Re: [PATCH v5 0/2] Enable legacy LBR support for guest

[Like Xu <like.xu.linux@gmail.com>]

Hi Weijiang,

On 23/1/2022 12:11 am, Yang Weijiang wrote:
> KVM legacy LBR patches have been merged in kernel 5.12, this patchset
> is to expose the feature to guest from the perf capability MSR. Qemu can
> add LBR format in cpu option to achieve it, e.g., -cpu host,lbr-fmt=0x5,

Some older Intel CPUs may have lbr-fmt=LBR_FORMAT_32 (which is 0), would
you help verify that KVM is supported on these platforms ? If so, how do we enable
guest LBR form the QEMU side, w/ -cpu host,lbr-fmt=0x0 ?

> the format should match host value in IA32_PERF_CAPABILITIES.
> 
> Note, KVM legacy LBR solution accelerates guest perf performace by LBR MSR
> passthrough so it requires guest cpu model matches that of host's, i.e.,

Would you help add live migration support across host/guest CPU models when
hosts at both ends have the same number of LBR entries and the same lbr-fmt ?

Thanks,
Like Xu

> only -cpu host is supported.
> 
> Change in v5:
> 	1. This patchset is rebased on tip : 6621441db5
> 	2. No functional change since v4.