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

[Yang, Weijiang]

On 2/9/2022 5:14 PM, Like Xu wrote:
> 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 ?

Hi, Like, do you know which cpu model or platform so that I can have a 
test on?

> > 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 ?
Yes, I'm working on this part for Arch LBR, then enable it for legacy 
LBR as well.
> 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.