[PATCH] KVM:SVM: Flush cache only on CPUs running SEV guest

Zheyun Shen
2024-03-01T02:39:17Z

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@sjtu.edu.cn>
---
arch/x86/include/asm/smp.h | 1 +
arch/x86/kvm/svm/sev.c | 28 ++++++++++++++++++++++++----
arch/x86/kvm/svm/svm.c | 4 ++++
arch/x86/kvm/svm/svm.h | 3 +++
arch/x86/lib/cache-smp.c | 7 +++++++
5 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index 4fab2ed45..19297202b 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -120,6 +120,7 @@ void native_play_dead(void);
void play_dead_common(void);
void wbinvd_on_cpu(int cpu);
int wbinvd_on_all_cpus(void);
+int wbinvd_on_cpus(struct cpumask *cpumask);

void smp_kick_mwait_play_dead(void);

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f760106c3..b6ed9a878 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -215,6 +215,21 @@ static void sev_asid_free(struct kvm_sev_info *sev)
sev->misc_cg = NULL;
}

+struct cpumask *sev_get_cpumask(struct kvm *kvm)
+{
+ struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
+
+ return &sev->cpumask;
+}
+
+void sev_clear_cpumask(struct kvm *kvm)
+{
+ struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
+
+ cpumask_clear(&sev->cpumask);
+}
+
+
static void sev_decommission(unsigned int handle)
{
struct sev_data_decommission decommission;
@@ -255,6 +270,7 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
if (unlikely(sev->active))
return ret;

+ cpumask_clear(&sev->cpumask);
sev->active = true;
sev->es_active = argp->id == KVM_SEV_ES_INIT;
asid = sev_asid_new(sev);
@@ -2048,7 +2064,8 @@ 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();
+ wbinvd_on_cpus(sev_get_cpumask(kvm));
+ sev_clear_cpumask(kvm);

__unregister_enc_region_locked(kvm, region);

@@ -2152,7 +2169,8 @@ 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();
+ wbinvd_on_cpus(sev_get_cpumask(kvm));
+ sev_clear_cpumask(kvm);

/*
* if userspace was terminated before unregistering the memory regions
@@ -2343,7 +2361,8 @@ static void sev_flush_encrypted_page(struct kvm_vcpu *vcpu, void *va)
return;

do_wbinvd:
- wbinvd_on_all_cpus();
+ wbinvd_on_cpus(sev_get_cpumask(vcpu->kvm));
+ sev_clear_cpumask(vcpu->kvm);
}

void sev_guest_memory_reclaimed(struct kvm *kvm)
@@ -2351,7 +2370,8 @@ void sev_guest_memory_reclaimed(struct kvm *kvm)
if (!sev_guest(kvm))
return;

- wbinvd_on_all_cpus();
+ wbinvd_on_cpus(sev_get_cpumask(kvm));
+ sev_clear_cpumask(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 e90b429c8..f9bfa6e57 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4107,6 +4107,10 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in

amd_clear_divider();

+ if (sev_guest(vcpu->kvm))
+ cpumask_set_cpu(smp_processor_id(), sev_get_cpumask(vcpu->kvm));
+
if (sev_es_guest(vcpu->kvm))
__svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted);
else
diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
index 8ef95139c..1577e200e 100644
--- a/arch/x86/kvm/svm/svm.h
+++ b/arch/x86/kvm/svm/svm.h
@@ -90,6 +90,7 @@ 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;
+ struct cpumask cpumask; /* CPU list to flush */
};

struct kvm_svm {
@@ -694,6 +695,8 @@ void sev_es_vcpu_reset(struct vcpu_svm *svm);
void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector);
void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa);
void sev_es_unmap_ghcb(struct vcpu_svm *svm);
+struct cpumask *sev_get_cpumask(struct kvm *kvm);
+void sev_clear_cpumask(struct kvm *kvm);

/* vmenter.S */

diff --git a/arch/x86/lib/cache-smp.c b/arch/x86/lib/cache-smp.c
index 7af743bd3..8806f53ba 100644
--- a/arch/x86/lib/cache-smp.c
+++ b/arch/x86/lib/cache-smp.c
@@ -20,3 +20,10 @@ int wbinvd_on_all_cpus(void)
return 0;
}
EXPORT_SYMBOL(wbinvd_on_all_cpus);
+
+int wbinvd_on_cpus(struct cpumask *mask)
+{
+ on_each_cpu_cond_mask(NULL, __wbinvd, NULL, 1, mask);
+ return 0;
+}
+EXPORT_SYMBOL(wbinvd_on_cpus);
--
2.34.1