Re: [PATCH 0/7] KVM: x86: APX reg prep work

[Dave Hansen]

On 4/2/26 16:19, Sean Christopherson wrote:
> Do we know what the compiler and/or kernel rules for using R16-R31 will be?
> E.g. if C code is allowed to use R16-R31 at will, then KVM will either need to
> swap R16-R31 in assembly, or annotate a pile of functions as "no_egpr" or
> whatever.

My _assumption_ is that the speedup from using the new GPRs as GPRs in
the kernel is going to be enough for us to support it. This is even
though those kernel binaries won't run on old hardware.

If I'm right, then we're going to have to handle the new GPRs just like
the existing ones and save them on kernel entry before we hit C code.
I'm not sure I want to be messing with XSAVE there. XSAVE requires
munging a header which means even if we used XSAVE we'd need to XSAVE
and then copy things over to pt_regs (assuming we continue using pt_regs).

That doesn't seem like loads of fun because we'll also need to copy out
to the XSAVE UABI spots, like PKRU times 32.