Re: [PATCH v2] KVM: arm64: Allow to limit number of PMU counters

[Alexander Graf <graf@amazon.com>]

Hey Drew!

On 09.09.20 08:25, Andrew Jones wrote:
> 
> On Tue, Sep 08, 2020 at 10:57:30PM +0200, Alexander Graf wrote:
>> We currently pass through the number of PMU counters that we have available
>> in hardware to guests. So if my host supports 10 concurrently active PMU
>> counters, my guest will be able to spawn 10 counters as well.
>>
>> This is undesireable if we also want to use the PMU on the host for
>> monitoring. In that case, we want to split the PMU between guest and
>> host.
>>
>> To help that case, let's add a PMU attr that allows us to limit the number
>> of PMU counters that we expose. With this patch in place, user space can
>> keep some counters free for host use.
> 
> Hi Alex,
> 
> Is there any reason to use the device API instead of just giving the user
> control over the necessary PMCR_EL0 bits through set/get-one-reg?

I mostly used the attr interface because I was in that particular mental 
mode after looking at the filtering bits :).

Today, the PMCR_EL0 register gets reset implicitly on every vcpu reset 
call. How would we persist the counter field across resets? Would we in 
the first place?

I'm slightly hazy how the ONE_REG API would look like here. Do you have 
recommendations?


Alex



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