linux-kernel - Re: [PATCH v7 3/3] KVM: SVM: Flush cache only on CPUs running SEV guest

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Message-ID: <Z75se_OZQvaeQE-4@google.com>
Date: Tue, 25 Feb 2025 17:20:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Zheyun Shen <szy0127@...u.edu.cn>
Cc: thomas.lendacky@....com, pbonzini@...hat.com, tglx@...utronix.de, 
	kevinloughlin@...gle.com, mingo@...hat.com, bp@...en8.de, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/3] KVM: SVM: Flush cache only on CPUs running SEV guest

On Tue, Jan 28, 2025, Zheyun Shen wrote:
> 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.
> 
> Signed-off-by: Zheyun Shen <szy0127@...u.edu.cn>
> ---
>  arch/x86/kvm/svm/sev.c | 30 +++++++++++++++++++++++++++---
>  arch/x86/kvm/svm/svm.c |  2 ++
>  arch/x86/kvm/svm/svm.h |  5 ++++-
>  3 files changed, 33 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 1ce67de9d..4b80ecbe7 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -252,6 +252,27 @@ static void sev_asid_free(struct kvm_sev_info *sev)
>  	sev->misc_cg = NULL;
>  }
>  
> +void sev_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

And now I'm very confused.

v1 and v2 marked the CPU dirty in pre_sev_run(), which AFAICT is exactly when a
CPU should be recorded as having dirtied memory.  v3 fixed a bug with using
get_cpu(), but otherwise was unchanged.  Tom even gave a Tested-by for v3.

Then v4 comes along, and without explanation, moved the code to vcpu_load().

  Changed the time of recording the CPUs from pre_sev_run() to vcpu_load().

Why?  If there's a good reason, then that absolutely, positively belongs in the
changelog and in the code as a comment.  If there's no good reason, then...

Unless I hear otherwise, my plan is to move this back to pre_sev_run().

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

A comment is definitely warranted, but I don't think we should mark it TODO.  I'm
not convinced the benefits justify the complexity, and I don't want someone trying
to "fix" the code because it has a TODO.

> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 43fa6a16e..82ec80cf4 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -112,6 +112,8 @@ struct kvm_sev_info {
>  	void *guest_req_buf;    /* Bounce buffer for SNP Guest Request input */
>  	void *guest_resp_buf;   /* Bounce buffer for SNP Guest Request output */
>  	struct mutex guest_req_mutex; /* Must acquire before using bounce buffers */
> +	/* CPUs invoked VMRUN call wbinvd after guest memory is reclaimed */
> +	struct cpumask *wbinvd_dirty_mask;

This needs to be cpumask_var_t, as the cpumask APIs expect the mask to be
statically allocated when CONFIG_CPUMASK_OFFSTACK=n.  E.g. this will hit a NULL
pointer deref.

  static __always_inline bool zalloc_cpumask_var(cpumask_var_t *mask, gfp_t flags)
  {
	cpumask_clear(*mask);
	return true;
  }

The wbinvd_dirty_mask name also turns out to be less than good.  In part because
of the looming WBNOINVD change, but also because it kinda sorta collides with
wbinvd_dirty_mask in kvm_vcpu_arch, which gets really confusing when trying to
read the code.

I don't have any great ideas, the best I came up with was have_run_cpus.