Re: [PATCH 4/4] OvmfPkg/PlatformPei: fix MTRR for low-RAM sizes that have many bits clear

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

On Sat, 4 May 2019 at 02:07, Laszlo Ersek <lersek@redhat.com> wrote:
>
> Assume that we boot OVMF in a QEMU guest with 1025 MB of RAM. The
> following assertion will fire:
>
> > ASSERT_EFI_ERROR (Status = Out of Resources)
> > ASSERT OvmfPkg/PlatformPei/MemDetect.c(696): !EFI_ERROR (Status)
>
> That's because the range [1025 MB, 4 GB) that we try to mark as
> uncacheable with MTRRs has size 3071 MB:
>
>    0x1_0000_0000
>   -0x0_4010_0000
>   --------------
>    0x0_BFF0_0000
>
> The integer that stands for the uncacheable area size has 11 (eleven) bits
> set to 1. As a result, covering this size requires 11 variable MTRRs (each
> MTRR must cover a naturally aligned, power-of-two sized area). But, if we
> need more variable MTRRs than the CPU can muster (such as 8), then
> MtrrSetMemoryAttribute() fails, and we refuse to continue booting (which
> is justified, in itself).
>
> Unfortunately, this is not difficult to trigger, and the error message is
> well-hidden from end-users, in the OVMF debug log. The following
> mitigation is inspired by SeaBIOS:
>
> Truncate the uncacheable area size to a power-of-two, while keeping the
> end fixed at 4 GB. Such an interval can be covered by just one variable
> MTRR.
>
> This may leave such an MMIO gap, between the end of low-RAM and the start
> of the uncacheable area, that is marked as WB (through the MTRR default).
> Raise the base of the 32-bit PCI MMIO aperture accordingly -- the gap will
> not be used for anything.
>
> On Q35, the minimal 32-bit PCI MMIO aperture (triggered by RAM size 2815
> MB) shrinks from
>
>   0xE000_0000 - 0xAFF0_0000 = 769 MB
>
> to
>
>   0xE000_0000 - 0xC000_0000 = 512 MB
>
> On i440fx, the minimal 32-bit PCI MMIO aperture (triggered by RAM size
> 3583 MB) shrinks from
>
>   0xFC00_0000 - 0xDFF0_0000 = 449 MB
>
> to
>
>   0xFC00_0000 - 0xE000_0000 = 448 MB
>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1666941
> Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1701710
> Signed-off-by: Laszlo Ersek <lersek@redhat.com>

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>


> ---
>  OvmfPkg/PlatformPei/Platform.h  |  2 ++
>  OvmfPkg/PlatformPei/MemDetect.c | 23 +++++++++++++++++---
>  OvmfPkg/PlatformPei/Platform.c  |  4 +---
>  3 files changed, 23 insertions(+), 6 deletions(-)
>
> diff --git a/OvmfPkg/PlatformPei/Platform.h b/OvmfPkg/PlatformPei/Platform.h
> index 81af8b71480f..4476ddd871cd 100644
> --- a/OvmfPkg/PlatformPei/Platform.h
> +++ b/OvmfPkg/PlatformPei/Platform.h
> @@ -114,4 +114,6 @@ 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 e890e36408a6..ae73c63d27d5 100644
> --- a/OvmfPkg/PlatformPei/MemDetect.c
> +++ b/OvmfPkg/PlatformPei/MemDetect.c
> @@ -42,6 +42,8 @@ STATIC UINT32 mS3AcpiReservedMemorySize;
>
>  STATIC UINT16 mQ35TsegMbytes;
>
> +UINT32 mQemuUc32Base;
> +
>  VOID
>  Q35TsegMbytesInitialization (
>    VOID
> @@ -663,6 +665,8 @@ QemuInitializeRam (
>    // cover it exactly.
>    //
>    if (IsMtrrSupported ()) {
> +    UINT32 Uc32Size;
> +
>      MtrrGetAllMtrrs (&MtrrSettings);
>
>      //
> @@ -689,11 +693,24 @@ QemuInitializeRam (
>
>      //
>      // Set memory range from the "top of lower RAM" (RAM below 4GB) to 4GB as
> -    // uncacheable
> +    // 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.
>      //
> -    Status = MtrrSetMemoryAttribute (LowerMemorySize,
> -               SIZE_4GB - LowerMemorySize, CacheUncacheable);
> +    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);
>      ASSERT_EFI_ERROR (Status);
> +  } else {
> +    mQemuUc32Base = (UINT32)LowerMemorySize;
>    }
>  }
>
> diff --git a/OvmfPkg/PlatformPei/Platform.c b/OvmfPkg/PlatformPei/Platform.c
> index fd8eccaf3e50..c064b4ed9b8f 100644
> --- a/OvmfPkg/PlatformPei/Platform.c
> +++ b/OvmfPkg/PlatformPei/Platform.c
> @@ -174,14 +174,12 @@ MemMapInitialization (
>    AddIoMemoryRangeHob (0x0A0000, BASE_1MB);
>
>    if (!mXen) {
> -    UINT32  TopOfLowRam;
>      UINT64  PciExBarBase;
>      UINT32  PciBase;
>      UINT32  PciSize;
>
> -    TopOfLowRam = GetSystemMemorySizeBelow4gb ();
>      PciExBarBase = 0;
> -    PciBase = (TopOfLowRam < BASE_2GB) ? BASE_2GB : TopOfLowRam;
> +    PciBase = (mQemuUc32Base < BASE_2GB) ? BASE_2GB : mQemuUc32Base;
>      if (mHostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
>        //
>        // The 32-bit PCI host aperture is expected to fall between the top of
> --
> 2.19.1.3.g30247aa5d201
>