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

["Yao, Jiewen" <jiewen.yao@intel.com>]

Yes. I am thinking the same thing.

[CC Flag memory location]
1) A general purpose register, such as EBP.

2) A global variable, such as 
.data
TeeFlags: DD 0

3) A fixed region in stack, such as
dword[STACK_TOP - 4]

4) A new CC common fixed region, such as 
dword[CC_COMMON_FLAGS]

5) A fixed region piggyback on existing CC working area, such as
dword[CC_WORKING_AREA]

Hi Brijesh/Min
Any preference?

[CC Indicator Flags]
Proposal: UINT8[4]

Byte [0] Version: 0
byte [1] Length: 4
byte [2] Type:
	0: legacy
	1: SEV
	2: TDX
byte [3] Sub Type: 
	If Type is 0 (legacy), then
		0: legacy
	If Type is 1 (SEV), then
		0: SEV
		1: SEV-ES
		2: SEV-SNP
	If Type is 2 (TDX), then
		0: TDX 1.0

Thank you
Yao Jiewen


> -----Original Message-----
> From: Xu, Min M <min.m.xu@intel.com>
> Sent: Wednesday, July 28, 2021 2:58 PM
> To: Yao, Jiewen <jiewen.yao@intel.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>; devel@edk2.groups.io; Ard
> Biesheuvel <ardb+tianocore@kernel.org>; Justen, Jordan L
> <jordan.l.justen@intel.com>; Erdem Aktas <erdemaktas@google.com>; James
> Bottomley <jejb@linux.ibm.com>; Tom Lendacky <thomas.lendacky@amd.com>
> Subject: RE: [PATCH V3 06/10] OvmfPkg: Add AmdSev.asm in ResetVector
> 
> 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;
> > >>