Re: [PATCH v3 10/10] target/i386/kvm: don't stop Intel PMU counters

[Zhao Liu <zhao1.liu@intel.com>]

On Sun, Mar 30, 2025 at 06:32:29PM -0700, Dongli Zhang wrote:
> Date: Sun, 30 Mar 2025 18:32:29 -0700
> From: Dongli Zhang <dongli.zhang@oracle.com>
> Subject: [PATCH v3 10/10] target/i386/kvm: don't stop Intel PMU counters
> X-Mailer: git-send-email 2.43.5
> 
> The kvm_put_msrs() sets the MSRs using KVM_SET_MSRS. The x86 KVM processes
> these MSRs one by one in a loop, only saving the config and triggering the
> KVM_REQ_PMU request. This approach does not immediately stop the event
> before updating PMC.

This is ture after KVM's 68fb4757e867 (v6.2). QEMU even supports v4.5
(docs/system/target-i386.rst)... I'm not sure whether it is outdated,
but it's better to mention the Linux version.

> In additional, PMU MSRs are set only at levels >= KVM_PUT_RESET_STATE,
> excluding runtime. Therefore, updating these MSRs without stopping events
> should be acceptable.

I agree.

> Finally, KVM creates kernel perf events with host mode excluded
> (exclude_host = 1). While the events remain active, they don't increment
> the counter during QEMU vCPU userspace mode.
> 
> No Fixed tag is going to be added for the commit 0d89436786b0 ("kvm:
> migrate vPMU state"), because this isn't a bugfix.
> 
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
>  target/i386/kvm/kvm.c | 9 ---------
>  1 file changed, 9 deletions(-)

Fine for me,

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>