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AJvYcCUywvKWOzFFiRKhMsDE6zp6NtjS2wqb2IsJ25ZppH76NNSlaXv1lTx5OnkGDFuw0ECWKOpRx/95xriRrqQ=@vger.kernel.org X-Gm-Message-State: AOJu0Yxhh8+D5909S7tg3qEvWaloxXW1ItntgW1s40zOZQ6Vllems2su FMwxzDjztow5lLgvnJbkKlOlRRBAboqlqmi4LZJDmWzbcDRoKz75RqZRByv6PA8Fr3jmftcmmTw ivA== X-Google-Smtp-Source: AGHT+IEVg2+55ehZPSbRnoql1bzyggGQpBBSHWR6pLtqveXC5L+JzJIrU9M7bu3d4x/fsGhxyIxVvEVIJ7U= X-Received: from pfbcd16.prod.google.com ([2002:a05:6a00:4210:b0:727:3c81:f42a]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:3d8e:b0:1ea:ddd1:2fb6 with SMTP id adf61e73a8af0-1eb2158170dmr34558526637.30.1737484348885; Tue, 21 Jan 2025 10:32:28 -0800 (PST) Date: Tue, 21 Jan 2025 10:32:27 -0800 In-Reply-To: <4f788368-b8da-4e22-8028-b609975806a0@xen.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20250118005552.2626804-1-seanjc@google.com> <20250118005552.2626804-5-seanjc@google.com> <4f788368-b8da-4e22-8028-b609975806a0@xen.org> Message-ID: Subject: Re: [PATCH 04/10] KVM: x86: Set PVCLOCK_GUEST_STOPPED only for kvmclock, not for Xen PV clock From: Sean Christopherson To: paul@xen.org Cc: Paolo Bonzini , David Woodhouse , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+352e553a86e0d75f5120@syzkaller.appspotmail.com, Paul Durrant , David Woodhouse , Vitaly Kuznetsov Content-Type: text/plain; charset="us-ascii" On Tue, Jan 21, 2025, Paul Durrant wrote: > On 21/01/2025 17:09, Sean Christopherson wrote: > > On Tue, Jan 21, 2025, Paul Durrant wrote: > > > > --- > > > > arch/x86/kvm/x86.c | 20 ++++++++++++++------ > > > > 1 file changed, 14 insertions(+), 6 deletions(-) > > > > > > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > > > > index d8ee37dd2b57..3c4d210e8a9e 100644 > > > > --- a/arch/x86/kvm/x86.c > > > > +++ b/arch/x86/kvm/x86.c > > > > @@ -3150,11 +3150,6 @@ static void kvm_setup_guest_pvclock(struct kvm_vcpu *v, > > > > /* retain PVCLOCK_GUEST_STOPPED if set in guest copy */ > > > > vcpu->hv_clock.flags |= (guest_hv_clock->flags & PVCLOCK_GUEST_STOPPED); > > > > - if (vcpu->pvclock_set_guest_stopped_request) { > > > > - vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; > > > > - vcpu->pvclock_set_guest_stopped_request = false; > > > > - } > > > > - > > > > memcpy(guest_hv_clock, &vcpu->hv_clock, sizeof(*guest_hv_clock)); > > > > if (force_tsc_unstable) > > > > @@ -3264,8 +3259,21 @@ static int kvm_guest_time_update(struct kvm_vcpu *v) > > > > if (use_master_clock) > > > > vcpu->hv_clock.flags |= PVCLOCK_TSC_STABLE_BIT; > > > > - if (vcpu->pv_time.active) > > > > + if (vcpu->pv_time.active) { > > > > + /* > > > > + * GUEST_STOPPED is only supported by kvmclock, and KVM's > > > > + * historic behavior is to only process the request if kvmclock > > > > + * is active/enabled. > > > > + */ > > > > + if (vcpu->pvclock_set_guest_stopped_request) { > > > > + vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; > > > > + vcpu->pvclock_set_guest_stopped_request = false; > > > > + } > > > > kvm_setup_guest_pvclock(v, &vcpu->pv_time, 0, false); > > > > + > > > > + vcpu->hv_clock.flags &= ~PVCLOCK_GUEST_STOPPED; > > > > > > Is this intentional? The line above your change in kvm_setup_guest_pvclock() > > > clearly keeps the flag enabled if it already set and, without this patch, I > > > don't see anything clearing it. > > > > Oh, I see what you're getting at. Hrm. Yes, clearing the flag is intentional, > > otherwise the patch wouldn't do what it claims to do (set PVCLOCK_GUEST_STOPPED > > only for kvmclock). > > > > Swapping the order of this patch and the next patch ("don't bleed ...") doesn't > > break the cycle because that would result in PVCLOCK_GUEST_STOPPED only being > > applied to the first active clock (kvmclock). > > > > The only way I can think of to fully isolate the changes would be to split this > > into two patches: (4a) hoist pvclock_set_guest_stopped_request processing into > > kvm_guest_time_update() and (4b) apply it only to kvmclock, and then make the > > ordering 4a, 5, 4b, i.e. "hoist", "don't bleed", "only kvmclock". > > > > 4a would be quite ugly, because to avoid introducing a functional change, it > > would need to be: > > > > if (vcpu->pv_time.active || vcpu->xen.vcpu_info_cache.active || > > vcpu->xen.vcpu_time_info_cache.active) { > > vcpu->hv_clock.flags |= PVCLOCK_GUEST_STOPPED; > > vcpu->pvclock_set_guest_stopped_request = false; > > } > > > > But it's not the worst intermediate code, so I'm not opposed to going that > > route. > > > > What about putting this change after patch 7. Then you could take a local > copy of hv_clock in which you could set PVCLOCK_GUEST_STOPPED and so avoid > bleeding the flag that way? But to preserve the current behavior of setting PVCLOCK_GUEST_STOPPED for all clocks, processing pvclock_set_guest_stopped_request needs to be moved out of kvm_setup_guest_pvclock() before said helper can make a copy of the reference.