[edk2-devel] [PATCH V3 06/10] OvmfPkg: Add AmdSev.asm in ResetVector

Wed Jul 28 08:34:21 UTC 2021

I noticed SEV has the memory region of SEV_ES_WORK_AREA (gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase) in MEMFD
The definition is below:
  typedef struct _SEC_SEV_ES_WORK_AREA {
    UINT8    SevEsEnabled;
    UINT8    Reserved1[7];
    UINT64   RandomData;
    UINT64   EncryptionMask;
  } SEC_SEV_ES_WORK_AREA;

In ResetVector SEV flag is recorded in the first byte of SEV_ES_WORK_AREA. In SecMain.c this flag is read to determine SEV.

I am thinking whether this memory region can be used by both TDX and SEV. (This is option 5)

SEV_ES_WORK_AREA will be added in tdx metadata so that it is initialized by host VMM and can be accessed in Tdx guest.
In SEV guest I believe SEV_ES_WORK_AREA can be accessed without any error.
The first 8 bytes of SEV_ES_WORK_AREA can be redefined as [CC Indicator Flags].

> -----Original Message-----
> From: Yao, Jiewen <jiewen.yao at intel.com>
> Sent: Wednesday, July 28, 2021 3:55 PM
> To: Xu, Min M <min.m.xu at intel.com>
> Cc: Brijesh Singh <brijesh.singh at amd.com>; devel at edk2.groups.io; Ard
> Biesheuvel <ardb+tianocore at kernel.org>; Justen, Jordan L
> <jordan.l.justen at intel.com>; Erdem Aktas <erdemaktas at google.com>;
> James Bottomley <jejb at linux.ibm.com>; Tom Lendacky
> <thomas.lendacky at amd.com>
> Subject: RE: [PATCH V3 06/10] OvmfPkg: Add AmdSev.asm in ResetVector
> 
> 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 at intel.com>
> > Sent: Wednesday, July 28, 2021 2:58 PM
> > To: Yao, Jiewen <jiewen.yao at intel.com>
> > Cc: Brijesh Singh <brijesh.singh at amd.com>; devel at edk2.groups.io; Ard
> > Biesheuvel <ardb+tianocore at kernel.org>; Justen, Jordan L
> > <jordan.l.justen at intel.com>; Erdem Aktas <erdemaktas at google.com>;
> > James Bottomley <jejb at linux.ibm.com>; Tom Lendacky
> > <thomas.lendacky at 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 at 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;
> > > >>

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