From: Sean Christopherson <seanjc@google.com>
To: Dongli Zhang <dongli.zhang@oracle.com>
Cc: kvm@vger.kernel.org, x86@kernel.org,
linux-kselftest@vger.kernel.org, pbonzini@redhat.com,
vkuznets@redhat.com, tglx@kernel.org, mingo@redhat.com,
bp@alien8.de, dave.hansen@linux.intel.com, shuah@kernel.org,
hpa@zytor.com, peterz@infradead.org, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
vschneid@redhat.com, kprateek.nayak@amd.com, jgross@suse.com,
dwmw2@infradead.org, joe.jin@oracle.com
Subject: Re: [PATCH 2/5] KVM: x86: Reset vcpu->arch.st.last_steal when enabling steal time
Date: Fri, 8 May 2026 15:40:45 -0700 [thread overview]
Message-ID: <af5mbWXpG5ofRdJ2@google.com> (raw)
In-Reply-To: <20260505003044.78693-3-dongli.zhang@oracle.com>
On Mon, May 04, 2026, Dongli Zhang wrote:
> KVM does not support vCPU hotplug. When a vCPU is removed, its
> corresponding data structures are not freed by KVM. Instead, QEMU destroys
> only the userspace state and the vCPU thread, while the KVM vCPU fd remains
> open and parked in QEMU.
>
> As a result, vcpu->arch.st.last_steal is not reset.
>
> If the same vCPU is later re-created by QEMU, last_steal retains its old
> value, while current->sched_info.run_delay starts from zero since a new
> vCPU thread is created. This causes
> current->sched_info.run_delay - vcpu->arch.st.last_steal to produce a
> large, bogus value.
>
> Fix this by resetting vcpu->arch.st.last_steal to
> current->sched_info.run_delay when KVM steal time is enabled.
This is quite arbitrary. E.g. if userspace hands the vCPU off to a different
task without going through QEMU's hotplug dance, then current->sched_info.run_delay
will also change.
Shouldn't x86 hook kvm_arch_vcpu_run_pid_change() and reset last_steal in there?
next prev parent reply other threads:[~2026-05-08 22:40 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-05 0:30 [PATCH 0/5] Fix and enhance KVM steal accounting for both guest and host Dongli Zhang
2026-05-05 0:30 ` [PATCH 1/5] x86/kvm: Reset prev_steal_time and prev_steal_time_rq when enabling steal time Dongli Zhang
2026-05-05 0:30 ` [PATCH 2/5] KVM: x86: Reset vcpu->arch.st.last_steal " Dongli Zhang
2026-05-08 22:40 ` Sean Christopherson [this message]
2026-05-10 17:09 ` David Woodhouse
2026-05-10 18:40 ` David Woodhouse
2026-05-05 0:30 ` [PATCH 3/5] KVM: x86: account KVM_SET_CLOCK downtime in " Dongli Zhang
2026-05-10 18:54 ` David Woodhouse
2026-05-10 19:11 ` H. Peter Anvin
2026-05-10 20:13 ` David Woodhouse
2026-05-05 0:30 ` [PATCH 4/5] KVM: selftests: Test steal time when re-adding a vCPU on a new thread Dongli Zhang
2026-05-05 0:30 ` [PATCH 5/5] KVM: selftests: Test KVM_SET_CLOCK downtime in steal time Dongli Zhang
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