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

[Sheng Yang <sheng@linux.intel.com>]

On Monday 28 June 2010 14:56:38 Avi Kivity wrote:
> On 06/28/2010 09:42 AM, Sheng Yang wrote:
> >>> +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.
> 
> Several milliseconds of non-responsiveness may not be acceptable for
> some applications.  So I think queue_work_on() and a clflush loop is
> better than an IPI and wbinvd.

OK... Would update it in the next version.

--
regards
Yang, Sheng

> 
> >>> +
> >>> 
> >>>    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.
> 
> Ah, yes.