Re: [RFC/PATCH 4/4] perf/x86: Relax privilege filter restriction on AMD IBS

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Aug 30, 2024 at 04:29:10PM -0700, Namhyung Kim wrote:
> While IBS is available for per-thread profiling, still regular users
> cannot open an event due to the default paranoid setting (2) which
> doesn't allow unprivileged users to get kernel samples.  That means
> it needs to set exclude_kernel bit in the attribute but IBS driver
> would reject it since it has PERF_PMU_CAP_NO_EXCLUDE.  This is not what
> we want and I've been getting requests to fix this issue.
> 
> This should be done in the hardware, but until we get the HW fix we may
> allow exclude_{kernel,user} in the attribute and silently drop the
> samples in the PMU IRQ handler.  It won't guarantee the sampling
> frequency or even it'd miss some with fixed period too.  Not ideal,
> but that'd still be helpful to regular users.

Urgh.... this is really rather bad. And I'm sure a bunch of people are
going to be spending a lot of time trying to figure out why their
results don't make sense.

I realize that having entry hooks to disable/enable the counters is also
not going to happen, this has a ton of problems too.

Also, that PMU passthrough patch set has guest hooks, so you can
actually do the exclude_host/guest nonsense with those, right?

> This uses perf_exclude_event() which checks regs->cs.  But it should be
> fine because set_linear_ip() also updates the CS according to the RIP
> provided by IBS.
> 
> Cc: Ravi Bangoria <ravi.bangoria@amd.com>
> Cc: Stephane Eranian <eranian@google.com>
> Signed-off-by: Namhyung Kim <namhyung@kernel.org>
> ---
>  arch/x86/events/amd/ibs.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/events/amd/ibs.c b/arch/x86/events/amd/ibs.c
> index e91970b01d62..e40e2255239a 100644
> --- a/arch/x86/events/amd/ibs.c
> +++ b/arch/x86/events/amd/ibs.c
> @@ -290,6 +290,11 @@ static int perf_ibs_init(struct perf_event *event)
>  	if (has_branch_stack(event))
>  		return -EOPNOTSUPP;
>  
> +	/* handle exclude_{user,kernel} in the IRQ handler */
> +	if (event->attr.exclude_hv || event->attr.exclude_idle ||
> +	    event->attr.exclude_host || event->attr.exclude_guest)
> +		return -EINVAL;
> +
>  	ret = validate_group(event);
>  	if (ret)
>  		return ret;
> @@ -667,7 +672,6 @@ static struct perf_ibs perf_ibs_fetch = {
>  		.start		= perf_ibs_start,
>  		.stop		= perf_ibs_stop,
>  		.read		= perf_ibs_read,
> -		.capabilities	= PERF_PMU_CAP_NO_EXCLUDE,
>  	},
>  	.msr			= MSR_AMD64_IBSFETCHCTL,
>  	.config_mask		= IBS_FETCH_CONFIG_MASK,
> @@ -691,7 +695,6 @@ static struct perf_ibs perf_ibs_op = {
>  		.start		= perf_ibs_start,
>  		.stop		= perf_ibs_stop,
>  		.read		= perf_ibs_read,
> -		.capabilities	= PERF_PMU_CAP_NO_EXCLUDE,
>  	},
>  	.msr			= MSR_AMD64_IBSOPCTL,
>  	.config_mask		= IBS_OP_CONFIG_MASK,
> @@ -1111,6 +1114,12 @@ static int perf_ibs_handle_irq(struct perf_ibs *perf_ibs, struct pt_regs *iregs)
>  		regs.flags |= PERF_EFLAGS_EXACT;
>  	}
>  
> +	if (perf_exclude_event(event, &regs)) {
> +		throttle = perf_event_account_interrupt(event);
> +		atomic64_inc(&event->dropped_samples);
> +		goto out;
> +	}
> +
>  	if (event->attr.sample_type & PERF_SAMPLE_RAW) {
>  		raw = (struct perf_raw_record){
>  			.frag = {
> -- 
> 2.46.0.469.g59c65b2a67-goog
>