Re: [PATCH RFC 3/7] kvm: x86: XSAVE state and XFD MSRs context switch

[Sean Christopherson]

On Mon, Feb 08, 2021, Paolo Bonzini wrote:
> On 07/02/21 16:42, Jing Liu wrote:
> > |In KVM, "guest_fpu" serves for any guest task working on this vcpu
> > during vmexit and vmenter. We provide a pre-allocated guest_fpu space
> > and entire "guest_fpu.state_mask" to avoid each dynamic features
> > detection on each vcpu task. Meanwhile, to ensure correctly
> > xsaves/xrstors guest state, set IA32_XFD as zero during vmexit and
> > vmenter.|
> 
> Most guests will not need the whole xstate feature set.  So perhaps you
> could set XFD to the host value | the guest value, trap #NM if the host XFD
> is zero, and possibly reflect the exception to the guest's XFD and XFD_ERR.
> 
> In addition, loading the guest XFD MSRs should use the MSR autoload feature
> (add_atomic_switch_msr).

Why do you say that?  I would strongly prefer to use the load lists only if they
are absolutely necessary.  I don't think that's the case here, as I can't
imagine accessing FPU state in NMI context is allowed, at least not without a
big pile of save/restore code.