Re: [PATCH V3 06/10] OvmfPkg: Add AmdSev.asm in ResetVector

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

On July 28, 2021 2:05 PM, Yao, Jiewen wrote:
> It does not necessary to be a working area.
> 
> We just need a common TEE flag to indicate if the system run in legacy, SEV, or
> TDX, right?
Right. We need somewhere to store this flag, either in a Register or in Memory. 
If it is memory, then in Tdx the memory region should be initialized by host VMM.
> 
> thank you!
> Yao, Jiewen
> 
> 
> > 在 2021年7月28日，下午1:07，Xu, Min M <min.m.xu@intel.com> 写道：
> >
> > On July 27, 2021 8:46 PM, Yao, Jiewen wrote:
> >> HI Min
> >> I agree with Brijesh.
> >>
> >> The basic rule is: SEV file shall never refer to TDX data structure.
> >> TDX file shall never refer to SEV data structure.
> >> These code should be isolated clearly.
> >>
> >> Do we still need that logic if we follow the new pattern?
> > I have replied to Brijesh's mail about the concern of the new pattern.
> >
> > I have some concern in the current pattern:
> > ====================
> >    OneTimeCall   PreMainFunctionHookSev
> >    OneTimeCall   PreMainFunctionHookTdx
> > MainFunction:
> >    XXXXXX
> >    OneTimeCall   PostMainFunctionHookSev
> >    OneTimeCall   PostMainFunctionHookTdx
> > ====================
> > The TEE function need implement a TEE check function (such as IsSev, or IsTdx).
> >
> > Tdx call CPUID(0x21) to determine if it is tdx guest in the very
> > beginning of ResetVector. Then 'TDXG' is set in TDX_WORK_AREA. SEV does
> the similar work which call CheckSevFeatures to set SEV_ES_WORK_AREA to 1.
> >
> > After that both TDX and SEV read the above WORK_AREA to check if it is TDX
> or SEV or legacy guest.
> >
> > In Tdx the access to SEV_ES_WORK_AREA will trigger error because
> SEV_ES_WORK_AREA is *NOT* initialized by host VMM.
> > In SEV-SNP I am afraid the access to TDX_WORK_AREA will trigger error too.
> >
> > I am wondering if TDX and SEV can use the same memory region (for example,
> TEE_WORK_AREA) as the work area?
> > So that this work area is guaranteed to be initialized in both TDX and
> > SEV. Structure of the TEE_WORK_AREA may look like this:
> >  typedef struct {
> >      UINT8  Flag[4];         'TDXG' or 'SEVG' or all-0
> >      UINT8  Others[];
> >  } TEE_WORK_AREA;
> >>