Re: [PATCH v2 1/1] OvmfPkg/PlatformInitLib: q35 mtrr setup fix

["Ard Biesheuvel" <ardb@kernel.org>]

On Thu, 22 Sept 2022 at 07:55, Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> Traditional q35 memory layout is 2.75 GB of low memory, leaving room
> for the pcie mmconfig at 0xb0000000 and the 32-bit pci mmio window at
> 0xc0000000.  Because of that OVMF tags the memory range above
> 0xb0000000 as uncachable via mtrr.
>
> A while ago qemu started to gigabyte-align memory by default (to make
> huge pages more effective) and q35 uses only 2G of low memory in that
> case.  Which effectively makes the 32-bit pci mmio window start at
> 0x80000000.
>
> This patch updates the mtrr setup code accordingly.
>
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

Reviewed-by: Ard Biesheuvel <ardb@kernel.org>

Merged as #3425



> ---
>  OvmfPkg/Library/PlatformInitLib/MemDetect.c | 26 ++++++++++++++-------
>  1 file changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> index 942eaf89cfcf..d1a4f4b20791 100644
> --- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> +++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> @@ -55,15 +55,25 @@ PlatformQemuUc32BaseInitialization (
>    }
>
>    if (PlatformInfoHob->HostBridgeDevId == INTEL_Q35_MCH_DEVICE_ID) {
> -    //
> -    // On q35, the 32-bit area that we'll mark as UC, through variable MTRRs,
> -    // starts at PcdPciExpressBaseAddress. The platform DSC is responsible for
> -    // setting PcdPciExpressBaseAddress such that describing the
> -    // [PcdPciExpressBaseAddress, 4GB) range require a very small number of
> -    // variable MTRRs (preferably 1 or 2).
> -    //
> +    LowerMemorySize = PlatformGetSystemMemorySizeBelow4gb (PlatformInfoHob);
>      ASSERT (PcdGet64 (PcdPciExpressBaseAddress) <= MAX_UINT32);
> -    PlatformInfoHob->Uc32Base = (UINT32)PcdGet64 (PcdPciExpressBaseAddress);
> +    ASSERT (PcdGet64 (PcdPciExpressBaseAddress) >= LowerMemorySize);
> +
> +    if (LowerMemorySize <= BASE_2GB) {
> +      // Newer qemu with gigabyte aligned memory,
> +      // 32-bit pci mmio window is 2G -> 4G then.
> +      PlatformInfoHob->Uc32Base = BASE_2GB;
> +    } else {
> +      //
> +      // On q35, the 32-bit area that we'll mark as UC, through variable MTRRs,
> +      // starts at PcdPciExpressBaseAddress. The platform DSC is responsible for
> +      // setting PcdPciExpressBaseAddress such that describing the
> +      // [PcdPciExpressBaseAddress, 4GB) range require a very small number of
> +      // variable MTRRs (preferably 1 or 2).
> +      //
> +      PlatformInfoHob->Uc32Base = (UINT32)PcdGet64 (PcdPciExpressBaseAddress);
> +    }
> +
>      return;
>    }
>
> --
> 2.37.3
>