Re: [PATCH v14 00/11] KVM: x86/pmu: Guest Last Branch Recording Enabling

Like Xu
2021-07-30T03:28:28Z

On 30/7/2021 11:15 am, Liuxiangdong wrote:
> Hi, like.
>
> Does it have requirement on CPU if we want to use LBR in Guest?

As long as you find valid output from the "dmesg| grep -i LBR" like
"XX-deep LBR",
you can use LBR on the host and theoretically on the most Intel guest.

But I don't have various Intel machine types for testing.

As far as I know, the guest LBR doesn't work on the platforms that
the MSR_LBR_SELECT is defined per physical core not logical core.

I will fix this issue by making KVM aware of the recent core scheduling
policy.

>
> I have tried linux-5.14-rc3 on different CPUs. And I can use lbr on
> Haswell, Broadwell, skylake and icelake, but I cannot use lbr on IvyBridge.

I suppose INTEL_FAM6_IVYBRIDGE and INTEL_FAM6_IVYBRIDGE_X do support LBR.

You may check the return values from x86_perf_get_lbr() or
cpuid_model_is_consistent() in the KVM for more details.

>
> Thanks!
>
>
> On 2021/7/29 20:40, Liuxiangdong wrote:
>> Hi, like.
>>
>> This patch set has been merged in 5.12 kernel tree so we can use LBR
>> in Guest.
>> Does it have requirement on CPU?
>> I can use lbr in guest on skylake and icelake, but cannot on IvyBridge.
>>
>> I can see lbr formats(000011b) in perf_capabilities msr(0x345), but
>> there is still
>> error when I try.
>>
>> $ perf record -b
>> Error:
>> cycles: PMU Hardware doesn't support sampling/overflow-interrupts. Try
>> 'perf stat'
>>
>> Host CPU:
>> Architecture:                    x86_64
>> CPU op-mode(s):                  32-bit, 64-bit
>> Byte Order:                      Little Endian
>> Address sizes:                   46 bits physical, 48 bits virtual
>> CPU(s):                          24
>> On-line CPU(s) list:             0-23
>> Thread(s) per core:              2
>> Core(s) per socket:              6
>> Socket(s):                       2
>> NUMA node(s):                    2
>> Vendor ID:                       GenuineIntel
>> CPU family:                      6
>> Model:                           62
>> Model name:                      Intel(R) Xeon(R) CPU E5-2620 v2 @
>> 2.10GHz
>> Stepping:                        4
>>
>>
>> Thanks!
>> Xiangdong Liu
>