Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

[Andi Kleen]

On Wed, Jan 10, 2024 at 01:26:39AM +0000, Kevin Loughlin wrote:
> SEV/SME code can execute prior to page table fixups for kernel

That seems to be fundamental problem.

> relocation. However, as with global variables accessed in
> __startup_64(), clang does not currently generate PC-relative accesses
> for SEV/SME global variables, causing certain flavors of SEV hosts and
> guests to crash.

And that's a hack to work around it.

> 
> While an attempt was made to force PC-relative addressing for certain
> global SEV/SME variables via inline assembly (see snp_cpuid_get_table()
> for example), PC-relative addressing must be pervasively-enforced for
> SEV/SME global variables that can be accessed prior to page table
> fixups.
> 
> To avoid the error-prone approach of manually referencing each SEV/SME
> global variable via a general form of snp_cpuid_get_table(), it is
> preferable to use compiler flags for position-independent code (ex:

But if gcc doesn't support it then it doesn't work.

It seems your approach with incompatible execution models between
the compilers is just a recipe for future patches only working
on one of the compilers because most code submitters probably
won't test both.

It would be better to at least use a unified execution model, if you want
to extend the hack and not fix the underlying issue.

-Andi