Re: [PATCH 3/4] KVM: x86: hyper-v: Track Hyper-V TSC page status

[Sean Christopherson <seanjc@google.com>]

On Tue, Mar 16, 2021, Vitaly Kuznetsov wrote:
> Vitaly Kuznetsov <vkuznets@redhat.com> writes:
> 
> > Sean Christopherson <seanjc@google.com> writes:
> >
> >> On Mon, Mar 15, 2021, Vitaly Kuznetsov wrote:
> >>> @@ -1193,8 +1199,13 @@ static int kvm_hv_set_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data,
> >>>  	}
> >>>  	case HV_X64_MSR_REFERENCE_TSC:
> >>>  		hv->hv_tsc_page = data;
> >>> -		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE)
> >>> +		if (hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE) {
> >>> +			if (!host)
> >>> +				hv->hv_tsc_page_status = HV_TSC_PAGE_GUEST_CHANGED;
> >>> +			else
> >>> +				hv->hv_tsc_page_status = HV_TSC_PAGE_HOST_CHANGED;
> >>
> >> Writing the status without taking hv->hv_lock could cause the update to be lost,
> >> e.g. if a different vCPU fails kvm_hv_setup_tsc_page() at the same time, its
> >> write to set status to HV_TSC_PAGE_BROKEN would race with this write.
> >>
> >
> > Oh, right you are, the lock was somewhere in my brain :-) Will do in
> > v2.
> 
> Actually no, kvm_hv_set_msr_pw() is only called from
> kvm_hv_set_msr_common() with hv->hv_lock held so we're already
> synchronized.
> 
> ... and of course I figured that our by putting another
> mutex_lock()/mutex_unlock() here and then wondering why everything hangs
> :-)

Doh, sorry :-/