Re: [PATCH] KVM: x86: switch hugepage recovery thread to vhost_task

Keith Busch
2025-01-15T19:03:30Z

On Wed, Jan 15, 2025 at 06:10:05PM +0100, Paolo Bonzini wrote:
> You can implement something like pthread_once():

...

> Where to put it I don't know. It doesn't belong in
> include/linux/once.h. I'm okay with arch/x86/kvm/call_once.h and just
> pull it with #include "call_once.h".

Thanks for the suggestion, I can work with that. As to where to put it,
I think the new 'struct once' needs to be a member of struct kvm_arch,
so I've put it in arch/x86/include/asm/.

Here's the result with that folded in. If this is okay, I'll send a v2,
and can split out the call_once as a prep patch with your attribution if
you like.

---
diff --git a/arch/x86/include/asm/kvm_call_once.h b/arch/x86/include/asm/kvm_call_once.h
new file mode 100644
index 0000000000000..451cc87084aa7
--- /dev/null
+++ b/arch/x86/include/asm/kvm_call_once.h
@@ -0,0 +1,44 @@
+#ifndef _LINUX_CALL_ONCE_H
+#define _LINUX_CALL_ONCE_H
+
+#include <linux/types.h>
+
+#define ONCE_NOT_STARTED 0
+#define ONCE_RUNNING 1
+#define ONCE_COMPLETED 2
+
+struct once {
+ atomic_t state;
+ struct mutex lock;
+};
+
+static inline void __once_init(struct once *once, const char *name,
+ struct lock_class_key *key)
+{
+ atomic_set(&once->state, ONCE_NOT_STARTED);
+ __mutex_init(&once->lock, name, key);
+}
+
+#define once_init(once) \
+do { \
+ static struct lock_class_key __key; \
+ __once_init((once), #once, &__key); \
+} while (0)
+
+static inline void call_once(struct once *once, void (*cb)(struct once *))
+{
+ /* Pairs with atomic_set_release() below. */
+ if (atomic_read_acquire(&once->state) == ONCE_COMPLETED)
+ return;
+
+ guard(mutex)(&once->lock);
+ WARN_ON(atomic_read(&once->state) == ONCE_RUNNING);
+ if (atomic_read(&once->state) != ONCE_NOT_STARTED)
+ return;
+
+ atomic_set(&once->state, ONCE_RUNNING);
+ cb(once);
+ atomic_set_release(&once->state, ONCE_COMPLETED);
+}
+
+#endif /* _LINUX_CALL_ONCE_H */
diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index e159e44a6a1b6..56c79958dc9cb 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -37,6 +37,7 @@
#include <asm/kvm_page_track.h>
#include <asm/kvm_vcpu_regs.h>
#include <asm/hyperv-tlfs.h>
+#include <asm/kvm_call_once.h>
#include <asm/reboot.h>

#define __KVM_HAVE_ARCH_VCPU_DEBUGFS
@@ -1445,6 +1446,7 @@ struct kvm_arch {
struct kvm_x86_pmu_event_filter __rcu *pmu_event_filter;
struct vhost_task *nx_huge_page_recovery_thread;
u64 nx_huge_page_last;
+ struct once nx_once;

#ifdef CONFIG_X86_64
/* The number of TDP MMU pages across all roots. */
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 2401606db2604..3c0c1f3647ceb 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -7411,20 +7411,28 @@ static bool kvm_nx_huge_page_recovery_worker(void *data)
return true;
}

-int kvm_mmu_post_init_vm(struct kvm *kvm)
+static void kvm_mmu_start_lpage_recovery(struct once *once)
{
- if (nx_hugepage_mitigation_hard_disabled)
- return 0;
+ struct kvm_arch *ka = container_of(once, struct kvm_arch, nx_once);
+ struct kvm *kvm = container_of(ka, struct kvm, arch);

kvm->arch.nx_huge_page_last = get_jiffies_64();
kvm->arch.nx_huge_page_recovery_thread = vhost_task_create(
kvm_nx_huge_page_recovery_worker, kvm_nx_huge_page_recovery_worker_kill,
kvm, "kvm-nx-lpage-recovery");

+ if (kvm->arch.nx_huge_page_recovery_thread)
+ vhost_task_start(kvm->arch.nx_huge_page_recovery_thread);
+}
+
+int kvm_mmu_post_init_vm(struct kvm *kvm)
+{
+ if (nx_hugepage_mitigation_hard_disabled)
+ return 0;
+
+ call_once(&kvm->arch.nx_once, kvm_mmu_start_lpage_recovery);
if (!kvm->arch.nx_huge_page_recovery_thread)
return -ENOMEM;
-
- vhost_task_start(kvm->arch.nx_huge_page_recovery_thread);
return 0;
}

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c79a8cc57ba42..23bf088fc4ae1 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -11463,6 +11463,10 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
struct kvm_run *kvm_run = vcpu->run;
int r;

+ r = kvm_mmu_post_init_vm(vcpu->kvm);
+ if (r)
+ return r;
+
vcpu_load(vcpu);
kvm_sigset_activate(vcpu);
kvm_run->flags = 0;
@@ -12742,7 +12746,8 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)

int kvm_arch_post_init_vm(struct kvm *kvm)
{
- return kvm_mmu_post_init_vm(kvm);
+ once_init(&kvm->arch.nx_once);
+ return 0;
}

static void kvm_unload_vcpu_mmu(struct kvm_vcpu *vcpu)
--