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Re: [Qemu-arm] [Qemu-devel] [PATCH V7 0/2] Add option to configure guest

From: Peter Maydell
Subject: Re: [Qemu-arm] [Qemu-devel] [PATCH V7 0/2] Add option to configure guest vPMU
Date: Tue, 25 Oct 2016 16:26:02 +0100

On 25 October 2016 at 08:33, Andrew Jones <address@hidden> wrote:
> On Mon, Oct 24, 2016 at 11:39:59PM -0500, Wei Huang wrote:
>> >> V6->V7:
>> >>   * change has_pmu variable type from OnOffAuto to Boolean
>> >>   * only add "pmu" property to CPU under kvm mode, default ON
>> >
>> > Hmm, if we don't allow the property with TCG then switching a guest from
>> > KVM to TCG will require more than just an accelerator switch. That's a
>> > bit annoying and I think we'd have to teach it to libvirt too. I'd prefer
>> >
>> >  -M virt-2.8,accel=tcg -cpu cortex-a57         NO     NO
>> >  -M virt-2.8,accel=tcg -cpu cortex-a57,pmu=off NO     NO
>> >  -M virt-2.8,accel=tcg -cpu cortex-a57,pmu=on  NO     "Warning: PMU not
>> > yet supported with TCG" (or something)
>> I am fine with this request. But note that, if we enforce
>> pmu-default=ON, we can't tell "-cpu cortex-a57" apart from
>> "cortex-a57,pmu=on", implying that we have to print the warning msg for
>> the case of "cortex-a57" as well (this was why we switch to tri-state
>> before). To solve this problem, we have to switch from pmu-default=ON to
>> pmu-default=OFF under TCG mode, something like:
>>     if (arm_feature(&cpu->env, ARM_FEATURE_PMU)) {
>>         qdev_property_add_static(DEVICE(obj),
>>             &arm_cpu_has_pmu_property, &error_abort);
>>         if (!kvm_enabled())
>>             object_property_set_bool(obj, false, "pmu", NULL);
>>     }
>> Then we do:
>>     if (cpu->has_pmu && !kvm_enabled()) {
>>         cpu->has_pmu = false;
>>         if (!pmu_warned && !qtest_enabled()) {
>>             error_report("warning: pmu not supported under TCG");
>>             pmu_warned = true;
>>         }
>>     }
>> This will work. Are you and Peter OK with this solution?
> Yes, I prefer it to not being able to easily switch a command line from
> KVM to TCG. Granted, right now we mostly need to run with -cpu host for
> KVM, which also needs to be switched for TCG. However that will hopefully
> change with Shannon's work.

If the TCG default is different from the KVM default then you
will have problems with switching it anyway, because it will
silently change behaviour underfoot. I think we should avoid that...

-- PMM

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