linux-kernel - Re: [PATCH v5 3/3] KVM: SVM: Flush cache only on CPUs running SEV guest

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Message-ID: <85frlcvjyo.fsf@amd.com>
Date: Tue, 21 Jan 2025 05:01:35 +0000
From: Nikunj A Dadhania <nikunj@....com>
To: Zheyun Shen <szy0127@...u.edu.cn>, <thomas.lendacky@....com>,
	<seanjc@...gle.com>, <pbonzini@...hat.com>, <tglx@...utronix.de>,
	<kevinloughlin@...gle.com>, <mingo@...hat.com>, <bp@...en8.de>
CC: <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>, Zheyun Shen
	<szy0127@...u.edu.cn>
Subject: Re: [PATCH v5 3/3] KVM: SVM: Flush cache only on CPUs running SEV
 guest

Zheyun Shen <szy0127@...u.edu.cn> writes:

> On AMD CPUs without ensuring cache consistency, each memory page
> reclamation in an SEV guest triggers a call to wbinvd_on_all_cpus(),
> thereby affecting the performance of other programs on the host.
>
> Typically, an AMD server may have 128 cores or more, while the SEV guest
> might only utilize 8 of these cores. Meanwhile, host can use qemu-affinity
> to bind these 8 vCPUs to specific physical CPUs.
>
> Therefore, keeping a record of the physical core numbers each time a vCPU
> runs can help avoid flushing the cache for all CPUs every time.
>
> Suggested-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Zheyun Shen <szy0127@...u.edu.cn>
> ---
>  arch/x86/kvm/svm/sev.c | 39 ++++++++++++++++++++++++++++++++++++---
>  arch/x86/kvm/svm/svm.c |  2 ++
>  arch/x86/kvm/svm/svm.h |  5 ++++-
>  3 files changed, 42 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 1ce67de9d..91469edd1 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -252,6 +252,36 @@ static void sev_asid_free(struct kvm_sev_info *sev)
>  	sev->misc_cg = NULL;
>  }
>  
> +static struct cpumask *sev_get_wbinvd_dirty_mask(struct kvm *kvm)
> +{
> +	struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;

There is a helper to get sev_info: to_kvm_sev_info(), if you use that,
sev_get_wbinvd_dirty_mask() helper will not be needed.

> +
> +	return sev->wbinvd_dirty_mask;
> +}
> +
> +void sev_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> +{
> +	/*
> +	 * To optimize cache flushes when memory is reclaimed from an SEV VM,
> +	 * track physical CPUs that enter the guest for SEV VMs and thus can
> +	 * have encrypted, dirty data in the cache, and flush caches only for
> +	 * CPUs that have entered the guest.
> +	 */
> +	cpumask_set_cpu(cpu, sev_get_wbinvd_dirty_mask(vcpu->kvm));
> +}
> +
> +static void sev_do_wbinvd(struct kvm *kvm)
> +{
> +	struct cpumask *dirty_mask = sev_get_wbinvd_dirty_mask(kvm);
> +
> +	/*
> +	 * TODO: Clear CPUs from the bitmap prior to flushing.  Doing so
> +	 * requires serializing multiple calls and having CPUs mark themselves
> +	 * "dirty" if they are currently running a vCPU for the VM.
> +	 */
> +	wbinvd_on_many_cpus(dirty_mask);
> +}

Something like the below

void sev_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
{
        /* ... */
        cpumask_set_cpu(cpu, to_kvm_sev_info(kvm)->wbinvd_dirty_mask);
}

static void sev_do_wbinvd(struct kvm *kvm)
{
        /* ... */
        wbinvd_on_many_cpus(to_kvm_sev_info(kvm)->wbinvd_dirty_mask);
}

Regards,
Nikunj