Re: [PATCH v2 15/18] perf: arm_spe: Handle guest/host exclusion flags

On 2019-12-23 12:10, Andrew Murray wrote:

On Sun, Dec 22, 2019 at 12:10:52PM +0000, Marc Zyngier wrote:

quoted

On Fri, 20 Dec 2019 14:30:22 +0000,
Andrew Murray [off-list ref] wrote:

quoted

A side effect of supporting the SPE in guests is that we prevent 

the

quoted

host from collecting data whilst inside a guest thus creating a 

black-out

quoted

window. This occurs because instead of emulating the SPE, we share 

it

quoted

with our guests.

Let's accurately describe our capabilities by using the perf 

exclude

quoted

flags to prevent !exclude_guest and exclude_host flags from being 

used.

quoted

Signed-off-by: Andrew Murray <redacted>
---
 drivers/perf/arm_spe_pmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/perf/arm_spe_pmu.c 

b/drivers/perf/arm_spe_pmu.c

quoted

index 2d24af4cfcab..3703dbf459de 100644

--- a/drivers/perf/arm_spe_pmu.c
+++ b/drivers/perf/arm_spe_pmu.c

@@ -679,6 +679,9 @@ static int arm_spe_pmu_event_init(struct 

perf_event *event)

quoted

 	if (attr->exclude_idle)
 		return -EOPNOTSUPP;

+	if (!attr->exclude_guest || attr->exclude_host)
+		return -EOPNOTSUPP;
+

I have the opposite approach. If the host decides to profile the
guest, why should that be denied? If there is a black hole, it 
should
take place in the guest. Today, the host does expect this to work, 
and
there is no way that we unconditionally allow it to regress.

That seems reasonable.

Upon entering the guest we'd have to detect if the host is using SPE, 
and if
so choose not to restore the guest registers. Instead we'd have to 
trap them
and let the guest read/write emulated values until the host has 
finished with
SPE - at which time we could restore the guest SPE registers to 
hardware.

Does that approach make sense?

Yes, this would be much better. All of this can be found out at 
vcpu_load()
time, and once you've moved most of the SPE sysreg handling there, it 
will
just follow the normal scheduling flow.

         M.
-- 
Jazz is not dead. It just smells funny...

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