Hey I am not sure why Min did not response to my latest email. I did give suggestion in my previous comment.
===== CcWorkArea.Type = 0; InitCcWorkAreaSev(); // set Type=1 if SEV InitCcWorkAreaTdx(); // set Type=2 if TDX ===== That is option 1. Thank you Yao Jiewen > -----Original Message----- > From: Xu, Min M <min.m...@intel.com> > Sent: Thursday, July 29, 2021 7:54 PM > To: Brijesh Singh <brijesh.si...@amd.com>; Yao, Jiewen > <jiewen....@intel.com>; devel@edk2.groups.io > Cc: Ard Biesheuvel <ardb+tianoc...@kernel.org>; Justen, Jordan L > <jordan.l.jus...@intel.com>; Erdem Aktas <erdemak...@google.com>; James > Bottomley <j...@linux.ibm.com>; Tom Lendacky <thomas.lenda...@amd.com> > Subject: RE: [edk2-devel] [PATCH V3 06/10] OvmfPkg: Add AmdSev.asm in > ResetVector > > On July 29, 2021 6:08 PM, Brijesh Singh wrote: > > On 7/29/21 1:07 AM, Xu, Min M wrote: > > > On July 29, 2021 12:29 PM, Brijesh Singh wrote: > > >> On 7/28/21 9:44 PM, Xu, Min M wrote: > > >>> Jiewen & Singh > > >>> > > >>> From the discussion I am thinking we have below rules to follow to > > >>> the design the structure of TEE_WORK_AREA: > > >>> 1. Design should be flexible but not too complicated 2. Reuse the > > >>> current SEV_ES_WORK_AREA (PcdSevEsWorkAreaBase) as > > TEE_WORK_AREA 3. > > >>> TEE_WORK_AREA should be initialized to all-0 at the beginning of > > >>> ResetVecotr 4. Reduce the changes to exiting code if possible > > >>> > > >>> So I try to make below conclusions below: (Please review) 1. > > >>> SEV_ES_WORK_AREA is used as the TEE_WORK_AREA by both TDX and > > SEV, > > >>> maybe in the future it can be used by other CC technologies. > > >>> > > >>> 2. In MEMFD, add below initial value. So that TEE_WORK_AREA is > > >>> guaranteed to be cleared in legacy guest. In TDX this memory region > > >>> is initialized to be all-0 by host VMM. In SEV the memory region is > > cleared as well. > > >>> 0x00B000|0x001000 > > >>> > > >> > > gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase|gUefiCpuPkgTokenSpa > > ce > > >> Guid.PcdSevEsWorkAreaSize > > >>> DATA = { > > >>> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, > > >>> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, > > >>> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, > > >>> 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 > > >>> } > > >> Hmm, I thought the contents of the data pages are controlled by the host > > VMM. > > >> If the backing pages are not zero filled then there is no guarantee > > >> that memory will be zero. To verify it: > > >> > > >> 1. I applied your above change in OvmfPkgX86.fdt. I modified the DATA > > >> values from 0x00 -> 0xCC > > >> > > >> 2. Modified the SecMain.c to dump the SevEsWorkArea on entry > > >> > > >> And dump does not contain the 0xcc. > > >> > > >> And to confirm further, I attached to the qemu with the GDB before > > >> the booting the OVMF, and modified the SevEsWorkArea with some > > >> garbage number and this time the dump printed garbage value I put > > through the debugger. > > >> > > >> In summary, the OVMF to zero the workarea memory on the entry and > > we > > >> cannot rely on the DATA={0x00, 0x00...} to zero the CCWorkArea. > > > So in legacy guest, CCWorkArea is cleared to all-0 without the > > DATA={0x00,0x00...}, right? > > > > Okay, maybe I was not able to communicate it correctly. > > > > The run I did is for the legacy guest. For the legacy guest, the contents > > of the > > CCWorkArea may *not* be always zero even when you use the DATA={0x00, > > 0x00...}. > > > > Currently, Qemu uses zero filled backing pages, so we will get a zero filled > > CCWorkArea; but nothing says that a backing page *must* be zero. > > Another VMM may choose to do things differently. In summary, the OVMF > > reset vector code must zero the CCWorkArea before calling SEV or TDX > > probes. > > > Ah, I see. > In current CheckSevFeatures, byte[SEV_ES_WORK_AREA] is cleared to0. > Then its values is set based on the result of SEV probe. > > There is a bug here. CheckTdxFeatures does the similar work and it sets the > WORK_AREA to 2. If CheckSevFeatures is called after CheckTdxFeatures, then > WORK_AREA is cleared and it is set to 0 because it is not SEV. The value is > override. > > I think there are 2 options: > Option 1: > Neither CheckTdxFeatures nor CheckSevFeatures should clear WORK_AREA. > Instead > It should be cleared to 0 outside and before these 2 calls. So in Main16 after > TransitionFromReal16To32BitFlat WORK_AREA is cleared to 0. In Tdx guest this > WORK_AREA > is initialized to 0 by host VMM. > > Option 2: > Another option is to figure out a mechanism that only one CheckXXXFeatures is > called. > Since there are 2 entry point in Main.asm: Main16 and Main32. > In Main16 CheckSevFeatures is called after TransitionFromReal16To32BitFlat. > (eax should > be saved because it is used in SetCr3ForPageTables64) > In Main32 CheckTdxFeatures is called after ReloadFlat32. > > What's your opinion? -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. 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