linux-kernel - Re: [PATCH v2] KVM:SVM: Flush cache only on CPUs running SEV guest

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Date: Tue, 12 Mar 2024 09:45:40 -0500
From: Tom Lendacky <thomas.lendacky@....com>
To: Zheyun Shen <szy0127@...u.edu.cn>, Sean Christopherson
 <seanjc@...gle.com>, pbonzini <pbonzini@...hat.com>,
 tglx <tglx@...utronix.de>
Cc: kvm <kvm@...r.kernel.org>, linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] KVM:SVM: Flush cache only on CPUs running SEV guest

On 3/6/24 20:14, 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.
> 
> Since the usage of sev_flush_asids() isn't tied to a single VM, we just
> replace all wbinvd_on_all_cpus() with sev_do_wbinvd() except for that
> in sev_flush_asids().
> 
> Signed-off-by: Zheyun Shen <szy0127@...u.edu.cn>

I'm unable to launch my SEV or SEV-ES guests with this patch (haven't 
tried an SEV-SNP guest, yet). Qemu segfaults at launch.

I'll try to dig into what is happening, but not sure when I'll be able to 
do that at the moment.

Thanks,
Tom

> ---
>   arch/x86/kvm/svm/sev.c | 27 +++++++++++++++++++++++----
>   arch/x86/kvm/svm/svm.h |  3 +++
>   2 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index f760106c3..743931e33 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -215,6 +215,24 @@ 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;
> +
> +	return &sev->wbinvd_dirty_mask;
> +}
> +
> +static void sev_do_wbinvd(struct kvm *kvm)
> +{
> +	int cpu;
> +	struct cpumask *dirty_mask = sev_get_wbinvd_dirty_mask(kvm);
> +
> +	for_each_possible_cpu(cpu) {
> +		if (cpumask_test_and_clear_cpu(cpu, dirty_mask))
> +			wbinvd_on_cpu(cpu);
> +	}
> +}
> +
>   static void sev_decommission(unsigned int handle)
>   {
>   	struct sev_data_decommission decommission;
> @@ -2048,7 +2066,7 @@ int sev_mem_enc_unregister_region(struct kvm *kvm,
>   	 * releasing the pages back to the system for use. CLFLUSH will
>   	 * not do this, so issue a WBINVD.
>   	 */
> -	wbinvd_on_all_cpus();
> +	sev_do_wbinvd(kvm);
>   
>   	__unregister_enc_region_locked(kvm, region);
>   
> @@ -2152,7 +2170,7 @@ void sev_vm_destroy(struct kvm *kvm)
>   	 * releasing the pages back to the system for use. CLFLUSH will
>   	 * not do this, so issue a WBINVD.
>   	 */
> -	wbinvd_on_all_cpus();
> +	sev_do_wbinvd(kvm);
>   
>   	/*
>   	 * if userspace was terminated before unregistering the memory regions
> @@ -2343,7 +2361,7 @@ static void sev_flush_encrypted_page(struct kvm_vcpu *vcpu, void *va)
>   	return;
>   
>   do_wbinvd:
> -	wbinvd_on_all_cpus();
> +	sev_do_wbinvd(vcpu->kvm);
>   }
>   
>   void sev_guest_memory_reclaimed(struct kvm *kvm)
> @@ -2351,7 +2369,7 @@ void sev_guest_memory_reclaimed(struct kvm *kvm)
>   	if (!sev_guest(kvm))
>   		return;
>   
> -	wbinvd_on_all_cpus();
> +	sev_do_wbinvd(kvm);
>   }
>   
>   void sev_free_vcpu(struct kvm_vcpu *vcpu)
> @@ -2648,6 +2666,7 @@ void pre_sev_run(struct vcpu_svm *svm, int cpu)
>   	sd->sev_vmcbs[asid] = svm->vmcb;
>   	svm->vmcb->control.tlb_ctl = TLB_CONTROL_FLUSH_ASID;
>   	vmcb_mark_dirty(svm->vmcb, VMCB_ASID);
> +	cpumask_set_cpu(get_cpu(), sev_get_wbinvd_dirty_mask(svm->vcpu.kvm));
>   }
>   
>   #define GHCB_SCRATCH_AREA_LIMIT		(16ULL * PAGE_SIZE)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 8ef95139c..de240a9e9 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -90,6 +90,9 @@ struct kvm_sev_info {
>   	struct list_head mirror_entry; /* Use as a list entry of mirrors */
>   	struct misc_cg *misc_cg; /* For misc cgroup accounting */
>   	atomic_t migration_in_progress;
> +
> +	/* CPUs invoked VMRUN should do wbinvd after guest memory is reclaimed */
> +	struct cpumask wbinvd_dirty_mask;
>   };
>   
>   struct kvm_svm {