Re: [edk2-devel] [PATCH] OvmfPkg: Set PciLib for TdxDxe driver

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 4/18/22 23:47, Yao, Jiewen wrote:
> Can SEV clear the C-bit in SEC phase?

Not really. IIRC, even if cleared in the SEC phase, the DXE phase replaces 
the page tables and it has to be cleared again.

Thanks,
Tom

> 
> I think that is right way to ensure PCI Express can always be accessed by anyone.
> 
> 
>> -----Original Message-----
>> From: Xu, Min M <min.m.xu@intel.com>
>> Sent: Tuesday, April 19, 2022 12:39 PM
>> To: Yao, Jiewen <jiewen.yao@intel.com>; devel@edk2.groups.io
>> Cc: Brijesh Singh <brijesh.singh@amd.com>; Aktas, Erdem
>> <erdemaktas@google.com>; James Bottomley <jejb@linux.ibm.com>; Tom
>> Lendacky <thomas.lendacky@amd.com>
>> Subject: RE: [edk2-devel] [PATCH] OvmfPkg: Set PciLib for TdxDxe driver
>>
>> In AmdSevDxe's entry point it clears the C-bit from PcdPciExpressBaseAddress
>> and other memory spaces if needed. Please see
>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Ftianocore%2Fedk2%2Fblob%2Fmaster%2FOvmfPkg%2FAmdSevDxe%2FAmdSev&amp;data=04%7C01%7Cthomas.lendacky%40amd.com%7Cc39c49fd4e944900bdb708da21bfac91%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637859404370071519%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000&amp;sdata=9sxJnGXyaiHTdIzS%2BTzziBnwTAsKvSLFRMmHT4HGe60%3D&amp;reserved=0
>> Dxe.c#L81-L95. After that OVMF can use PCI express.
>>
>> This broken is caused by the call sequence of TdxDxe driver and AmdSevDxe
>> driver. Currently TdxDxe driver is loaded before AmdSevDxe, so in SEV-ES guest
>> the C-bit of PcdPciExpressBaseAddress hasn't been cleared. In this situation the
>> access to PciExpressBaseAddress trigger exceptions (lib constructor in TdxDxe).
>>
>> There are 2 options to fix this issue.
>> 1. Adjust the load sequence of AmdSevDxe and TdxDxe (Load AmdSevDxe before
>> TdxDxe)
>> 2. Make TdxDxe to import BasePciLibCf8.inf instead of DxePciLibI440FxQ35.inf
>> (just like AmdSevDxe)
>>
>> Tom and I tested above 2 options in SEV and TDX and all work.
>>
>>> -----Original Message-----
>>> From: Yao, Jiewen <jiewen.yao@intel.com>
>>> Sent: Tuesday, April 19, 2022 12:16 PM
>>> To: Xu, Min M <min.m.xu@intel.com>; devel@edk2.groups.io
>>> Cc: Brijesh Singh <brijesh.singh@amd.com>; Aktas, Erdem
>>> <erdemaktas@google.com>; James Bottomley <jejb@linux.ibm.com>; Tom
>>> Lendacky <thomas.lendacky@amd.com>
>>> Subject: RE: [edk2-devel] [PATCH] OvmfPkg: Set PciLib for TdxDxe driver
>>>
>>> Do you mean, with SEV introduced, OVMF cannot use PCI express any more?
>>>
>>> Thank you
>>> Yao Jiewen
>>>
>>>
>>>> -----Original Message-----
>>>> From: Xu, Min M <min.m.xu@intel.com>
>>>> Sent: Tuesday, April 19, 2022 11:05 AM
>>>> To: Yao, Jiewen <jiewen.yao@intel.com>; devel@edk2.groups.io
>>>> Cc: Brijesh Singh <brijesh.singh@amd.com>; Aktas, Erdem
>>>> <erdemaktas@google.com>; James Bottomley <jejb@linux.ibm.com>; Tom
>>>> Lendacky <thomas.lendacky@amd.com>
>>>> Subject: RE: [edk2-devel] [PATCH] OvmfPkg: Set PciLib for TdxDxe
>>>> driver
>>>>
>>>> On April 19, 2022 10:54 AM, Yao Jiewen wrote:
>>>>>
>>>>> Why does TdxDxe call TdxMailbox in an SEV platform?
>>>>> Or why does TdxMailbox call SynchronizationLib in an SEV platform?
>>>>>
>>>> TdxDxe will not call TdxMailbox/SynchronizationLib in SEV platform.
>>>> The problem is in the lib constructor. When TdxDxe driver is loaded,
>>>> before its entry point is called, the lib constructors will be called even in a
>>> SEV platform.
>>>>>
>>>>> There are many places we can do CcProbe to stop action. Why we need
>>>>> do it in DSC?
>>>> So we cannot stop the lib constructor with CcProbe in this case.
>>>>
>>>> Thanks
>>>> Min