Re: [edk2-devel] [PATCH 03/16] MdePkg/BaseLib: Add a new VMGEXIT instruction invocation for SVSM

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

On Mon, Jan 29, 2024 at 09:51:47AM -0600, Tom Lendacky wrote:
> On 1/29/24 07:22, Gerd Hoffmann wrote:
> > > +global ASM_PFX(AsmVmgExitSvsm)
> > > +ASM_PFX(AsmVmgExitSvsm):
> > > +;
> > > +; NASM doesn't support the vmmcall instruction in 32-bit mode and NASM versions
> > > +; before 2.12 cannot translate the 64-bit "rep vmmcall" instruction into elf32
> > > +; format. Given that VMGEXIT does not make sense on IA32, provide a stub
> > > +; implementation that is identical to CpuBreakpoint(). In practice,
> > > +; AsmVmgExitSvsm() should never be called on IA32.
> > > +;
> > > +    int  3
> > > +    ret
> > 
> > Why?  I assume because the calling convention is only defined for X64?
> > 
> > Why do we need this in the first place if this is never called on IA32?
> > 
> > Wouldn't it be better to have an ASSERT(FALSE) for IA32 in the code
> > calling this?
> 
> An ASSERT() only works for DEBUG code.
> 
> I was following in the steps of AsmVmgExit, where we did this. I suppose we
> could just not have it at all for IA32 and cause a build failure if someone
> was to try and call this from 32-bit code. Thoughts?

See also patch #13 sub-thread.  Maybe just move the 32-bit build to the
Null versions of the Libraries.

SEV-SNP + SVSM apparently is 64-bit only already (there is no 32-bit
calling convention).

Over the last year Intel landed full 64-bit PEI support in edk2, which
means the pure 64-bit build (OvmfPkgX64.dsc) has feature parity with
the 32-bit PEI / 64-bit DXE build (OvmfPkgIa32X64.dsc).

So maybe we can simply leave 32-bit support behind and remove SEV
support from OvmfPkgIa32X64.dsc and OvmfPkgIa32.dsc builds ...

take care,
  Gerd



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