On 2019-12-23 12:10, Andrew Murray wrote:
On Sun, Dec 22, 2019 at 12:10:52PM +0000, Marc Zyngier wrote:
On Fri, 20 Dec 2019 14:30:22 +0000,
Andrew Murray <andrew.murray@xxxxxxx> wrote:
>
> 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 <andrew.murray@xxxxxxx>
> ---
> 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?
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...
