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

[Marc Zyngier]

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...