Re: [edk2-devel] [PATCH 15/16] Ovmfpkg/CcExitLib: Provide SVSM discovery support

["Gerd Hoffmann" <kraxel@redhat.com>]

On Mon, Jan 29, 2024 at 12:04:51PM -0600, Tom Lendacky wrote:
> On 1/29/24 09:23, Gerd Hoffmann via groups.io wrote:
> > On Fri, Jan 26, 2024 at 04:13:14PM -0600, Tom Lendacky wrote:
> > > BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4654
> > > 
> > > The SVSM specification documents an alternative method of discovery for
> > > the SVSM using a reserved CPUID bit and a reserved MSR.
> > > 
> > > For the CPUID support, the #VC handler of an SEV-SNP guest should modify
> > > the returned value in the EAX register for the 0x8000001f CPUID function
> > > by setting bit 28 when an SVSM is present.
> > > 
> > > For the MSR support, new reserved MSR 0xc001f000 has been defined. A #VC
> > > should be generated when accessing this MSR. The #VC handler is expected
> > > to ignore writes to this MSR and return the physical calling area address
> > > (CAA) on reads of this MSR.
> > 
> > I'm wondering why this is handled by the ovmf #vc handler instead of the
> > svsm #vc handler?
> 
> OVMF will be running in guest VMPL1 context, not SVSM VMPL0 context, when
> the CPUID instruction / MSR access is performed. In that case, it is the
> guest VMPL1 #VC handler that will get control and have to respond.

IIRC vmpl0 can handle #vc exceptions for vmpl1.

But maybe only in case vmpl1 hasn't a #vc handler, i.e. vmpl1 can't
cherry-pick which exceptions it handles itself and which it delegates
to the SVSM.

What happens once the linux kernel called ExitBootServices?  Will the
edk2 #vc handler stay alive?  Or will the linux kernel install its own
handler?

take care,
  Gerd



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