linux-kernel - Re: [PATCH 1/2] KVM: SVM: don't check have_run_cpus in sev_writeback

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Message-ID: <aJpXh3dQNZpmUlHL@google.com>
Date: Mon, 11 Aug 2025 13:50:15 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Zheyun Shen <szy0127@...u.edu.cn>
Subject: Re: [PATCH 1/2] KVM: SVM: don't check have_run_cpus in sev_writeback_caches()

On Mon, Aug 11, 2025, Yury Norov wrote:
> From: Yury Norov (NVIDIA) <yury.norov@...il.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.

> While there, move function-wide comment on top of the function.
> 
> Fixes: 6f38f8c57464 ("KVM: SVM: Flush cache only on CPUs running SEV guest")
> Signed-off-by: Yury Norov (NVIDIA) <yury.norov@...il.com>
> ---
>  arch/x86/kvm/svm/sev.c | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 2fbdebf79fbb..49d7557de8bc 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -716,15 +716,12 @@ static void sev_clflush_pages(struct page *pages[], unsigned long npages)
>  	}
>  }
>  
> +/*
> + * The caller is responsible for ensuring correctness if the mask
> + * can be modified, e.g. if a CPU could be doing VMRUN.
> + */
>  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
> -- 
> 2.43.0
>