Re: [Qemu-devel] [PATCH RFC 1/1] arm64: add an option to turn on/off vpmu support

[Peter Maydell <peter.maydell@linaro.org>]

On 28 July 2016 at 17:38, Wei Huang <wei@redhat.com> wrote:
> This patch adds a pmu=[on/off] option to enable/disable vpmu support
> in guest vm. There are several reasons to justify this option. First
> vpmu can be problematic for cross-migration between different SoC as
> perf counters is architecture-dependent. It is more flexible to
> have an option to turn it on/off. Secondly it matches the -cpu pmu
> option in libivrt. This patch has been tested on both DT/ACPI modes.


What particular two systems are you trying to migrate between?
In general we don't support migrating between different CPU
types at the moment, so the PMU sholud be the same on both ends.

(If we ever do get to supporting cross-cpu-type migration
then it will probably involve a very long and detailed command
line to specify exactly a whole lot of things like pmu yes/no,
number of hw breakpoints/watchpoints, and everything else that
can differ between implementations.)

That said, I don't have any objection to making the PMU
presence controllable (especially if we have similar
control on x86).

> --- a/target-arm/cpu.h
> +++ b/target-arm/cpu.h
> @@ -579,8 +579,9 @@ struct ARMCPU {
>      bool powered_off;
>      /* CPU has security extension */
>      bool has_el3;
> -    /* CPU has PMU (Performance Monitor Unit) */
> -    bool has_pmu;
> +
> +    /* CPU has vPMU (Performance Monitor Unit) support */
> +    bool enable_pmu;

Why rename the flag? has_foo is what we use for other features,
as you can see in the context of this bit of the patch.

>
>      /* CPU has memory protection unit */
>      bool has_mpu;

thanks
-- PMM