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

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
host from collecting data whilst inside a guest thus creating a black-out
window. This occurs because instead of emulating the SPE, we share it
with our guests.

Let's accurately describe our capabilities by using the perf exclude
flags to prevent !exclude_guest and exclude_host flags from being used.

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
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)
 	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?

Thanks,

Andrew Murray

	M.

-- 
Jazz is not dead, it just smells funny.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel