Re: [PATCH v3] KVM: VMX: Execute WBINVD to keep data consistency with assigned devices

Sheng Yang
2010-06-28T06:44:26Z

On Monday 28 June 2010 11:56:08 Avi Kivity wrote:
> On 06/28/2010 06:36 AM, Sheng Yang wrote:
> > Some guest device driver may leverage the "Non-Snoop" I/O, and explicitly
> > WBINVD or CLFLUSH to a RAM space. Since migration may occur before WBINVD
> > or
>
> > CLFLUSH, we need to maintain data consistency either by:
> Don't we always force enable snooping? Or is that only for the
> processor, and you're worried about devices?

We only force enabling snooping for capable VT-d engine(with
KVM_IOMMU_CACHE_COHERENCY flag, on most recent server board). And you're right,
with the snooping capable VT-d engine we don't need to do all these. Would address
it in the next version.

> > 1: flushing cache (wbinvd) when the guest is scheduled out if there is no
> > wbinvd exit, or
> > 2: execute wbinvd on all dirty physical CPUs when guest wbinvd exits.
> >
> > /* fields used by HYPER-V emulation */
> > u64 hv_vapic;
> >
> > +
> > + cpumask_t wbinvd_dirty_mask;
> >
> > };
>
> Need alloc_cpumask_var()/free_cpumask_var() for very large hosts.

OK.
>
> > +static void wbinvd_ipi(void *garbage)
> > +{
> > + wbinvd();
> > +}
>
> Like Jan mentioned, this is quite heavy. What about a clflush() loop
> instead? That may take more time, but at least it's preemptible. Of
> course, it isn't preemptible in an IPI.

I think this kind of behavior happened rarely, and most recent processor should
have WBINVD exit which means it's an IPI... So I think it's maybe acceptable here.

> > +
> >
> > void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > {
> >
> > + /* Address WBINVD may be executed by guest */
> > + if (vcpu->kvm->arch.iommu_domain) {
> > + if (kvm_x86_ops->has_wbinvd_exit())
> > + cpu_set(cpu, vcpu->arch.wbinvd_dirty_mask);
> > + else if (vcpu->cpu != -1)
> > + smp_call_function_single(vcpu->cpu,
> > + wbinvd_ipi, NULL, 1);
>
> Is there any point to doing this if !has_wbinvd_exit()? The vcpu might
> not have migrated in time, so the cache is flushed too late.

For the !has_wbinvd_exit(), the instruction would be executed by guest and flush
the current processor immediately. And we can ensure that it's clean in the last
CPU, so we're fine.

> > + }
> > +
> >
> > kvm_x86_ops->vcpu_load(vcpu, cpu);
> > if (unlikely(per_cpu(cpu_tsc_khz, cpu) == 0)) {
> >
> > unsigned long khz = cpufreq_quick_get(cpu);
> >
> > @@ -3650,6 +3664,21 @@ int emulate_invlpg(struct kvm_vcpu *vcpu, gva_t
> > address)
> >
> > return X86EMUL_CONTINUE;
> >
> > }
> >
> > +int kvm_emulate_wbinvd(struct kvm_vcpu *vcpu)
> > +{
> > + if (!vcpu->kvm->arch.iommu_domain)
> > + return X86EMUL_CONTINUE;
> > +
> > + if (kvm_x86_ops->has_wbinvd_exit()) {
> > + smp_call_function_many(&vcpu->arch.wbinvd_dirty_mask,
> > + wbinvd_ipi, NULL, 1);
> > + cpus_clear(vcpu->arch.wbinvd_dirty_mask);
>
> Race - a migration may set a new bit in wbinvd_dirty_mask after the
> s_c_f_m().
>
> However, it's probably benign, since we won't be entering the guest in
> that period.

Yes. :)

--
regards
Yang, Sheng

>
> > + } else
> > + wbinvd();
> > + return X86EMUL_CONTINUE;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_emulate_wbinvd);
> > +
> >
> > int emulate_clts(struct kvm_vcpu *vcpu)
> > {
> >
> > kvm_x86_ops->set_cr0(vcpu, kvm_read_cr0_bits(vcpu, ~X86_CR0_TS));