[edk2-devel] [PATCH v1] OvmfPkg/PlatformInitLib: Don't override user specified PciMmio64Size

["Vivek Kasireddy" <vivek.kasireddy@intel.com>]

If the user specified a size for the PCI MMIO window via the option:
-fw_cfg name=opt/ovmf/X-PciMmio64Mb,string=32768
then this patch ensures that the mmio window is not resized again.

Essentially, this prevents the change introduced in the following
patch from taking effect:
commit ecb778d0ac62560aa172786ba19521f27bc3f650
Author: Gerd Hoffmann <kraxel@redhat.com>
Date:   Tue Oct 4 15:47:27 2022 +0200

    OvmfPkg/PlatformInitLib: dynamic mmio window size

    In case we have a reliable PhysMemAddressWidth use that to dynamically
    size the 64bit address window.  Allocate 1/8 of the physical address
    space and place the window at the upper end of the address space.

The problem this patch is trying to solve is the VFIO mapping failures:
VFIO_MAP_DMA failed: Invalid argument
vfio_dma_map(0x557b2f2736d0, 0x380000000000, 0x1000000, 0x7f98ac400000) = -22 (Invalid argument)
that occur when we try to passthrough the graphics device to the guest:
qemu-system-x86_64 -m 4096 -enable-kvm -cpu host -smp cores=4,threads=2,sockets=1
-device vfio-pci,host=0000:00:02.0 -bios OVMF.fd -nographic

The above failures seem to occur because of a mismatch between the
PhysMemAddressWidth and the Host IOMMU address width. More specifically,
if the PhysMemAddressWidth is bigger than the IOMMU address width,
VFIO fails to map the MMIO regions as the IOVAs would be larger
than the IOMMU aperture regions. When tested on modern Intel platforms
such as ADL, MTL, etc, OVMF determines PhysMemAddressWidth = 46 which
matches the Host address width but the IOMMU address width seems to
range anywhere from 38 to 48 depending on the IOMMU hardware
capabilities, version, etc.

One way to address this issue is if we ensure that PhysMemAddressWidth
matches IOMMU address width:
-cpu host,host-phys-bits=on,host-phys-bits-limit=<IOMMU address width>
However, this requires the user to figure out the IOMMU address width;
which can be determined by looking at the 16-21 bits of the cap value:
cat /sys/devices/virtual/iommu/dmar0/intel-iommu/cap
or by reading the DMAR_CAP_REG register. But this does not seem like
a reasonable approach to solve this problem.

Therefore, this problem requires an OVMF specific solution to retain
the prior behavior. To this end, this patch reuses the X-PciMmio64Mb
option to opt-out of the behavior introduced in the above commit
instead of adding a new option or mechanism.

Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>
---
 OvmfPkg/Include/Library/PlatformInitLib.h   | 1 +
 OvmfPkg/Library/PlatformInitLib/MemDetect.c | 4 +++-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/OvmfPkg/Include/Library/PlatformInitLib.h b/OvmfPkg/Include/Library/PlatformInitLib.h
index 57b18b94d9..e8ea3defa2 100644
--- a/OvmfPkg/Include/Library/PlatformInitLib.h
+++ b/OvmfPkg/Include/Library/PlatformInitLib.h
@@ -55,6 +55,7 @@ typedef struct {
   BOOLEAN              QemuFwCfgChecked;
   BOOLEAN              QemuFwCfgSupported;
   BOOLEAN              QemuFwCfgDmaSupported;
+  BOOLEAN              QemuFwCfgSizeSpecified;
 } EFI_HOB_PLATFORM_INFO;
 #pragma pack()
 
diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
index 662e7e85bb..a53a1e24e4 100644
--- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
+++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
@@ -485,6 +485,7 @@ PlatformGetFirstNonAddress (
     case EFI_SUCCESS:
       if (FwCfgPciMmio64Mb <= 0x1000000) {
         PlatformInfoHob->PcdPciMmio64Size = LShiftU64 (FwCfgPciMmio64Mb, 20);
+        PlatformInfoHob->QemuFwCfgSizeSpecified = TRUE;
         break;
       }
 
@@ -861,7 +862,8 @@ PlatformAddressWidthInitialization (
   }
 
   PlatformAddressWidthFromCpuid (PlatformInfoHob, TRUE);
-  if (PlatformInfoHob->PhysMemAddressWidth != 0) {
+  if (PlatformInfoHob->PhysMemAddressWidth != 0 &&
+      !PlatformInfoHob->QemuFwCfgSizeSpecified) {
     // physical address width is known
     PlatformDynamicMmioWindow (PlatformInfoHob);
     return;
-- 
2.39.2



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