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

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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




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