Hi Leo,
在 2023/8/16 11:04, Leo Yan 写道:
On Tue, Aug 15, 2023 at 07:32:40AM +0100, Marc Zyngier wrote:
On Mon, 14 Aug 2023 08:16:27 +0100,
Leo Yan <leo.yan@xxxxxxxxxx> wrote:
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.
Maybe unclear to you, but rather clear to me (and most people else on
Cc).
I have to admit this it true.
In the example shared by Shijie in [1], the cycle counter is enabled
for both host and guest
No. You're misreading the example. We're profiling the guest from the
host, and the guest has no PMU access.
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.
Who says that we're counting cycles using the cycle counter? This is
an event like any other, and it can be counted on any counter.
Sorry for noise.
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.
We have the root cause. You just need to think a bit harder.
Let me elaborate a bit more for my concern. The question is how we can
know the exactly the host and the guest have the different counter
enabling?
Shijie's patch relies on perf event rotation to trigger syncing for
PMU PMEVTYPER and PMCCFILTR registers. The perf event rotation will
enable and disable some events, but it doesn't mean the host and the
guest enable different counters. If we use the perf event rotation to
In the event rotation, it does mean the host and guest use different
counters.
Please read my previoust email which shows an example. :)
Thanks
Huang Shijie
trigger syncing, there must introduce redundant operations.
