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

[Sean Christopherson]

On Thu, Mar 12, 2026, Chang S. Bae wrote:
> On 3/11/2026 12:01 PM, Paolo Bonzini wrote:
> > 
> >   On the other hand, the extra 16 regs[] entries would be more or less
> > unused, the ugly switch statements wouldn't go away.  In other words,
> > most of your remarks to Changseok's patches would remain...
> 
> I think so...
> 
> If the host kernel ever starts using EGPRs, the state would need to be
> switched in the entry code. At that point, they would likely be saved
> somewhere other than XSAVE buffer. In turn, the guest state would also need
> to be saved to regs[] on VM exit.
> 
> However, that is sort of what-if scenarios at best. The host kernel still
> manages EGPR context switching through XSAVE. Saving EGPRs into regs[] would
> introduce an oddity to synchronize between two buffers: regs[] and
> gfpu->fpstate, which looks like unnecessary complexity.
> 
> So while ugly, the switch statements are a bit of a trade-off here. Also
> bits 16-31 in the extended regs_avail will remain unset with APX=y.

Have you measured performance/latency overhead if KVM goes straight to context
switching R16-R31 at entry/exit?  With PUSH2/POP2, it's "only" 8 more instructions
on each side.

If the overhead is in the noise, I'd be very strongly inclined to say KVM should
swap at entry/exit regardless of kernel behavior so that we don't have to special
case accesses on the back end.