Re: [PATCH v5 4/9] KVM-HV: KVM Steal time implementation

[Avi Kivity]

On 07/07/2011 08:07 PM, Glauber Costa wrote:
> > > +static void record_steal_time(struct kvm_vcpu *vcpu)
> > > +{
> > > +    u64 delta;
> > > +
> > > +    if (!(vcpu->arch.st.msr_val&  KVM_MSR_ENABLED))
> > > +        return;
> > > +
> > > +    if (unlikely(kvm_read_guest_cached(vcpu->kvm,&vcpu->arch.st.stime,
> > > + &vcpu->arch.st.steal, sizeof(struct kvm_steal_time))))
> > > +        return;
> >
> > The guest memory page is not pinned, sleeping via
> > __copy_from_user/to_user is not allowed in vcpu_load context. Either pin
> > it or use atomic acessors.
>
>
> I do recognize the problem.
> Avi, what's your take here?

The easiest solution is to set a KVM_REQ bit in atomic context, and move 
the sleepy code to vcpu_enter_guest().

> > > +    case MSR_KVM_STEAL_TIME:
> > > +        vcpu->arch.st.msr_val = data;
> > > +
> > > +        if (!(data&  KVM_MSR_ENABLED)) {
> > > +            break;
> > > +        }
> >
> > On failure below this point, msr_val should be cleared of 
> > KVM_MSR_ENABLED?
> No, msr_val has to hold whatever the guest wrote into it.
> We should probably use an independent variable here to indicate that 
> we failed to activate it.

If we fail, we return a #GP to the guest (and don't write any value into 
the msr).

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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