Re: [PATCH V9 2/4] OvmfPkg: Clear WORK_AREA_GUEST_TYPE in Main.asm

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

On 10/11/21 9:37 PM, Min Xu wrote:
> RFC： https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D3429&data=04%7C01%7Cthomas.lendacky%40amd.com%7Cc4c4ac9654a940ada92308d98d2994d0%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637696032012206979%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=1SVRKXztfFcaVVer1AYOhLIhs6sVW%2BwtYQNxuuHHbTE%3D&reserved=0
> 
> Previously WORK_AREA_GUEST_TYPE was cleared in SetCr3ForPageTables64.
> This is workable for Legacy guest and SEV guest. But it doesn't work
> after Intel TDX is introduced. It is because all TDX CPUs (BSP and APs)
> start to run from 0xfffffff0, thus WORK_AREA_GUEST_TYPE will be cleared
> multi-times if it is TDX guest. So the clearance of WORK_AREA_GUEST_TYPE
> is moved to Main16 entry point in Main.asm.
> Note: WORK_AREA_GUEST_TYPE is only defined for ARCH_X64.
> 
> For Intel TDX, its corresponding entry point is Main32 (which will be
> introduced in next commit in this patch-set). WORK_AREA_GUEST_TYPE will
> be cleared there.
> 
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
> Signed-off-by: Min Xu <min.m.xu@intel.com>
> ---
>   OvmfPkg/ResetVector/Ia32/PageTables64.asm | 4 ----
>   OvmfPkg/ResetVector/Main.asm              | 8 ++++++++
>   2 files changed, 8 insertions(+), 4 deletions(-)
> 
> diff --git a/OvmfPkg/ResetVector/Ia32/PageTables64.asm b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> index 07b6ca070909..02528221e560 100644
> --- a/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> +++ b/OvmfPkg/ResetVector/Ia32/PageTables64.asm
> @@ -42,10 +42,6 @@ BITS    32
>   ;
>   SetCr3ForPageTables64:
>   
> -    ; Clear the WorkArea header. The SEV probe routines will populate the
> -    ; work area when detected.
> -    mov     byte[WORK_AREA_GUEST_TYPE], 0
> -
>       ; Check whether the SEV is active and populate the SevEsWorkArea
>       OneTimeCall   CheckSevFeatures
>   
> diff --git a/OvmfPkg/ResetVector/Main.asm b/OvmfPkg/ResetVector/Main.asm
> index ae90a148fce7..a501fbe880f2 100644
> --- a/OvmfPkg/ResetVector/Main.asm
> +++ b/OvmfPkg/ResetVector/Main.asm
> @@ -36,6 +36,14 @@ Main16:
>   
>   BITS    32
>   
> +%ifdef ARCH_X64

A regular SEV guest can be built in the hybrid IA32 and X64 configuration, 
so this will break existing SEV firmwares built in that manner. Only 
SEV-ES and SEV-SNP require the full X64 confguration.

Thanks,
Tom

> +
> +    ; Clear the WorkArea header. The SEV probe routines will populate the
> +    ; work area when detected.
> +    mov     byte[WORK_AREA_GUEST_TYPE], 0
> +
> +%endif
> +
>       ;
>       ; Search for the Boot Firmware Volume (BFV)
>       ;
>