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

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

Can SEV clear the C-bit in SEC phase?

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://github.com/tianocore/edk2/blob/master/OvmfPkg/AmdSevDxe/AmdSev
> 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