Re: [edk2-devel] [PATCH V2 4/4] OvmfPkg/ResetVector: Update ResetVector to support Tdx

["Min Xu" <min.m.xu@intel.com>]

On July 23, 2021 9:35 PM, Lendacky, Thomas wrote:
> On 7/22/21 5:58 PM, Xu, Min M wrote:
> > On July 23, 2021 1:08 AM, Tom Lendacky wrote:
> >> On 7/22/21 12:52 AM, Min Xu wrote:
> >>> RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429
> >>>
> >>> diff --git a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> >>> b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> >>> index c6d0d898bcd1..2206ca719593 100644
> >>> --- a/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> >>> +++ b/OvmfPkg/ResetVector/Ia32/Flat32ToFlat64.asm
> >>> @@ -17,6 +17,9 @@ Transition32FlatTo64Flat:
> >>>
> >>>      OneTimeCall SetCr3ForPageTables64
> >>>
> >>> +    cmp     dword[TDX_WORK_AREA], 0x47584454 ; 'TDXG'
> >>> +    jz      TdxTransition32FlatTo64Flat
> >>> +
> >>
> >> Is the memory area guaranteed to be zeroed for legacy guests?
> >> Hopefully, this won't trip up a non-TDX guest with a false match (highly
> unlikely, though).
> >>
> > TDX_WORK_AREA is piece of TdxMailbox which is located in the MEMFD
> > started from PcdOvmfSecGhcbBackupBase. In Td guest, this memory region
> > is initialized to all-0 by host VMM. In legacy guests, I am not sure
> > what's the initialized value it is. So 'TDXG' is checked to guarantee it is Td-
> guest or not.
> > Since Tdx re-use the memory region (PcdOvmfSecGhcbBackupBase) as the
> > TDX_WORK_AREA, and @Tom Lendacky you should be the original owner of
> > PcdOvmfSecGhcbBackupBase, can this area be cleared in the beginning of
> > ResetVector in legacy guests? Or I should better create a TDX specific
> > work area in MEMFD to guarantee the Td And Non-Td check?
> 
> I believe PcdOvmfSecGhcbBackupBase can be cleared early. For SEV-ES, it isn't
> shared with the hypervisor, so clearing it before activating the pagetables can
> be done (it will be treated as encrypted before paging is enabled and mapped
> as encrypted after paging is enabled) and for a legacy guest the mapping
> doesn't matter. It isn't required to be cleared today, so if you do add
> something, be sure to put a comment in there about why it's being done. No
> need for a new area.
> 
> The possibility of random data being there that matches 'TDXG' is extremely
> low. But better safe than sorry, I guess.
>
Thanks Tom. I will update it in the next version.
>

Thanks!
Xu, Min