Re: [RFC PATCH] perf/core: don't sample kernel regs upon skid

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Jul 09, 2018 at 06:42:29PM -0400, Boris Ostrovsky wrote:
> On 07/02/2018 12:02 PM, Mark Rutland wrote:
> > On Mon, Jul 02, 2018 at 05:46:55PM +0200, Peter Zijlstra wrote:
> >> On Mon, Jul 02, 2018 at 04:12:50PM +0100, Mark Rutland wrote:
> >>> +static struct pt_regs *perf_get_sample_regs(struct perf_event *event,
> >>> +					    struct pt_regs *regs)
> >>> +{
> >>> +	/*
> >>> +	 * Due to interrupt latency (AKA "skid"), we may enter the kernel
> >>> +	 * before taking an overflow, even if the PMU is only counting user
> >>> +	 * events.
> >>> +	 *
> >>> +	 * If we're not counting kernel events, always use the user regs when
> >>> +	 * sampling.
> >>> +	 *
> >>> +	 * TODO: what do we do about sampling a guest's registers? The IP is
> >>> +	 * special-cased, but for the rest of the regs they'll get the
> >>> +	 * user/kernel regs depending on whether exclude_kernel is set, which
> >>> +	 * is nonsensical.
> >>> +	 *
> >>> +	 * We can't get at the full set of regs in all cases (e.g. Xen's PV PMU
> >>> +	 * can't provide the GPRs), so should we just zero the GPRs when in a
> >>> +	 * guest? Or skip outputting the regs in perf_output_sample?
> >> Seems daft Xen cannot provide registers; why is that? Boris?
> > The xen_pmu_regs structure simply doesn't have them, so I assume there's
> > no API to get them.
> >
> > Given we don't currently sample the guest regs, I'd be tempted to just
> > zero them for now, or skip the sample at output time (if that doesn't
> > break some other case).
> 
> (Was out on vacation, couldn't respond earlier)
> 
> Yes, PV guests only get a limited set of registers passed to the handler
> by the hypervisor. GPRs are not part of this set.

Is that also true for Dom0?

> I think we need do
> 
> diff --git a/arch/x86/xen/pmu.c b/arch/x86/xen/pmu.c
> index 7d00d4a..95997e6 100644
> --- a/arch/x86/xen/pmu.c
> +++ b/arch/x86/xen/pmu.c
> @@ -478,7 +478,7 @@ static void xen_convert_regs(const struct
> xen_pmu_regs *xen_regs,
>  irqreturn_t xen_pmu_irq_handler(int irq, void *dev_id)
>  {
>         int err, ret = IRQ_NONE;
> -       struct pt_regs regs;
> +       struct pt_regs regs = {0};
>         const struct xen_pmu_data *xenpmu_data = get_xenpmu_data();
>         uint8_t xenpmu_flags = get_xenpmu_flags();
> 
> 
> Do you want me to submit a separate patch or can you make this part of
> yours?

I think this is going to become a series rather than a single patch, but I can
have a go. I need to get my head around how the various cases interact with
each other.

Thanks,
Mark.