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[54.240.197.233]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-ab384f22c75sm765478766b.88.2025.01.21.08.58.26 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Jan 2025 08:58:27 -0800 (PST) From: Paul Durrant X-Google-Original-From: Paul Durrant Message-ID: <06482c0e-e519-47ca-9f70-da3ab12ed2e4@xen.org> Date: Tue, 21 Jan 2025 16:58:26 +0000 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Reply-To: paul@xen.org Subject: Re: [PATCH 06/10] KVM: x86/xen: Use guest's copy of pvclock when starting timer To: Sean Christopherson , Paolo Bonzini , David Woodhouse Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+352e553a86e0d75f5120@syzkaller.appspotmail.com, Paul Durrant , David Woodhouse , Vitaly Kuznetsov References: <20250118005552.2626804-1-seanjc@google.com> <20250118005552.2626804-7-seanjc@google.com> Content-Language: en-US Organization: Xen Project In-Reply-To: <20250118005552.2626804-7-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 18/01/2025 00:55, Sean Christopherson wrote: > Use the guest's copy of its pvclock when starting a Xen timer, as KVM's > reference copy may not be up-to-date, i.e. may yield a false positive of > sorts. In the unlikely scenario that the guest is starting a Xen timer > and has used a Xen pvclock in the past, but has since but turned it "off", > then vcpu->arch.hv_clock may be stale, as KVM's reference copy is updated > if and only if at least pvclock is enabled. > > Furthermore, vcpu->arch.hv_clock is currently used by three different > pvclocks: kvmclock, Xen, and Xen compat. While it's extremely unlikely a > guest would ever enable multiple pvclocks, effectively sharing KVM's > reference clock could yield very weird behavior. Using the guest's active > Xen pvclock instead of KVM's reference will allow dropping KVM's > reference copy. > > Fixes: 451a707813ae ("KVM: x86/xen: improve accuracy of Xen timers") > Cc: Paul Durrant > Cc: David Woodhouse > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/xen.c | 58 ++++++++++++++++++++++++++++++++++++++++++---- > 1 file changed, 53 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c > index a909b817b9c0..b82c28223585 100644 > --- a/arch/x86/kvm/xen.c > +++ b/arch/x86/kvm/xen.c > @@ -150,11 +150,46 @@ static enum hrtimer_restart xen_timer_callback(struct hrtimer *timer) > return HRTIMER_NORESTART; > } > > +static int xen_get_guest_pvclock(struct kvm_vcpu *vcpu, > + struct pvclock_vcpu_time_info *hv_clock, > + struct gfn_to_pfn_cache *gpc, > + unsigned int offset) > +{ > + struct pvclock_vcpu_time_info *guest_hv_clock; > + unsigned long flags; > + int r; > + > + read_lock_irqsave(&gpc->lock, flags); > + while (!kvm_gpc_check(gpc, offset + sizeof(*guest_hv_clock))) { > + read_unlock_irqrestore(&gpc->lock, flags); > + > + r = kvm_gpc_refresh(gpc, offset + sizeof(*guest_hv_clock)); > + if (r) > + return r; > + > + read_lock_irqsave(&gpc->lock, flags); > + } > + I guess I must be missing something subtle... What is setting guest_hv_clock to point at something meaningful before this line? > + memcpy(hv_clock, guest_hv_clock, sizeof(*hv_clock)); > + read_unlock_irqrestore(&gpc->lock, flags); > + > + /* > + * Sanity check TSC shift+multiplier to verify the guest's view of time > + * is more or less consistent. > + */ > + if (hv_clock->tsc_shift != vcpu->arch.hv_clock.tsc_shift || > + hv_clock->tsc_to_system_mul != vcpu->arch.hv_clock.tsc_to_system_mul) > + return -EINVAL; > + return 0; > +} > + > static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, > bool linux_wa) > { > + struct kvm_vcpu_xen *xen; > int64_t kernel_now, delta; > uint64_t guest_now; > + int r = -EOPNOTSUPP; > > /* > * The guest provides the requested timeout in absolute nanoseconds > @@ -173,10 +208,22 @@ static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, > * the absolute CLOCK_MONOTONIC time at which the timer should > * fire. > */ > - if (vcpu->arch.hv_clock.version && vcpu->kvm->arch.use_master_clock && > - static_cpu_has(X86_FEATURE_CONSTANT_TSC)) { > + do { > + struct pvclock_vcpu_time_info hv_clock; > uint64_t host_tsc, guest_tsc; > > + if (!static_cpu_has(X86_FEATURE_CONSTANT_TSC) || > + !vcpu->kvm->arch.use_master_clock) > + break; > + > + if (xen->vcpu_info_cache.active) > + r = xen_get_guest_pvclock(vcpu, &hv_clock, &xen->vcpu_info_cache, > + offsetof(struct compat_vcpu_info, time)); > + else if (xen->vcpu_time_info_cache.active) > + r = xen_get_guest_pvclock(vcpu, &hv_clock, &xen->vcpu_time_info_cache, 0); > + if (r) > + break; > + > if (!IS_ENABLED(CONFIG_64BIT) || > !kvm_get_monotonic_and_clockread(&kernel_now, &host_tsc)) { > /* > @@ -197,9 +244,10 @@ static void kvm_xen_start_timer(struct kvm_vcpu *vcpu, u64 guest_abs, > > /* Calculate the guest kvmclock as the guest would do it. */ > guest_tsc = kvm_read_l1_tsc(vcpu, host_tsc); > - guest_now = __pvclock_read_cycles(&vcpu->arch.hv_clock, > - guest_tsc); > - } else { > + guest_now = __pvclock_read_cycles(&hv_clock, guest_tsc); > + } while (0); > + > + if (r) { > /* > * Without CONSTANT_TSC, get_kvmclock_ns() is the only option. > *