Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

[Paolo Bonzini]

On 14/10/21 13:30, Liu, Jing2 wrote:
> I guess we're worrying about is when KVM is sched_out, a nonzero XFD_ERR
> can be lost by other host thread. We can save guest XFD_ERR in sched_out
> and restore before next vmenter. Kernel is assumed not using AMX thus
> softirq won't make it lost.
> I think this solves the problem. So we can directly passthrough RW of it,
> and no need to rdmsr(XFD_ERR) in vmexit.

Correct; you can also use the "user-return MSRs" machinery (until Linux 
starts using AMX in the kernel, but that shouldn't happen too soon).

Paolo