single Physical Function (PF) to 166 support several Virtual Functions (VFs). Registers in the PF's SR-IOV 167 Capability control the number of VFs and whether they are enabled. 168 169 When VFs are enabled, they appear in Configuration Space like normal 170 PCI devices, but the BARs in VF config space headers are unusual. For 171 a non-VF device, software uses BARs in the config space header to 172 discover the BAR sizes and assign addresses for them. For VF devices, 173 software uses VF BAR registers in the *PF* SR-IOV Capability to 174 discover sizes and assign addresses. The BARs in the VF's config space 175 header are read-only zeros. 176 177 When a VF BAR in the PF SR-IOV Capability is programmed, it sets the 178 base address for all the corresponding VF(n) BARs. For example, if the 179 PF SR-IOV Capability is programmed to enable eight VFs, and it has a 180 1MB VF BAR0, the address in that VF BAR sets the base of an 8MB region. 181 This region is divided into eight contiguous 1MB regions, each of which 182 is a BAR0 for one of the VFs. Note that even though the VF BAR 183 describes an 8MB region, the alignment requirement is for a single VF, 184 i.e., 1MB in this example. 185 186 There are several strategies for isolating VFs in PEs: 187 188 - M32 window: There's one M32 window, and it is split into 256 189 equally-sized segments. The finest granularity possible is a 256MB 190 window with 1MB segments. VF BARs that are 1MB or larger could be 191 mapped to separate PEs in this window. Each segment can be 192 individually mapped to a PE via the lookup table, so this is quite 193 flexible, but it works best when all the VF BARs are the same size. If 194 they are different sizes, the entire window has to be small enough that 195 the segment size matches the smallest VF BAR, which means larger VF 196 BARs span several segments. 197 198 - Non-segmented M64 window: A non-segmented M64 window is mapped entirely 199 to a single PE, so it could only isolate one VF. 200 201 - Single segmented M64 windows: A segmented M64 window could be used just 202 like the M32 window, but the segments can't be individually mapped to 203 PEs (the segment number is the PE#), so there isn't as much 204 flexibility. A VF with multiple BARs would have to be in a "domain" of 205 multiple PEs, which is not as well isolated as a single PE. 206 207 - Multiple segmented M64 windows: As usual, each window is split into 256 208 equally-sized segments, and the segment number is the PE#. But if we 209 use several M64 windows, they can be set to different base addresses 210 and different segment sizes. If we have VFs that each have a 1MB BAR 211 and a 32MB BAR, we could use one M64 window to assign 1MB segments and 212 another M64 window to assign 32MB segments. 213 214 Finally, the plan to use M64 windows for SR-IOV, which will be described 215 more in the next two sections. For a given VF BAR, we need to 216 effectively reserve the entire 256 segments (256 * VF BAR size) and 217 position the VF BAR to start at the beginning of a free range of 218 segments/PEs inside that M64 window. 219 220 The goal is of course to be able to give a separate PE for each VF. 221 222 The IODA2 platform has 16 M64 windows, which are used to map MMIO 223 range to PE#. Each M64 window defines one MMIO range and this range is 224 divided into 256 segments, with each segment corresponding to one PE. 225 226 We decide to leverage this M64 window to map VFs to individual PEs, since 227 SR-IOV VF BARs are all the same size. 228 229 But doing so introduces another problem: total_VFs is usually smaller 230 than the number of M64 window segments, so if we map one VF BAR directly 231 to one M64 window, some part of the M64 window will map to another 232 device's MMIO range. 233 234 IODA supports 256 PEs, so segmented windows contain 256 segments, so if 235 total_VFs is less than 256, we have the situation in Figure 1.0, where 236 segments [total_VFs, 255] of the M64 window may map to some MMIO range on 237 other devices:: 238 239 0 1 total_VFs - 1 240 +------+------+- -+------+------+ 241 | | | ... | | | 242 +------+------+- -+------+------+ 243 244 VF(n) BAR space 245 246 0 1 total_VFs - 1 255 247 +------+------+- -+------+------+- -+------+------+ 248 | | | ... | | | ... | | | 249 +------+------+- -+------+------+- -+------+------+ 250 251 M64 window 252 253 Figure 1.0 Direct map VF(n) BAR space 254 255 Our current solution is to allocate 256 segments even if the VF(n) BAR 256 space doesn't need that much, as shown in Figure 1.1:: 257 258 0 1 total_VFs - 1 255 259 +------+------+- -+------+------+- -+------+------+ 260 | | | ... | | | ... | | | 261 +------+------+- -+------+------+- -+------+------+ 262 263 VF(n) BAR space + extra 264 265 0 1 total_VFs - 1 255 266 +------+------+- -+------+------+- -+------+------+ 267 | | | ... | | | ... | | | 268 +------+------+- -+------+------+- -+------+------+ 269 270 M64 window 271 272 Figure 1.1 Map VF(n) BAR space + extra 273 274 Allocating the extra space ensures that the entire M64 window will be 275 assigned to this one SR-IOV device and none of the space will be 276 available for other devices. Note that this only expands the space 277 reserved in software; there are still only total_VFs VFs, and they only 278 respond to segments [0, total_VFs - 1]. There's nothing in hardware that 279 responds to segments [total_VFs, 255]. 280 2814. Implications for the Generic PCI Code 282======================================== 283 284The PCIe SR-IOV spec requires that the base of the VF(n) BAR space be 285aligned to the size of an individual VF BAR. 286 287In IODA2, the MMIO address determines the PE#. If the address is in an M32 288window, we can set the PE# by updating the table that translates segments 289to PE#s. Similarly, if the address is in an unsegmented M64 window, we can 290set the PE# for the window. But if it's in a segmented M64 window, the 291segment number is the PE#. 292 293Therefore, the only way to control the PE# for a VF is to change the base 294of the VF(n) BAR space in the VF BAR. If the PCI core allocates the exact 295amount of space required for the VF(n) BAR space, the VF BAR value is fixed 296and cannot be changed. 297 298On the other hand, if the PCI core allocates additional space, the VF BAR 299value can be changed as long as the entire VF(n) BAR space remains inside 300the space allocated by the core. 301 302Ideally the segment size will be the same as an individual VF BAR size. 303Then each VF will be in its own PE. The VF BARs (and therefore the PE#s) 304are contiguous. If VF0 is in PE(x), then VF(n) is in PE(x+n). If we 305allocate 256 segments, there are (256 - numVFs) choices for the PE# of VF0. 306 307If the segment size is smaller than the VF BAR size, it will take several 308segments to cover a VF BAR, and a VF will be in several PEs. This is 309possible, but the isolation isn't as good, and it reduces the number of PE# 310choices because instead of consuming only numVFs segments, the VF(n) BAR 311space will consume (numVFs * n) segments. That means there aren't as many 312available segments for adjusting base of the VF(n) BAR space.