From: Leo Yan <leo.yan@linaro.org>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
kvm@vger.kernel.org, James Morse <james.morse@arm.com>,
Suzuki K Poulose <suzuki.poulose@arm.com>,
Oliver Upton <oliver.upton@linux.dev>,
Zenghui Yu <yuzenghui@huawei.com>,
Huang Shijie <shijie@os.amperecomputing.com>,
Mark Rutland <mark.rutland@arm.com>,
Will Deacon <will@kernel.org>
Subject: Re: [PATCH] KVM: arm64: pmu: Resync EL0 state on counter rotation
Date: Mon, 14 Aug 2023 15:16:27 +0800 [thread overview]
Message-ID: <20230814071627.GA3963214@leoy-huanghe> (raw)
In-Reply-To: <20230811180520.131727-1-maz@kernel.org>
On Fri, Aug 11, 2023 at 07:05:20PM +0100, Marc Zyngier wrote:
> Huang Shijie reports that, when profiling a guest from the host
> with a number of events that exceeds the number of available
> counters, the reported counts are wildly inaccurate. Without
> the counter oversubscription, the reported counts are correct.
>
> Their investigation indicates that upon counter rotation (which
> takes place on the back of a timer interrupt), we fail to
> re-apply the guest EL0 enabling, leading to the counting of host
> events instead of guest events.
Seems to me, it's not clear for why the counter rotation will cause
the issue.
In the example shared by Shijie in [1], the cycle counter is enabled for
both host and guest, and cycle counter is a dedicated event which does
not share counter with other events. Even there have counter rotation,
it should not impact the cycle counter.
I mean if we cannot explain clearly for this part, we don't find the
root cause, and this patch (and Shijie's patch) just walks around the
issue.
Thanks,
Leo
[1] https://lore.kernel.org/lkml/20230810072906.4007-1-shijie@os.amperecomputing.com/
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next prev parent reply other threads:[~2023-08-14 7:17 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-11 18:05 [PATCH] KVM: arm64: pmu: Resync EL0 state on counter rotation Marc Zyngier
2023-08-14 2:58 ` Shijie Huang
2023-08-15 6:27 ` Marc Zyngier
2023-08-15 7:24 ` Shijie Huang
2023-08-14 5:03 ` kernel test robot
2023-08-14 7:16 ` Leo Yan [this message]
2023-08-14 8:12 ` Shijie Huang
2023-08-14 8:47 ` Leo Yan
2023-08-14 9:15 ` Shijie Huang
2023-08-14 9:29 ` Shijie Huang
2023-08-14 10:01 ` Leo Yan
2023-08-15 2:59 ` Shijie Huang
2023-08-15 6:32 ` Marc Zyngier
2023-08-16 3:04 ` Leo Yan
2023-08-16 3:31 ` Shijie Huang
2023-08-16 21:15 ` kernel test robot
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