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

[Chang S. Bae]

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.