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

From: [email-masked] (Will Deacon)
Date: Wed, 6 Aug 2014 15:24:39 +0100

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 <[email-masked]>
> Signed-off-by: Anup Patel <[email-masked]>
> ---
>  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?

Will