Re: [edk2-devel] [PATCH for-edk2-stable201905 1/6] Revert "OvmfPkg/PlatformPei: fix MTRR for low-RAM sizes that have many bits clear"

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 5/29/19 5:12 PM, Laszlo Ersek wrote:
> This reverts commit 39b9a5ffe6618b7870be2a54fe7725000249c33a.
> 
> The original fix for <https://bugzilla.tianocore.org/show_bug.cgi?id=1814>
> triggered a bug / incorrect assumption in QEMU.
> 
> QEMU assumes that the PCIEXBAR is below the 32-bit PCI window, not above
> it. When the firmware doesn't satisfy this assumption, QEMU generates an
> \_SB.PCI0._CRS object in the ACPI DSDT that does not reflect the
> firmware's 32-bit MMIO BAR assignments. This causes OSes to re-assign
> 32-bit MMIO BARs.
> 
> Working around the problem in the firmware looks less problematic than
> fixing QEMU. Revert the original changes first, before implementing an
> alternative fix.
> 
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1859
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>

> ---
>  OvmfPkg/PlatformPei/Platform.h  |  2 --
>  OvmfPkg/PlatformPei/MemDetect.c | 23 +++-----------------
>  OvmfPkg/PlatformPei/Platform.c  |  4 +++-
>  3 files changed, 6 insertions(+), 23 deletions(-)
> 
> diff --git a/OvmfPkg/PlatformPei/Platform.h b/OvmfPkg/PlatformPei/Platform.h
> index 4476ddd871cd..81af8b71480f 100644
> --- a/OvmfPkg/PlatformPei/Platform.h
> +++ b/OvmfPkg/PlatformPei/Platform.h
> @@ -114,6 +114,4 @@ extern UINT32 mMaxCpuCount;
>  
>  extern UINT16 mHostBridgeDevId;
>  
> -extern UINT32 mQemuUc32Base;
> -
>  #endif // _PLATFORM_PEI_H_INCLUDED_
> diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
> index ae73c63d27d5..e890e36408a6 100644
> --- a/OvmfPkg/PlatformPei/MemDetect.c
> +++ b/OvmfPkg/PlatformPei/MemDetect.c
> @@ -42,8 +42,6 @@ STATIC UINT32 mS3AcpiReservedMemorySize;
>  
>  STATIC UINT16 mQ35TsegMbytes;
>  
> -UINT32 mQemuUc32Base;
> -
>  VOID
>  Q35TsegMbytesInitialization (
>    VOID
> @@ -665,8 +663,6 @@ QemuInitializeRam (
>    // cover it exactly.
>    //
>    if (IsMtrrSupported ()) {
> -    UINT32 Uc32Size;
> -
>      MtrrGetAllMtrrs (&MtrrSettings);
>  
>      //
> @@ -693,24 +689,11 @@ QemuInitializeRam (
>  
>      //
>      // Set memory range from the "top of lower RAM" (RAM below 4GB) to 4GB as
> -    // uncacheable. Make sure one variable MTRR suffices by truncating the size
> -    // to a whole power of two. This will round the base *up*, and a gap (not
> -    // used for either RAM or MMIO) may stay in the middle, marked as
> -    // cacheable-by-default.
> +    // uncacheable
>      //
> -    Uc32Size = GetPowerOfTwo32 ((UINT32)(SIZE_4GB - LowerMemorySize));
> -    mQemuUc32Base = (UINT32)(SIZE_4GB - Uc32Size);
> -    if (mQemuUc32Base != LowerMemorySize) {
> -      DEBUG ((DEBUG_VERBOSE, "%a: rounded UC32 base from 0x%x up to 0x%x, for "
> -        "an UC32 size of 0x%x\n", __FUNCTION__, (UINT32)LowerMemorySize,
> -        mQemuUc32Base, Uc32Size));
> -    }
> -
> -    Status = MtrrSetMemoryAttribute (mQemuUc32Base, Uc32Size,
> -               CacheUncacheable);
> +    Status = MtrrSetMemoryAttribute (LowerMemorySize,
> +               SIZE_4GB - LowerMemorySize, CacheUncacheable);
>      ASSERT_EFI_ERROR (Status);
> -  } else {
> -    mQemuUc32Base = (UINT32)LowerMemorySize;
>    }
>  }
>  
> diff --git a/OvmfPkg/PlatformPei/Platform.c b/OvmfPkg/PlatformPei/Platform.c
> index c064b4ed9b8f..fd8eccaf3e50 100644
> --- a/OvmfPkg/PlatformPei/Platform.c
> +++ b/OvmfPkg/PlatformPei/Platform.c
> @@ -174,12 +174,14 @@ MemMapInitialization (
>    AddIoMemoryRangeHob (0x0A0000, BASE_1MB);
>  
>    if (!mXen) {
> +    UINT32  TopOfLowRam;
>      UINT64  PciExBarBase;
>      UINT32  PciBase;
>      UINT32  PciSize;
>  
> +    TopOfLowRam = GetSystemMemorySizeBelow4gb ();
>      PciExBarBase = 0;
> -    PciBase = (mQemuUc32Base < BASE_2GB) ? BASE_2GB : mQemuUc32Base;
> +    PciBase = (TopOfLowRam < BASE_2GB) ? BASE_2GB : TopOfLowRam;
>      if (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
>        //
>        // The 32-bit PCI host aperture is expected to fall between the top of
>