[RFC PATCH 2/6] ARM64: perf: Re-enable overflow interrupt from interrupt handler

[Anup Patel]

On 6 August 2014 19:54, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, Aug 05, 2014 at 10:24:11AM +0100, Anup Patel wrote:
>> A hypervisor will typically mask the overflow interrupt before
>> forwarding it to Guest Linux hence we need to re-enable the overflow
>> interrupt after clearing it in Guest Linux. Also, this re-enabling
>> of overflow interrupt does not harm in non-virtualized scenarios.
>>
>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar at linaro.org>
>> Signed-off-by: Anup Patel <anup.patel at linaro.org>
>> ---
>>  arch/arm64/kernel/perf_event.c |    8 ++++++++
>>  1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
>> index 47dfb8b..19fb140 100644
>> --- a/arch/arm64/kernel/perf_event.c
>> +++ b/arch/arm64/kernel/perf_event.c
>> @@ -1076,6 +1076,14 @@ static irqreturn_t armv8pmu_handle_irq(int irq_num, void *dev)
>>               if (!armv8pmu_counter_has_overflowed(pmovsr, idx))
>>                       continue;
>>
>> +             /*
>> +              * If we are running under a hypervisor such as KVM then
>> +              * hypervisor will mask the interrupt before forwarding
>> +              * it to Guest Linux hence re-enable interrupt for the
>> +              * overflowed counter.
>> +              */
>> +             armv8pmu_enable_intens(idx);
>> +
>
> Really? This is a giant bodge in the guest to work around short-comings in
> the hypervisor. Why can't we fix this properly using something like Marc's
> irq forwarding code?

This change is in accordance with our previous RFC thread about
PMU virtualization where Marc Z had suggest to do interrupt
mask/unmask dance similar to arch-timer.

I have not tried Marc'z irq forwarding series. In next revision of this
patchset, I will try to use Marc's irq forwarding approach.

>
> Will

--
Anup