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Tue, 16 Mar 2021 08:20:49 -0700 (PDT) Date: Tue, 16 Mar 2021 08:20:43 -0700 From: Sean Christopherson To: Vitaly Kuznetsov Cc: kvm@vger.kernel.org, Paolo Bonzini , Wanpeng Li , Jim Mattson , Marcelo Tosatti Subject: Re: [PATCH 3/4] KVM: x86: hyper-v: Track Hyper-V TSC page status Message-ID: References: <20210315143706.859293-1-vkuznets@redhat.com> <20210315143706.859293-4-vkuznets@redhat.com> <87lfao8m7s.fsf@vitty.brq.redhat.com> <87a6r38exq.fsf@vitty.brq.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87a6r38exq.fsf@vitty.brq.redhat.com> Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Mar 16, 2021, Vitaly Kuznetsov wrote: > Vitaly Kuznetsov writes: > > > Sean Christopherson 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 :-/