Re: [edk2-devel] [PATCH v2 2/4] OvmfPkg/PlatformPei: consider AP stacks for pei memory cap

["Laszlo Ersek" <lersek@redhat.com>]

On 2/2/24 11:47, Gerd Hoffmann wrote:
> Needed to avoid running out of memory when booting
> with a large (~2048) number of vcpus.
> 
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> ---
>  OvmfPkg/PlatformPei/MemDetect.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/OvmfPkg/PlatformPei/MemDetect.c b/OvmfPkg/PlatformPei/MemDetect.c
> index 1684a3783546..83f1c1d02a26 100644
> --- a/OvmfPkg/PlatformPei/MemDetect.c
> +++ b/OvmfPkg/PlatformPei/MemDetect.c
> @@ -187,6 +187,8 @@ GetPeiMemoryCap (
>    UINT32   Pml4Entries;
>    UINT32   PdpEntries;
>    UINTN    TotalPages;
> +  UINT64   ApStacks;
> +  UINT64   MemoryCap;
>  
>    //
>    // If DXE is 32-bit, then just return the traditional 64 MB cap.
> @@ -234,12 +236,21 @@ GetPeiMemoryCap (
>                 (PdpEntries + 1) * Pml4Entries + 1;
>    ASSERT (TotalPages <= 0x40201);
>  
> +  //
> +  // With 32k stacks and 4096 vcpus this lands at 128 MB (far away
> +  // from MAX_UINT32).
> +  //
> +  ApStacks = PlatformInfoHob->PcdCpuMaxLogicalProcessorNumber * PcdGet32 (PcdCpuApStackSize);
> +
>    //
>    // Add 64 MB for miscellaneous allocations. Note that for
> -  // PhysMemAddressWidth values close to 36, the cap will actually be
> -  // dominated by this increment.
> +  // PhysMemAddressWidth values close to 36 and a small number of
> +  // CPUs, the cap will actually be dominated by this increment.
>    //
> -  return (UINT32)(EFI_PAGES_TO_SIZE (TotalPages) + SIZE_64MB);
> +  MemoryCap = EFI_PAGES_TO_SIZE (TotalPages) + ApStacks + SIZE_64MB;
> +
> +  ASSERT (MemoryCap <= MAX_UINT32);
> +  return (UINT32)MemoryCap;
>  }
>  
>  /**

Yes, this looks good. At this point, EFI_PAGES_TO_SIZE (TotalPages) is
~1GB at most, and ApStacks is empirically <= 128 MB, plus ApStacks is
UINT64. Therefore on IA32X64 we have

  UINT64 = (UINT32 + UINT64) + INT32
->
  UINT64 = (UINT64 + UINT64) + INT32
->
  UINT64 = UINT64 + INT32
->
  UINT64 = UINT64 + UINT64
->
  UINT64 = UINT64

OK!

Reviewed-by: Laszlo Ersek <lersek@redhat.com>




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