Re: [PATCH 3/4] arm64: arm_pmu: Exclude EL1,2 with :G :H perf attributes

[Andrew Murray <andrew.murray@arm.com>]

On Thu, Nov 15, 2018 at 02:57:59PM +0000, Suzuki K Poulose wrote:
> Hi Andrew,
> 
> On 15/11/2018 12:55, Andrew Murray wrote:
> > When using VHE, EL1 is unused by the host and EL2 is unused by the
> > guest - therefore we can filter out these events with the PMU as per
> > the 'exclude_host' and 'exclude_guest' attributes.
> > 
> > With both VHE and non-VHE we switch the counters between host/guest
> > at EL2. With non-VHE when using 'exclude_host' we filter out EL2.
> > 
> > These changes eliminate counters counting host events on the
> > boundaries of guest entry/exit when using :G. However when using :H
> > unless exclude_hv is set on non-VHE then there is a small blackout
> > window at the guest entry/exit where host events are not captured.
> > 
> > Signed-off-by: Andrew Murray <andrew.murray@arm.com>
> > ---
> >   arch/arm64/kernel/perf_event.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/perf_event.c b/arch/arm64/kernel/perf_event.c
> > index 89d444f..c079c1f 100644
> > --- a/arch/arm64/kernel/perf_event.c
> > +++ b/arch/arm64/kernel/perf_event.c
> > @@ -971,12 +971,14 @@ static int armv8pmu_set_event_filter(struct hw_perf_event *event,
> >   	 * with other architectures (x86 and Power).
> >   	 */
> >   	if (is_kernel_in_hyp_mode()) {
> > -		if (!attr->exclude_kernel)
> > +		if (!attr->exclude_kernel && !attr->exclude_host)
> >   			config_base |= ARMV8_PMU_INCLUDE_EL2;
> > +		if (attr->exclude_guest)
> > +			config_base |= ARMV8_PMU_EXCLUDE_EL1;
> 
> Do we really need this ? exclude_guest also implies you need to
> exclude guest EL0. We anyway disable the events when we enter the
> guest. So the above is not necessary and could possibly create
> confusion.

You're right, we disable the event anyway prior to entering EL1
and visa versa so there is no gain. It felt nice to utilise the
hardware as extra security but it does complicate this already
complex bit of logic. I'll drop it.

> 
> >   	} else {
> >   		if (attr->exclude_kernel)
> >   			config_base |= ARMV8_PMU_EXCLUDE_EL1;
> > -		if (!attr->exclude_hv)
> > +		if (!attr->exclude_hv && !attr->exclude_host)
> >   			config_base |= ARMV8_PMU_INCLUDE_EL2;
> >   	}
> >   	if (attr->exclude_user)
> > 
> 
> 
> I think this can be folded into the previous patch, which adds the
> support for exclude_host/guest support. :G, :H are nothing but
> the those exclude_ flags.

Thanks I'll do that.

Andrew Murray

> 
> Otherwise looks good to me.
> 
> Cheers
> Suzuki