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

["Mi, Dapeng" <dapeng1.mi@linux.intel.com>]

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.

>
> Thanks,
> Namhyung
>
>> Reported-by: Kevin Tian <kevin.tian@intel.com>
>> Fixes: 634d36f82517 ("perf record: Just use "cycles:P" as the default event")
>> Signed-off-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
>> ---
>>  tools/perf/arch/x86/util/kvm-stat.c | 46 +++++++++++++++++++++++++++++
>>  1 file changed, 46 insertions(+)
>>
>> diff --git a/tools/perf/arch/x86/util/kvm-stat.c b/tools/perf/arch/x86/util/kvm-stat.c
>> index 424716518b75..cdb5f3e1b5be 100644
>> --- a/tools/perf/arch/x86/util/kvm-stat.c
>> +++ b/tools/perf/arch/x86/util/kvm-stat.c
>> @@ -3,9 +3,11 @@
>>  #include <string.h>
>>  #include "../../../util/kvm-stat.h"
>>  #include "../../../util/evsel.h"
>> +#include "../../../util/env.h"
>>  #include <asm/svm.h>
>>  #include <asm/vmx.h>
>>  #include <asm/kvm.h>
>> +#include <subcmd/parse-options.h>
>>  
>>  define_exit_reasons_table(vmx_exit_reasons, VMX_EXIT_REASONS);
>>  define_exit_reasons_table(svm_exit_reasons, SVM_EXIT_REASONS);
>> @@ -211,3 +213,47 @@ int cpu_isa_init(struct perf_kvm_stat *kvm, const char *cpuid)
>>  
>>  	return 0;
>>  }
>> +
>> +/*
>> + * 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.
>> + *
>> + * So, to avoid this issue explicitly use "cycles" instead of "cycles:P" event
>> + * by default to sample guest on Intel platforms.
>> + */
>> +int kvm_add_default_arch_event(int *argc, const char **argv)
>> +{
>> +	const char **tmp;
>> +	bool event = false;
>> +	int i, j = *argc;
>> +
>> +	const struct option event_options[] = {
>> +		OPT_BOOLEAN('e', "event", &event, NULL),
>> +		OPT_END()
>> +	};
>> +
>> +	if (!x86__is_intel_cpu())
>> +		return 0;
>> +
>> +	tmp = calloc(j + 1, sizeof(char *));
>> +	if (!tmp)
>> +		return -EINVAL;
>> +
>> +	for (i = 0; i < j; i++)
>> +		tmp[i] = argv[i];
>> +
>> +	parse_options(j, tmp, event_options, NULL, PARSE_OPT_KEEP_UNKNOWN);
>> +	if (!event) {
>> +		argv[j++] = strdup("-e");
>> +		argv[j++] = strdup("cycles");
>> +		*argc += 2;
>> +	}
>> +
>> +	free(tmp);
>> +	return 0;
>> +}
>> -- 
>> 2.34.1
>>