Re: [PATCH 1/2] KVM: SVM: don't check have_run_cpus in sev_writeback_caches()

[Yury Norov <yury.norov@gmail.com>]

On Mon, Aug 11, 2025 at 02:21:44PM -0700, Sean Christopherson wrote:
> On Mon, Aug 11, 2025, Yury Norov wrote:
> > On Mon, Aug 11, 2025 at 01:50:15PM -0700, Sean Christopherson wrote:
> > > On Mon, Aug 11, 2025, Yury Norov wrote:
> > > > From: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> > > > 
> > > > Before calling wbnoinvd_on_cpus_mask(), the function checks the cpumask
> > > > for emptiness. It's useless, as the following wbnoinvd_on_cpus_mask()
> > > > ends up with smp_call_function_many_cond(), which handles empty cpumask
> > > > correctly.
> > > 
> > > I don't agree that it's useless.  The early check avoids disabling/enabling
> > > preemption (which is cheap, but still), and IMO it makes the KVM code more obviously
> > > correct.  E.g. it takes quite a bit of digging to understand that invoking
> > > wbnoinvd_on_cpus_mask() with an empty mask is ok/fine.
> > > 
> > > I'm not completely opposed to this change, but I also don't see the point.
> > 
> > So, there's a tradeoff between useless preemption cycling, which is
> > O(1) and cpumask_empty(), which is O(N).
> 
> Oh, that argument I buy.  I had it in my head that the check is going to be O(1)
> in practice, because never running vCPU0 would be all kinds of bizarre.  But the
> mask tracks physical CPUs, not virtual CPUs.  E.g. a 2-vCPU VM that's pinned to
> the last 2 pCPUs in the system could indeed trigger several superfluous loads and
> checks.
> 
> > I have no measurements that can support one vs another. But the
> > original patch doesn't discuss it anyhow, as well. So, with the
> > lack of any information on performance impact, I'd stick with the 
> > version that brings less code.
> > 
> > Agree?
> 
> Not sure I agree that less code is always better, but I do agree that dropping
> the check makes sense.  :-)
> 
> How about this?  No need for a v2 (unless you disagree on the tweaks), I'll happily
> fixup when applying.

Sure deal! Thanks.

> --
> From: Yury Norov <yury.norov@gmail.com>
> Date: Mon, 11 Aug 2025 16:30:39 -0400
> Subject: [PATCH] KVM: SEV: don't check have_run_cpus in sev_writeback_caches()
> 
> Drop KVM's check on an empty cpumask when flushing caches when memory is
> being reclaimed from an SEV VM, as smp_call_function_many_cond() naturally
> (and correctly) handles and empty cpumask.  This avoids an extra O(n)
> lookup in the common case where at least one pCPU has enterred the guest,
> which could be noticeable in some setups, e.g. if a small VM is pinned to
> the last few pCPUs in the system.
> 
> Fixes: 6f38f8c57464 ("KVM: SVM: Flush cache only on CPUs running SEV guest")
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
> [sean: rewrite changelog to capture performance angle]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/svm/sev.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 2fbdebf79fbb..0635bd71c10e 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -718,13 +718,6 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
>  
>  static void sev_writeback_caches(struct kvm *kvm)
>  {
> -	/*
> -	 * Note, the caller is responsible for ensuring correctness if the mask
> -	 * can be modified, e.g. if a CPU could be doing VMRUN.
> -	 */
> -	if (cpumask_empty(to_kvm_sev_info(kvm)->have_run_cpus))
> -		return;
> -
>  	/*
>  	 * Ensure that all dirty guest tagged cache entries are written back
>  	 * before releasing the pages back to the system for use.  CLFLUSH will
> @@ -739,6 +732,9 @@ static void sev_writeback_caches(struct kvm *kvm)
>  	 * serializing multiple calls and having responding CPUs (to the IPI)
>  	 * mark themselves as still running if they are running (or about to
>  	 * run) a vCPU for the VM.
> +	 *
> +	 * Note, the caller is responsible for ensuring correctness if the mask
> +	 * can be modified, e.g. if a CPU could be doing VMRUN.
>  	 */
>  	wbnoinvd_on_cpus_mask(to_kvm_sev_info(kvm)->have_run_cpus);
>  }
> 
> base-commit: 8f5ae30d69d7543eee0d70083daf4de8fe15d585
> --