Re: [PATCH 4/5] perf tools kvm: Use "cycles" to sample guest for "kvm record" on Intel

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Aug 07, 2025 at 11:08:11AM +0800, Mi, Dapeng wrote:
> 
> On 8/7/2025 8:08 AM, Namhyung Kim wrote:
> > On Tue, Aug 05, 2025 at 08:46:32AM +0800, Dapeng Mi wrote:
> >> After KVM supports PEBS for guest on Intel platforms
> >> (https://lore.kernel.org/all/20220411101946.20262-1-likexu@tencent.com/),
> >> host loses the capability to sample guest with PEBS since all PEBS related
> >> MSRs are switched to guest value after vm-entry, like IA32_DS_AREA MSR is
> >> switched to guest GVA at vm-entry. This would lead to "perf kvm record"
> >> fails to sample guest on Intel platforms since "cycles:P" event is used to
> >> sample guest by default as below case shows.
> > Do you mean we cannot use "cycles:PG" for perf kvm record?
> 
> Yes. Here is the output on Intel Sapphire rapids.
> 
> sudo ./perf record -e cycles:PG -a
> ^C[ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.801 MB perf.data ]
> 
> No guest records are captured with PEBS, and guest PEBS records can be
> sampled only without PEBS.
> 
> sudo ./perf record -e cycles:G -a
> ^C[ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.798 MB perf.data (60 samples) ]
> 
> 
> >
> >> sudo perf kvm record -a
> >> ^C[ perf record: Woken up 1 times to write data ]
> >> [ perf record: Captured and wrote 0.787 MB perf.data.guest ]
> >>
> >> So to ensure guest record can be sampled successfully, use "cycles"
> >> instead of "cycles:P" to sample guest record by default on Intel
> >> platforms. With this patch, the guest record can be sampled
> >> successfully.
> >>
> >> sudo perf kvm record -a
> >> ^C[ perf record: Woken up 1 times to write data ]
> >> [ perf record: Captured and wrote 0.783 MB perf.data.guest (23 samples) ]
> > What if user already gave some events in the command line?  I think you
> > need to check if "-e" or "--event" (and "--pfm-events" too) is in the
> > argv[] before adding these.
> 
> kvm_add_default_arch_event() would detect if user already sets events explicitly. If so, it won't add "cycles" event any more. Thanks.

Oh, ok.  I can see you called parse_options to check the option.
You'd better to check "--pfm-events" as well.

Thanks,
Namhyung