Re: [PATCH v2] KVM: x86: Rate-limit global clock updates on vCPU load

[Sean Christopherson <seanjc@google.com>]

On Wed, May 06, 2026, Thorsten Leemhuis wrote:
> On 4/9/26 21:21, Sean Christopherson wrote:
> > On Thu, Apr 09, 2026, Lei Chen wrote:
> >> commit 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
> >> dropped the rate limiting for KVM_REQ_GLOBAL_CLOCK_UPDATE.
> >>
> >> As a result, kvm_arch_vcpu_load() can queue global clock update requests
> >> every time a vCPU is scheduled when the master clock is disabled or when
> >> the vCPU is loaded for the first time.
> >>
> >> Restore the throttling with a per-VM ratelimit state and gate
> >> KVM_REQ_GLOBAL_CLOCK_UPDATE through __ratelimit(), so frequent vCPU
> >> scheduling does not generate a steady stream of redundant clock update
> >> requests.
> >>
> >> Fixes: 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
> >> Signed-off-by: Lei Chen <lei.chen@smartx.com>
> >> Reported-by: Jaroslav Pulchart <jaroslav.pulchart@gooddata.com>
> >> Closes: https://lore.kernel.org/all/CAK8fFZ5gY8_Mw2A=iZVFNVKQNrXQzVsn-HTd+Me9K6ZfmdgA+Q@mail.gmail.com/
> 
> Was this performance regression ever addressed?

Nope, not yet.

> Looks like this fall through the cracks, but it's easy to miss something.

It's in my list of patches to apply (probably for 7.2?).  I didn't want to squeeze
it into the initial 7.1 pull request for a variety of reasons.