Re: [edk2-devel] [PATCH v2 2/2] UefiCpuPkg/MpInitLib: Ensure SEV-SNP VMSA allocations are not 2MB aligned

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 3/28/23 13:09, Lendacky, Thomas via groups.io wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4353
> 
> Due to AMD erratum #1467, an SEV-SNP VMSA should not be 2MB aligned. To
> work around this issue, allocate two pages instead of one. Because of the
> way that page allocation is implemented, always try to use the second
> page. If the second page is not 2MB aligned, free the first page and use
> the second page. If the second page is 2MB aligned, free the second page
> and use the first page. Freeing in this way reduces holes in the memory
> map.
> 
> Fixes: 06544455d0d4 ("UefiCpuPkg/MpInitLib: Use SEV-SNP AP Creation ...")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>   UefiCpuPkg/Library/MpInitLib/X64/AmdSev.c | 26 ++++++++++++++++++++---
>   1 file changed, 23 insertions(+), 3 deletions(-)
> 
> diff --git a/UefiCpuPkg/Library/MpInitLib/X64/AmdSev.c b/UefiCpuPkg/Library/MpInitLib/X64/AmdSev.c
> index 509be9b41757..c9f0984f41a2 100644
> --- a/UefiCpuPkg/Library/MpInitLib/X64/AmdSev.c
> +++ b/UefiCpuPkg/Library/MpInitLib/X64/AmdSev.c
> @@ -13,6 +13,8 @@
>   #include <Register/Amd/Fam17Msr.h>
>   #include <Register/Amd/Ghcb.h>
>   
> +#define _IS_ALIGNED(x, y)  (ALIGN_POINTER((x), (y)) == (x))

I haven't seen any movement on Gerd's series to create common macros for 
alignment checks, but can't wait forever. So here I use an underscore in 
front of IS_ALIGNED() so that there won't be a conflict either before or 
after Gerd's series is applied. Once Gerd's series is applied, I can go 
back and submit another patch to use the new macros.

Thanks,
Tom

> +
>   /**
>     Perform the requested AP Creation action.
>   
> @@ -121,6 +123,7 @@ SevSnpCreateSaveArea (
>     UINT32          ApicId
>     )
>   {
> +  UINT8             *Pages;
>     SEV_ES_SAVE_AREA  *SaveArea;
>     IA32_CR0          ApCr0;
>     IA32_CR0          ResetCr0;
> @@ -131,13 +134,30 @@ SevSnpCreateSaveArea (
>   
>     if (CpuData->SevEsSaveArea == NULL) {
>       //
> -    // Allocate a single page for the SEV-ES Save Area and initialize it.
> +    // Allocate a page for the SEV-ES Save Area and initialize it. Due to AMD
> +    // erratum #1467 (VMSA cannot be on a 2MB boundary), allocate an extra page
> +    // to choose from to work around the issue.
>       //
> -    SaveArea = AllocateReservedPages (1);
> -    if (!SaveArea) {
> +    Pages = AllocateReservedPages (2);
> +    if (!Pages) {
>         return;
>       }
>   
> +    //
> +    // Since page allocation works by allocating downward in the address space,
> +    // try to always free the first (lower address) page to limit possible holes
> +    // in the memory map. So, if the address of the second page is 2MB aligned,
> +    // then use the first page and free the second page. Otherwise, free the
> +    // first page and use the second page.
> +    //
> +    if (_IS_ALIGNED (Pages + EFI_PAGE_SIZE, SIZE_2MB)) {
> +      SaveArea = (SEV_ES_SAVE_AREA *)Pages;
> +      FreePages (Pages + EFI_PAGE_SIZE, 1);
> +    } else {
> +      SaveArea = (SEV_ES_SAVE_AREA *)(Pages + EFI_PAGE_SIZE);
> +      FreePages (Pages, 1);
> +    }
> +
>       CpuData->SevEsSaveArea = SaveArea;
>     } else {
>       SaveArea = CpuData->SevEsSaveArea;