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Message-ID: <20180711055927.wcfcfkninfjwox3n@salmiak>
Date: Wed, 11 Jul 2018 06:59:28 +0100
From: Mark Rutland <mark.rutland@....com>
To: Boris Ostrovsky <boris.ostrovsky@...cle.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
Jin Yao <yao.jin@...ux.intel.com>
Subject: Re: [RFC PATCH] perf/core: don't sample kernel regs upon skid
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.
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