Re: [edk2-devel] [PATCH for-edk2-stable201905 5/6] OvmfPkg: raise the PCIEXBAR base to 2816 MB on Q35

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

On 5/29/19 5:12 PM, Laszlo Ersek wrote:
> (This is a replacement for commit 75136b29541b, "OvmfPkg/PlatformPei:
> reorder the 32-bit PCI window vs. the PCIEXBAR on q35", 2019-05-16).
> 
> Commit 7b8fe63561b4 ("OvmfPkg: PlatformPei: enable PCIEXBAR (aka MMCONFIG
> / ECAM) on Q35", 2016-03-10) claimed that,
> 
>   On Q35 machine types that QEMU intends to support in the long term, QEMU
>   never lets the RAM below 4 GB exceed 2 GB.
> 
> Alas, this statement came from a misunderstanding that occurred while we
> worked out the interface contract. In fact QEMU does allow the 32-bit RAM
> extend up to 0xB000_0000 (exclusive), in case the RAM size falls in the
> range (0x8000_0000, 0xB000_0000) (i.e., the RAM size is greater than
> 2048MB and smaller than 2816MB).
> 
> In turn, such a RAM size (justifiedly) triggers
> 
>   ASSERT (TopOfLowRam <= PciExBarBase);
> 
> in MemMapInitialization(), because we placed the 256MB PCIEXBAR at
> 0x8000_0000 (2GB) exactly, relying on the interface contract. (And, the
> 32-bit PCI window would follow the PCIEXBAR, covering the [0x9000_0000,
> 0xFC00_0000) range.)
> 
> In order to fix this, place the PCIEXBAR at 2816MB (0xB000_0000), and
> start the 32-bit PCI window at 3 GB (0xC000_0000). This shrinks the 32-bit
> PCI window to
> 
>   0xFC00_0000 - 0xC000_0000 = 0x3C00_0000 = 960 MB.

This fix is simpler.

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

> 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>
> ---
>  OvmfPkg/OvmfPkgIa32.dsc    | 4 ++--
>  OvmfPkg/OvmfPkgIa32X64.dsc | 4 ++--
>  OvmfPkg/OvmfPkgX64.dsc     | 4 ++--
>  3 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
> index 578fc6c98ec8..e74a9d5a5149 100644
> --- a/OvmfPkg/OvmfPkgIa32.dsc
> +++ b/OvmfPkg/OvmfPkgIa32.dsc
> @@ -492,8 +492,8 @@ [PcdsFixedAtBuild]
>    # the PCIEXBAR register.
>    #
>    # On Q35 machine types that QEMU intends to support in the long term, QEMU
> -  # never lets the RAM below 4 GB exceed 2 GB.
> -  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
> +  # never lets the RAM below 4 GB exceed 2816 MB.
> +  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
>  
>  !ifdef $(SOURCE_DEBUG_ENABLE)
>    gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
> diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
> index eade8f62d3de..67ac015991fd 100644
> --- a/OvmfPkg/OvmfPkgIa32X64.dsc
> +++ b/OvmfPkg/OvmfPkgIa32X64.dsc
> @@ -497,8 +497,8 @@ [PcdsFixedAtBuild]
>    # the PCIEXBAR register.
>    #
>    # On Q35 machine types that QEMU intends to support in the long term, QEMU
> -  # never lets the RAM below 4 GB exceed 2 GB.
> -  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
> +  # never lets the RAM below 4 GB exceed 2816 MB.
> +  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
>  
>  !ifdef $(SOURCE_DEBUG_ENABLE)
>    gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
> diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
> index 733a4c9d8a43..68073ef55b4d 100644
> --- a/OvmfPkg/OvmfPkgX64.dsc
> +++ b/OvmfPkg/OvmfPkgX64.dsc
> @@ -497,8 +497,8 @@ [PcdsFixedAtBuild]
>    # the PCIEXBAR register.
>    #
>    # On Q35 machine types that QEMU intends to support in the long term, QEMU
> -  # never lets the RAM below 4 GB exceed 2 GB.
> -  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0x80000000
> +  # never lets the RAM below 4 GB exceed 2816 MB.
> +  gEfiMdePkgTokenSpaceGuid.PcdPciExpressBaseAddress|0xB0000000
>  
>  !ifdef $(SOURCE_DEBUG_ENABLE)
>    gEfiSourceLevelDebugPkgTokenSpaceGuid.PcdDebugLoadImageMethod|0x2
>