linux-kernel - Re: [PATCH v2 2/3] KVM: x86: Include host suspended time in steal time.

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <CABCjUKAV0ycQH9YFXwLAsJDO22=STxFqONAqO=DY3F6bi+3xAA@mail.gmail.com>
Date: Fri, 23 Aug 2024 14:43:00 +0900
From: Suleiman Souhlal <suleiman@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Sean Christopherson <seanjc@...gle.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ssouhlal@...ebsd.org
Subject: Re: [PATCH v2 2/3] KVM: x86: Include host suspended time in steal time.

On Fri, Aug 23, 2024 at 2:25 PM Chao Gao <chao.gao@...el.com> wrote:
>
> On Fri, Aug 23, 2024 at 01:17:31PM +0900, Suleiman Souhlal wrote:
> >On Wed, Aug 21, 2024 at 3:31 PM Chao Gao <chao.gao@...el.com> wrote:
> >>
> >> On Tue, Aug 20, 2024 at 01:35:42PM +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. This can be particularly noticeable
> >> >if the guest task was RT, as it can end up getting throttled for a
> >> >long time.
> >> >
> >> >To mitigate this issue, we include the time that the host was
> >> >suspended in steal time, which lets the guest subtract the duration from
> >> >the tasks' runtime.
> >> >
> >> >Note that the case of a suspend happening during a VM migration
> >> >might not be accounted.
> >> >
> >> >Signed-off-by: Suleiman Souhlal <suleiman@...gle.com>
> >> >---
> >> > arch/x86/include/asm/kvm_host.h |  1 +
> >> > arch/x86/kvm/x86.c              | 11 ++++++++++-
> >> > 2 files changed, 11 insertions(+), 1 deletion(-)
> >> >
> >> >diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> >> >index 4a68cb3eba78f8..728798decb6d12 100644
> >> >--- a/arch/x86/include/asm/kvm_host.h
> >> >+++ b/arch/x86/include/asm/kvm_host.h
> >> >@@ -898,6 +898,7 @@ struct kvm_vcpu_arch {
> >> >               u8 preempted;
> >> >               u64 msr_val;
> >> >               u64 last_steal;
> >> >+              u64 last_suspend_ns;
> >> >               struct gfn_to_hva_cache cache;
> >> >       } st;
> >> >
> >> >diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> >> >index 70219e4069874a..104f3d318026fa 100644
> >> >--- a/arch/x86/kvm/x86.c
> >> >+++ b/arch/x86/kvm/x86.c
> >> >@@ -3654,7 +3654,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
> >> >       struct kvm_steal_time __user *st;
> >> >       struct kvm_memslots *slots;
> >> >       gpa_t gpa = vcpu->arch.st.msr_val & KVM_STEAL_VALID_BITS;
> >> >-      u64 steal;
> >> >+      u64 steal, suspend_ns;
> >> >       u32 version;
> >> >
> >> >       if (kvm_xen_msr_enabled(vcpu->kvm)) {
> >> >@@ -3735,6 +3735,14 @@ 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;
> >> >+      /*
> >> >+       * Include the time that the host was suspended in steal time.
> >> >+       * Note that the case of a suspend happening during a VM migration
> >> >+       * might not be accounted.
> >> >+       */
> >> >+      suspend_ns = kvm_total_suspend_ns();
> >> >+      steal += suspend_ns - vcpu->arch.st.last_suspend_ns;
> >> >+      vcpu->arch.st.last_suspend_ns = suspend_ns;
> >>
> >> The document in patch 3 states:
> >>
> >>   Time during which the vcpu is idle, will not be reported as steal time
> >>
> >> I'm wondering if all host suspend time should be reported as steal time,
> >> or if the suspend time during a vCPU halt should be excluded.
> >
> >I think the statement about idle time not being reported as steal isn't
> >completely accurate, so I'm not sure if it's worth the extra complexity.
> >
> >>
> >> >       unsafe_put_user(steal, &st->steal, out);
> >> >
> >> >       version += 1;
> >> >@@ -12280,6 +12288,7 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
> >> >
> >> >       vcpu->arch.arch_capabilities = kvm_get_arch_capabilities();
> >> >       vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
> >> >+      vcpu->arch.st.last_suspend_ns = kvm_total_suspend_ns();
> >>
> >> is this necessary? I doubt this because KVM doesn't capture
> >> current->sched_info.run_delay here.
> >
> >Isn't run_delay being captured by the scheduler at all time?
>
> I meant KVM doesn't do:
>
>         vcpu->arch.st.last_steal = current->sched_info.run_delay;
>
> at vCPU creation time.

I think for run_delay it's different because run_delay is a time
difference. It's something that gets added to steal, not relative
to the previous steal value.
>From what I can tell, it's correct for last_steal to be initialized to 0.

>
> >
> >We need to initialize last_suspend_ns otherwise the first call to
> >record_steal_time() for a VCPU would report a wrong value if
> >the VCPU is started after the host has already had a suspend.
>
> But initializing last_suspend_ns here doesn't guarantee KVM won't report a
> "wrong" value because a suspend can happen after vCPU creation and before
> its first VM-enter.

I see what you're saying.
I'm not sure how much this matters in practice.

-- Suleiman