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Re: [Qemu-arm] [PATCH V6 0/2] Add option to configure guest vPMU

From: Peter Maydell
Subject: Re: [Qemu-arm] [PATCH V6 0/2] Add option to configure guest vPMU
Date: Thu, 13 Oct 2016 17:19:37 +0100

On 4 October 2016 at 22:38, Wei Huang <address@hidden> wrote:
> This patchset adds a pmu=[on/off] option to enable/disable vPMU support
> for guest VM. There are several reasons to justify this option. First,
> vPMU can be problematic for cross-migration between different SoC as perf
> counters are architecture-dependent. It is more flexible to have an option
> to turn it on/off. Secondly this option matches the "pmu" option as
> supported in libvirt. To make sure backward compatible, a PMU-related
> property is added to mach-virt machine types.
> The following are testing results with this patchset. Other combinations
> should have similar results:
>   CONFIG (qemu-system-aarch64)                       vPMU   WARNING
>   -M virt-2.8/virt,accel=kvm -cpu host               NO     NO

What's the rationale for defaulting to no-pmu? Other cpu features
we have tend to default to present unless you ask not to have them.

>   -M virt-2.8/virt,accel=kvm -cpu host,pmu=off       NO     NO
>   -M virt-2.8/virt,accel=kvm -cpu host,pmu=on        YES    NO
>   -M virt-2.7,accel=kvm -cpu host                    YES    NO
>   -M virt-2.7,accel=kvm -cpu host,pmu=off            NO     NO
>   -M virt-2.7,accel=kvm -cpu host,pmu=on             YES    NO

in virt-2.7 you always got a pmu regardless, so why does pmu=off here give
you no pmu?

>   -M virt-2.6,accel=kvm -cpu host                    NO     NO
>   -M virt-2.6,accel=kvm -cpu host,pmu=off            NO     NO
>   -M virt-2.6,accel=kvm -cpu host,pmu=on             YES    NO
>   -M virt-2.8/virt,accel=tcg -cpu cortex-a57         NO     NO
>   -M virt-2.8/virt,accel=tcg -cpu cortex-a57,pmu=off NO     NO
>   -M virt-2.8/virt,accel=tcg -cpu cortex-a57,pmu=on  NO     "No KVM"
>   -M virt-2.7,accel=tcg -cpu cortex-a57              NO     NO
>   -M virt-2.7,accel=tcg -cpu cortex-a57,pmu=off      NO     NO
>   -M virt-2.7,accel=tcg -cpu cortex-a57,pmu=on       NO     "No KVM"
>   -M virt-2.6,accel=tcg -cpu cortex-a57              NO     NO
>   -M virt-2.6,accel=tcg -cpu cortex-a57,pmu=off      NO     NO
>   -M virt-2.6,accel=tcg -cpu cortex-a57,pmu=on       NO     "No KVM"

This patchset doesn't actually change whether the pmu is present
or not under TCG so this is all a little bit odd.

>   -M virt-2.8/virt,accel=tcg -cpu cortex-a15         NO     NO
>   -M virt-2.8/virt,accel=tcg -cpu cortex-a15,pmu=off NO     "No PMU property"
>   -M virt-2.8/virt,accel=tcg -cpu cortex-a15,pmu=on  NO     "No PMU property"
>   -M virt-2.7,accel=tcg -cpu cortex-a15              NO     NO
>   -M virt-2.7,accel=tcg -cpu cortex-a15,pmu=off      NO     "No PMU property"
>   -M virt-2.7,accel=tcg -cpu cortex-a15,pmu=on       NO     "No PMU property"
>   -M virt-2.6,accel=tcg -cpu cortex-a15              NO     NO
>   -M virt-2.6,accel=tcg -cpu cortex-a15,pmu=off      NO     "No PMU property"
>   -M virt-2.6,accel=tcg -cpu cortex-a15,pmu=on       NO     "No PMU property"
>   * "NO KVM" msg
>     warning: pmu can't be enabled without KVM acceleration
>   * "No PMU property" msg
>     can't apply global cortex-a15-arm-cpu.pmu=off: Property '.pmu' not found

I'm really not a fan of the tristate has_pmu, so I'm hoping
there's a setup which lets us avoid it...

-- PMM

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