Re: [PATCH v13 10/12] KVM: x86: Introduce new KVM_FEATURE_SEV_LIVE_MIGRATION feature & Custom MSR.

[Paolo Bonzini]

On 20/04/21 01:06, Sean Christopherson wrote:
> > diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
> > index 950afebfba88..f6bfa138874f 100644
> > --- a/arch/x86/include/uapi/asm/kvm_para.h
> > +++ b/arch/x86/include/uapi/asm/kvm_para.h
> > @@ -33,6 +33,7 @@
> >   #define KVM_FEATURE_PV_SCHED_YIELD	13
> >   #define KVM_FEATURE_ASYNC_PF_INT	14
> >   #define KVM_FEATURE_MSI_EXT_DEST_ID	15
> > +#define KVM_FEATURE_SEV_LIVE_MIGRATION	16
> >   
> >   #define KVM_HINTS_REALTIME      0
> >   
> > @@ -54,6 +55,7 @@
> >   #define MSR_KVM_POLL_CONTROL	0x4b564d05
> >   #define MSR_KVM_ASYNC_PF_INT	0x4b564d06
> >   #define MSR_KVM_ASYNC_PF_ACK	0x4b564d07
> > +#define MSR_KVM_SEV_LIVE_MIGRATION	0x4b564d08
> >   
> >   struct kvm_steal_time {
> >   	__u64 steal;
> > @@ -136,4 +138,6 @@ struct kvm_vcpu_pv_apf_data {
> >   #define KVM_PV_EOI_ENABLED KVM_PV_EOI_MASK
> >   #define KVM_PV_EOI_DISABLED 0x0
> >   
> > +#define KVM_SEV_LIVE_MIGRATION_ENABLED BIT_ULL(0)
>
> Even though the intent is to "force" userspace to intercept the MSR, I think KVM
> should at least emulate the legal bits as a nop.  Deferring completely to
> userspace is rather bizarre as there's not really anything to justify KVM
> getting involved.  It would also force userspace to filter the MSR just to
> support the hypercall.

I think this is the intention, the hypercall by itself cannot do much if
you cannot tell userspace that it's up-to-date.

On the other hand it is kind of wrong that KVM_GET_SUPPORTED_CPUID
returns the feature, but the MSR is not supported.

> Somewhat of a nit, but I think we should do something like s/ENABLED/READY,

Agreed.  I'll send a patch that puts everything together.

Paolo