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Re: [Qemu-arm] [Qemu-devel] [PATCH V8 0/2] Add option to configure guest

From: Peter Maydell
Subject: Re: [Qemu-arm] [Qemu-devel] [PATCH V8 0/2] Add option to configure guest vPMU
Date: Fri, 28 Oct 2016 13:01:45 +0100

On 27 October 2016 at 21:53, Wei Huang <address@hidden> wrote:
> On 10/27/2016 12:15 PM, Peter Maydell wrote:
>> Right, but at the moment the commit message says there's no
>> PMU in TCG at all. What's missing that causes there not to be one?
>> (I think maybe we just don't report it in the feature register,
>> which might be a bug.)
> This sounds like a bug (or incomplete). In current version of QEMU, if I
> forced vPMU declaration in the feature register under TCG mode, PMU
> driver will be loaded in VM. But as soon as "perf stat" is executed,
> guest VM panic. I think some features, like PMU IRQ, is still missing
> for TCG to turn vPMU on. I will debug it further.

You're right, we don't have the PMU IRQ implemented.
I wouldn't expect that to cause the guest to panic, though...

> Let me clarify. Assuming we can get TCG mode to support VM's PMU driver
> (see discussion above, only cycle counter supported for TCG), will the
> following be sufficient?
> (1) ,pmu=on
> perf driver can be loaded in VM, and "perf" command can use HW counters
> to profile.
> (2) ,pmu=off
> perf feature can't be detected, so the perf driver can't be loaded
> inside VM. "perf" can't use HW perf counters. But PMU registers (PMCR,
> PMCCNTR, ...) can still be accessed inside VM.

I think that's what we get today (with this patch set). But it
would seem more consistent if pmu=off didn't provide the PMCR,
PMCCNTR, etc to the guest under TCG in the same way that you
don't get them under KVM if you say pmu=off.

Anyway, I think that these patches are the right thing for KVM,
so I'm going to apply them to target-arm.next. We should probably
look at straightening out the TCG situation, but we can do that
with a separate patchset (and perhaps not for 2.8).

Thanks for your patience in working through all the revisions
of this series.

-- PMM

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