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

[Sean Christopherson]

On Fri, Apr 03, 2026, Dave Hansen wrote:
> 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.

Ooof, one nasty wrinkle to prepare for is an NMI that arrives after VM-Exit on
Intel CPUs.  Unless Intel extends VMX to context switch XCR0 at VM-Entry/VM-Exit,
and/or provides GIF-like functionality (which would be awesome!), it will be
possible for an NMI to be taken with the guest's XCR0 loaded, i.e. with XCR0.APX=0
even when APX is fully enabled in the host.

> 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.