* [PATCH v4 0/2] KVM: x86: Include host suspended time in steal time @ 2025-02-21 5:39 Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 1/2] KVM: x86: Advance guest TSC after deep suspend Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal 0 siblings, 2 replies; 5+ messages in thread From: Suleiman Souhlal @ 2025-02-21 5:39 UTC (permalink / raw) To: Paolo Bonzini, Sean Christopherson Cc: Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H. Peter Anvin, Chao Gao, David Woodhouse, Sergey Senozhatsky, kvm, linux-kernel, ssouhlal, Suleiman Souhlal This series makes it so that the time that the host is suspended is included in guests' steal time. When the host resumes from a suspend, the guest thinks any task that was running during the suspend ran for a long time, even though the effective run time was much shorter, which can end up having negative effects with scheduling. To mitigate this issue, we include the time that the host was suspended in steal time, which lets the guest can subtract the duration from the tasks' runtime. In addition, we make the guest TSC behavior consistent whether the host TSC went backwards or not. v4: - Advance guest TSC on suspends where host TSC goes backwards. - Block vCPUs from running until resume notifier. - Move suspend duration accounting out of machine-independent kvm to x86. - Merge code and documentation patches. - Reworded documentation. v3: https://lore.kernel.org/kvm/Z5AB-6bLRNLle27G@google.com/T/ - Use PM notifier instead of syscore ops (kvm_suspend()/kvm_resume()), because the latter doesn't get called on shallow suspend. - Don't call function under UACCESS. - Whitespace. v2: https://lore.kernel.org/lkml/20241118043745.1857272-1-suleiman@google.com/ - Accumulate suspend time at machine-independent kvm layer and track per-VCPU instead of per-VM. - Document changes. v1: https://lore.kernel.org/kvm/20240710074410.770409-1-suleiman@google.com/ Suleiman Souhlal (2): KVM: x86: Advance guest TSC after deep suspend. KVM: x86: Include host suspended time in steal time Documentation/virt/kvm/x86/msr.rst | 9 +++- arch/x86/include/asm/kvm_host.h | 7 +++ arch/x86/kvm/x86.c | 84 +++++++++++++++++++++++++++++- 3 files changed, 97 insertions(+), 3 deletions(-) -- 2.48.1.601.g30ceb7b040-goog ^ permalink raw reply [flat|nested] 5+ messages in thread
* [PATCH v4 1/2] KVM: x86: Advance guest TSC after deep suspend. 2025-02-21 5:39 [PATCH v4 0/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal @ 2025-02-21 5:39 ` Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal 1 sibling, 0 replies; 5+ messages in thread From: Suleiman Souhlal @ 2025-02-21 5:39 UTC (permalink / raw) To: Paolo Bonzini, Sean Christopherson Cc: Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H. Peter Anvin, Chao Gao, David Woodhouse, Sergey Senozhatsky, kvm, linux-kernel, ssouhlal, Suleiman Souhlal Advance guest TSC to current time after suspend when the host TSCs went backwards. This makes the behavior consistent between suspends where host TSC resets and suspends where it doesn't, such as suspend-to-idle, where in the former case if the host TSC resets, the guests' would previously be "frozen" due to KVM's backwards TSC prevention, while in the latter case they would advance. Suggested-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Suleiman Souhlal <suleiman@google.com> --- arch/x86/include/asm/kvm_host.h | 1 + arch/x86/kvm/x86.c | 33 ++++++++++++++++++++++++++++++++- 2 files changed, 33 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 0b7af5902ff757..452dd0204609af 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -1398,6 +1398,7 @@ struct kvm_arch { u64 cur_tsc_offset; u64 cur_tsc_generation; int nr_vcpus_matched_tsc; + bool host_was_suspended; u32 default_tsc_khz; bool user_set_tsc; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 02159c967d29e5..06464ec0d1c8d2 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4971,7 +4971,37 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu) /* Apply any externally detected TSC adjustments (due to suspend) */ if (unlikely(vcpu->arch.tsc_offset_adjustment)) { - adjust_tsc_offset_host(vcpu, vcpu->arch.tsc_offset_adjustment); + unsigned long flags; + struct kvm *kvm; + bool advance; + u64 kernel_ns, l1_tsc, offset, tsc_now; + + kvm = vcpu->kvm; + advance = kvm_get_time_and_clockread(&kernel_ns, + &tsc_now); + raw_spin_lock_irqsave(&kvm->arch.tsc_write_lock, flags); + /* + * Advance the guest's TSC to current time instead of only + * preventing it from going backwards, while making sure + * all the vCPUs use the same offset. + */ + if (kvm->arch.host_was_suspended && advance) { + l1_tsc = nsec_to_cycles(vcpu, + vcpu->kvm->arch.kvmclock_offset + + kernel_ns); + offset = kvm_compute_l1_tsc_offset(vcpu, + l1_tsc); + kvm->arch.cur_tsc_offset = offset; + kvm_vcpu_write_tsc_offset(vcpu, offset); + } else if (advance) + kvm_vcpu_write_tsc_offset(vcpu, + vcpu->kvm->arch.cur_tsc_offset); + else + adjust_tsc_offset_host(vcpu, + vcpu->arch.tsc_offset_adjustment); + kvm->arch.host_was_suspended = 0; + raw_spin_unlock_irqrestore(&kvm->arch.tsc_write_lock, + flags); vcpu->arch.tsc_offset_adjustment = 0; kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu); } @@ -12638,6 +12668,7 @@ int kvm_arch_enable_virtualization_cpu(void) kvm_make_request(KVM_REQ_MASTERCLOCK_UPDATE, vcpu); } + kvm->arch.host_was_suspended = 1; /* * We have to disable TSC offset matching.. if you were * booting a VM while issuing an S4 host suspend.... -- 2.48.1.601.g30ceb7b040-goog ^ permalink raw reply related [flat|nested] 5+ messages in thread
* [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time 2025-02-21 5:39 [PATCH v4 0/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 1/2] KVM: x86: Advance guest TSC after deep suspend Suleiman Souhlal @ 2025-02-21 5:39 ` Suleiman Souhlal 2025-03-01 17:21 ` Konrad Rzeszutek Wilk 1 sibling, 1 reply; 5+ messages in thread From: Suleiman Souhlal @ 2025-02-21 5:39 UTC (permalink / raw) To: Paolo Bonzini, Sean Christopherson Cc: Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H. Peter Anvin, Chao Gao, David Woodhouse, Sergey Senozhatsky, kvm, linux-kernel, ssouhlal, Suleiman Souhlal When the host resumes from a suspend, the guest thinks any task that was running during the suspend ran for a long time, even though the effective run time was much shorter, which can end up having negative effects with scheduling. To mitigate this issue, the time that the host was suspended is included in steal time, which lets the guest can subtract the duration from the tasks' runtime. In order to implement this behavior, once the suspend notifier fires, vCPUs trying to run block until the resume notifier finishes. This is because the freezing of userspace tasks happens between these two points, which means that vCPUs could otherwise run and get their suspend steal time misaccounted, particularly if a vCPU would run after resume before the resume notifier. Incidentally, doing this also addresses a potential race with the suspend notifier setting PVCLOCK_GUEST_STOPPED, which could then get cleared before the suspend actually happened. One potential caveat is that in the case of a suspend happening during a VM migration, the suspend time might not be accounted. A workaround would be for the VMM to ensure that the guest is entered with KVM_RUN after resuming from suspend. Signed-off-by: Suleiman Souhlal <suleiman@google.com> --- Documentation/virt/kvm/x86/msr.rst | 10 ++++-- arch/x86/include/asm/kvm_host.h | 6 ++++ arch/x86/kvm/x86.c | 51 ++++++++++++++++++++++++++++++ 3 files changed, 65 insertions(+), 2 deletions(-) diff --git a/Documentation/virt/kvm/x86/msr.rst b/Documentation/virt/kvm/x86/msr.rst index 3aecf2a70e7b43..48f2a8ca519548 100644 --- a/Documentation/virt/kvm/x86/msr.rst +++ b/Documentation/virt/kvm/x86/msr.rst @@ -294,8 +294,14 @@ data: steal: the amount of time in which this vCPU did not run, in - nanoseconds. Time during which the vcpu is idle, will not be - reported as steal time. + nanoseconds. This includes the time during which the host is + suspended. Time during which the vcpu is idle, might not be + reported as steal time. The case where the host suspends + during a VM migration might not be accounted if VCPUs aren't + entered post-resume, because KVM does not currently support + suspend/resuming the associated metadata. A workaround would + be for the VMM to ensure that the guest is entered with + KVM_RUN after resuming from suspend. preempted: indicate the vCPU who owns this struct is running or diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h index 452dd0204609af..007656ceac9a71 100644 --- a/arch/x86/include/asm/kvm_host.h +++ b/arch/x86/include/asm/kvm_host.h @@ -124,6 +124,7 @@ #define KVM_REQ_HV_TLB_FLUSH \ KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP) #define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE KVM_ARCH_REQ(34) +#define KVM_REQ_WAIT_FOR_RESUME KVM_ARCH_REQ(35) #define CR0_RESERVED_BITS \ (~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \ @@ -916,8 +917,13 @@ struct kvm_vcpu_arch { struct { u8 preempted; + bool host_suspended; u64 msr_val; u64 last_steal; + u64 last_suspend; + u64 suspend_ns; + u64 last_suspend_ns; + wait_queue_head_t resume_waitq; struct gfn_to_hva_cache cache; } st; diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 06464ec0d1c8d2..f34edcf77cca0a 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -3717,6 +3717,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu) steal += current->sched_info.run_delay - vcpu->arch.st.last_steal; vcpu->arch.st.last_steal = current->sched_info.run_delay; + steal += vcpu->arch.st.suspend_ns - vcpu->arch.st.last_suspend_ns; + vcpu->arch.st.last_suspend_ns = vcpu->arch.st.suspend_ns; unsafe_put_user(steal, &st->steal, out); version += 1; @@ -6930,6 +6932,19 @@ long kvm_arch_vm_compat_ioctl(struct file *filp, unsigned int ioctl, } #endif +static void wait_for_resume(struct kvm_vcpu *vcpu) +{ + wait_event_interruptible(vcpu->arch.st.resume_waitq, + vcpu->arch.st.host_suspended == 0); + + /* + * This might happen if we blocked here before the freezing of tasks + * and we get woken up by the freezer. + */ + if (vcpu->arch.st.host_suspended) + kvm_make_request(KVM_REQ_WAIT_FOR_RESUME, vcpu); +} + #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER static int kvm_arch_suspend_notifier(struct kvm *kvm) { @@ -6939,6 +6954,19 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm) mutex_lock(&kvm->lock); kvm_for_each_vcpu(i, vcpu, kvm) { + vcpu->arch.st.last_suspend = ktime_get_boottime_ns(); + /* + * Tasks get thawed before the resume notifier has been called + * so we need to block vCPUs until the resume notifier has run. + * Otherwise, suspend steal time might get applied too late, + * and get accounted to the wrong guest task. + * This also ensures that the guest paused bit set below + * doesn't get checked and cleared before the host actually + * suspends. + */ + vcpu->arch.st.host_suspended = 1; + kvm_make_request(KVM_REQ_WAIT_FOR_RESUME, vcpu); + if (!vcpu->arch.pv_time.active) continue; @@ -6954,12 +6982,32 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm) return ret ? NOTIFY_BAD : NOTIFY_DONE; } +static int kvm_arch_resume_notifier(struct kvm *kvm) +{ + struct kvm_vcpu *vcpu; + unsigned long i; + + mutex_lock(&kvm->lock); + kvm_for_each_vcpu(i, vcpu, kvm) { + vcpu->arch.st.host_suspended = 0; + vcpu->arch.st.suspend_ns += ktime_get_boottime_ns() - + vcpu->arch.st.last_suspend; + wake_up_interruptible(&vcpu->arch.st.resume_waitq); + } + mutex_unlock(&kvm->lock); + + return NOTIFY_DONE; +} + int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state) { switch (state) { case PM_HIBERNATION_PREPARE: case PM_SUSPEND_PREPARE: return kvm_arch_suspend_notifier(kvm); + case PM_POST_HIBERNATION: + case PM_POST_SUSPEND: + return kvm_arch_resume_notifier(kvm); } return NOTIFY_DONE; @@ -10813,6 +10861,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) r = 1; goto out; } + if (kvm_check_request(KVM_REQ_WAIT_FOR_RESUME, vcpu)) + wait_for_resume(vcpu); if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu)) record_steal_time(vcpu); if (kvm_check_request(KVM_REQ_PMU, vcpu)) @@ -12341,6 +12391,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu) if (r) goto free_guest_fpu; + init_waitqueue_head(&vcpu->arch.st.resume_waitq); kvm_xen_init_vcpu(vcpu); vcpu_load(vcpu); kvm_vcpu_after_set_cpuid(vcpu); -- 2.48.1.601.g30ceb7b040-goog ^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time 2025-02-21 5:39 ` [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal @ 2025-03-01 17:21 ` Konrad Rzeszutek Wilk 2025-03-05 4:17 ` Suleiman Souhlal 0 siblings, 1 reply; 5+ messages in thread From: Konrad Rzeszutek Wilk @ 2025-03-01 17:21 UTC (permalink / raw) To: Suleiman Souhlal Cc: Paolo Bonzini, Sean Christopherson, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H. Peter Anvin, Chao Gao, David Woodhouse, Sergey Senozhatsky, kvm, linux-kernel, ssouhlal On Fri, Feb 21, 2025 at 02:39:27PM +0900, Suleiman Souhlal wrote: > When the host resumes from a suspend, the guest thinks any task > that was running during the suspend ran for a long time, even though > the effective run time was much shorter, which can end up having > negative effects with scheduling. > > To mitigate this issue, the time that the host was suspended is included > in steal time, which lets the guest can subtract the duration from the s/can// > tasks' runtime. > > In order to implement this behavior, once the suspend notifier fires, > vCPUs trying to run block until the resume notifier finishes. This is s/run/run will/ > because the freezing of userspace tasks happens between these two points, Full stop at the end of that ^ > which means that vCPUs could otherwise run and get their suspend steal > time misaccounted, particularly if a vCPU would run after resume before > the resume notifier. s/notifier/notifier fires/ > Incidentally, doing this also addresses a potential race with the > suspend notifier setting PVCLOCK_GUEST_STOPPED, which could then get > cleared before the suspend actually happened. > > One potential caveat is that in the case of a suspend happening during > a VM migration, the suspend time might not be accounted. s/accounted/accounted for./ > A workaround would be for the VMM to ensure that the guest is entered > with KVM_RUN after resuming from suspend. So ..does that mean there is a QEMU patch as well? > > Signed-off-by: Suleiman Souhlal <suleiman@google.com> > --- > Documentation/virt/kvm/x86/msr.rst | 10 ++++-- > arch/x86/include/asm/kvm_host.h | 6 ++++ > arch/x86/kvm/x86.c | 51 ++++++++++++++++++++++++++++++ > 3 files changed, 65 insertions(+), 2 deletions(-) > > diff --git a/Documentation/virt/kvm/x86/msr.rst b/Documentation/virt/kvm/x86/msr.rst > index 3aecf2a70e7b43..48f2a8ca519548 100644 > --- a/Documentation/virt/kvm/x86/msr.rst > +++ b/Documentation/virt/kvm/x86/msr.rst > @@ -294,8 +294,14 @@ data: > > steal: > the amount of time in which this vCPU did not run, in > - nanoseconds. Time during which the vcpu is idle, will not be > - reported as steal time. > + nanoseconds. This includes the time during which the host is > + suspended. Time during which the vcpu is idle, might not be > + reported as steal time. The case where the host suspends > + during a VM migration might not be accounted if VCPUs aren't > + entered post-resume, because KVM does not currently support > + suspend/resuming the associated metadata. A workaround would > + be for the VMM to ensure that the guest is entered with > + KVM_RUN after resuming from suspend. > > preempted: > indicate the vCPU who owns this struct is running or > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index 452dd0204609af..007656ceac9a71 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -124,6 +124,7 @@ > #define KVM_REQ_HV_TLB_FLUSH \ > KVM_ARCH_REQ_FLAGS(32, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP) > #define KVM_REQ_UPDATE_PROTECTED_GUEST_STATE KVM_ARCH_REQ(34) > +#define KVM_REQ_WAIT_FOR_RESUME KVM_ARCH_REQ(35) > > #define CR0_RESERVED_BITS \ > (~(unsigned long)(X86_CR0_PE | X86_CR0_MP | X86_CR0_EM | X86_CR0_TS \ > @@ -916,8 +917,13 @@ struct kvm_vcpu_arch { > > struct { > u8 preempted; > + bool host_suspended; > u64 msr_val; > u64 last_steal; > + u64 last_suspend; > + u64 suspend_ns; > + u64 last_suspend_ns; > + wait_queue_head_t resume_waitq; > struct gfn_to_hva_cache cache; > } st; > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 06464ec0d1c8d2..f34edcf77cca0a 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -3717,6 +3717,8 @@ static void record_steal_time(struct kvm_vcpu *vcpu) > steal += current->sched_info.run_delay - > vcpu->arch.st.last_steal; > vcpu->arch.st.last_steal = current->sched_info.run_delay; > + steal += vcpu->arch.st.suspend_ns - vcpu->arch.st.last_suspend_ns; > + vcpu->arch.st.last_suspend_ns = vcpu->arch.st.suspend_ns; > unsafe_put_user(steal, &st->steal, out); > > version += 1; > @@ -6930,6 +6932,19 @@ long kvm_arch_vm_compat_ioctl(struct file *filp, unsigned int ioctl, > } > #endif > > +static void wait_for_resume(struct kvm_vcpu *vcpu) > +{ > + wait_event_interruptible(vcpu->arch.st.resume_waitq, > + vcpu->arch.st.host_suspended == 0); > + > + /* > + * This might happen if we blocked here before the freezing of tasks > + * and we get woken up by the freezer. > + */ > + if (vcpu->arch.st.host_suspended) > + kvm_make_request(KVM_REQ_WAIT_FOR_RESUME, vcpu); > +} > + > #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER > static int kvm_arch_suspend_notifier(struct kvm *kvm) > { > @@ -6939,6 +6954,19 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm) > > mutex_lock(&kvm->lock); > kvm_for_each_vcpu(i, vcpu, kvm) { > + vcpu->arch.st.last_suspend = ktime_get_boottime_ns(); > + /* > + * Tasks get thawed before the resume notifier has been called > + * so we need to block vCPUs until the resume notifier has run. > + * Otherwise, suspend steal time might get applied too late, > + * and get accounted to the wrong guest task. > + * This also ensures that the guest paused bit set below > + * doesn't get checked and cleared before the host actually > + * suspends. > + */ > + vcpu->arch.st.host_suspended = 1; > + kvm_make_request(KVM_REQ_WAIT_FOR_RESUME, vcpu); > + > if (!vcpu->arch.pv_time.active) > continue; > > @@ -6954,12 +6982,32 @@ static int kvm_arch_suspend_notifier(struct kvm *kvm) > return ret ? NOTIFY_BAD : NOTIFY_DONE; > } > > +static int kvm_arch_resume_notifier(struct kvm *kvm) > +{ > + struct kvm_vcpu *vcpu; > + unsigned long i; > + > + mutex_lock(&kvm->lock); > + kvm_for_each_vcpu(i, vcpu, kvm) { > + vcpu->arch.st.host_suspended = 0; > + vcpu->arch.st.suspend_ns += ktime_get_boottime_ns() - > + vcpu->arch.st.last_suspend; > + wake_up_interruptible(&vcpu->arch.st.resume_waitq); > + } > + mutex_unlock(&kvm->lock); > + > + return NOTIFY_DONE; > +} > + > int kvm_arch_pm_notifier(struct kvm *kvm, unsigned long state) > { > switch (state) { > case PM_HIBERNATION_PREPARE: > case PM_SUSPEND_PREPARE: > return kvm_arch_suspend_notifier(kvm); > + case PM_POST_HIBERNATION: > + case PM_POST_SUSPEND: > + return kvm_arch_resume_notifier(kvm); > } > > return NOTIFY_DONE; > @@ -10813,6 +10861,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) > r = 1; > goto out; > } > + if (kvm_check_request(KVM_REQ_WAIT_FOR_RESUME, vcpu)) > + wait_for_resume(vcpu); > if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu)) > record_steal_time(vcpu); > if (kvm_check_request(KVM_REQ_PMU, vcpu)) > @@ -12341,6 +12391,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu) > if (r) > goto free_guest_fpu; > > + init_waitqueue_head(&vcpu->arch.st.resume_waitq); > kvm_xen_init_vcpu(vcpu); > vcpu_load(vcpu); > kvm_vcpu_after_set_cpuid(vcpu); > -- > 2.48.1.601.g30ceb7b040-goog > > ^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time 2025-03-01 17:21 ` Konrad Rzeszutek Wilk @ 2025-03-05 4:17 ` Suleiman Souhlal 0 siblings, 0 replies; 5+ messages in thread From: Suleiman Souhlal @ 2025-03-05 4:17 UTC (permalink / raw) To: Konrad Rzeszutek Wilk Cc: Paolo Bonzini, Sean Christopherson, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen, x86, H. Peter Anvin, Chao Gao, David Woodhouse, Sergey Senozhatsky, kvm, linux-kernel, ssouhlal On Sun, Mar 2, 2025 at 2:22 AM Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote: > > On Fri, Feb 21, 2025 at 02:39:27PM +0900, Suleiman Souhlal wrote: > > When the host resumes from a suspend, the guest thinks any task > > that was running during the suspend ran for a long time, even though > > the effective run time was much shorter, which can end up having > > negative effects with scheduling. > > > > To mitigate this issue, the time that the host was suspended is included > > in steal time, which lets the guest can subtract the duration from the > > s/can// > > tasks' runtime. > > > > In order to implement this behavior, once the suspend notifier fires, > > vCPUs trying to run block until the resume notifier finishes. This is > > s/run/run will/ > > because the freezing of userspace tasks happens between these two points, > Full stop at the end of that ^ > > which means that vCPUs could otherwise run and get their suspend steal > > time misaccounted, particularly if a vCPU would run after resume before > > the resume notifier. > > s/notifier/notifier fires/ > > > Incidentally, doing this also addresses a potential race with the > > suspend notifier setting PVCLOCK_GUEST_STOPPED, which could then get > > cleared before the suspend actually happened. > > > > One potential caveat is that in the case of a suspend happening during > > a VM migration, the suspend time might not be accounted. > > s/accounted/accounted for./ > > A workaround would be for the VMM to ensure that the guest is entered > > with KVM_RUN after resuming from suspend. > > So ..does that mean there is a QEMU patch as well? No, I am not planning on making a QEMU patch. A QEMU patch would only be needed if you cared about the caveat mentioned there. Thanks, -- Suleiman ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2025-03-05 4:17 UTC | newest] Thread overview: 5+ messages (download: mbox.gz follow: Atom feed -- links below jump to the message on this page -- 2025-02-21 5:39 [PATCH v4 0/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 1/2] KVM: x86: Advance guest TSC after deep suspend Suleiman Souhlal 2025-02-21 5:39 ` [PATCH v4 2/2] KVM: x86: Include host suspended time in steal time Suleiman Souhlal 2025-03-01 17:21 ` Konrad Rzeszutek Wilk 2025-03-05 4:17 ` Suleiman Souhlal
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