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Message-ID: <fce6d203-6822-7bb6-ca9b-4427d0772312@amd.com>
Date: Thu, 6 Feb 2025 16:05:25 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Zheyun Shen <szy0127@...u.edu.cn>, 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
Subject: Re: [PATCH v7 3/3] KVM: SVM: Flush cache only on CPUs running SEV
guest
On 1/27/25 19:53, 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>
Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> ---
> 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)
> +{
> + /*
> + * 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.
> + */
> + wbinvd_on_many_cpus(to_kvm_sev_info(kvm)->wbinvd_dirty_mask);
> +}
> +
> static void sev_decommission(unsigned int handle)
> {
> struct sev_data_decommission decommission;
> @@ -448,6 +469,8 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
> ret = sev_platform_init(&init_args);
> if (ret)
> goto e_free;
> + if (!zalloc_cpumask_var(&sev->wbinvd_dirty_mask, GFP_KERNEL_ACCOUNT))
> + goto e_free;
>
> /* This needs to happen after SEV/SNP firmware initialization. */
> if (vm_type == KVM_X86_SNP_VM) {
> @@ -2778,7 +2801,7 @@ 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);
>
> @@ -2926,6 +2949,7 @@ void sev_vm_destroy(struct kvm *kvm)
> }
>
> sev_asid_free(sev);
> + free_cpumask_var(sev->wbinvd_dirty_mask);
> }
>
> void __init sev_set_cpu_caps(void)
> @@ -3129,7 +3153,7 @@ 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)
> @@ -3143,7 +3167,7 @@ void sev_guest_memory_reclaimed(struct kvm *kvm)
> if (!sev_guest(kvm) || sev_snp_guest(kvm))
> return;
>
> - wbinvd_on_all_cpus();
> + sev_do_wbinvd(kvm);
> }
>
> void sev_free_vcpu(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index dd15cc635..f3b03b0d8 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1565,6 +1565,8 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> }
> if (kvm_vcpu_apicv_active(vcpu))
> avic_vcpu_load(vcpu, cpu);
> + if (sev_guest(vcpu->kvm))
> + sev_vcpu_load(vcpu, cpu);
> }
>
> static void svm_vcpu_put(struct kvm_vcpu *vcpu)
> 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;
> };
>
> struct kvm_svm {
> @@ -763,6 +765,7 @@ void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu);
> int sev_gmem_prepare(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
> void sev_gmem_invalidate(kvm_pfn_t start, kvm_pfn_t end);
> int sev_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn);
> +void sev_vcpu_load(struct kvm_vcpu *vcpu, int cpu);
> #else
> static inline struct page *snp_safe_alloc_page_node(int node, gfp_t gfp)
> {
> @@ -793,7 +796,7 @@ static inline int sev_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
> {
> return 0;
> }
> -
> +static inline void sev_vcpu_load(struct kvm_vcpu *vcpu, int cpu) {}
> #endif
>
> /* vmenter.S */
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