diff mbox series

[v12,17/20] KVM: xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()

Message ID 20240115125707.1183-18-paul@xen.org
State New
Headers
Series KVM: xen: update shared_info and vcpu_info handling |

Commit Message

Paul Durrant Jan. 15, 2024, 12:57 p.m. UTC 
  From: Paul Durrant <pdurrant@amazon.com>

As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
There is only actually blocking with PREEMPT_RT because the locks will
turned into mutexes. There is no 'raw' version of rwlock_t that can be used
to avoid that, so use read_trylock() and treat failure to lock the same as
an invalid cache.

[1] https://lore.kernel.org/lkml/99771ef3a4966a01fefd3adbb2ba9c3a75f97cf2.camel@infradead.org/T/#mbd06e5a04534ce9c0ee94bd8f1e8d942b2d45bd6

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: x86@kernel.org

v11:
 - Amended the commit comment.

v10:
 - New in this version.
---
 arch/x86/kvm/xen.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)

Comments

Sean Christopherson Feb. 7, 2024, 4:17 a.m. UTC | #1 
On Mon, Jan 15, 2024, Paul Durrant wrote:
> From: Paul Durrant <pdurrant@amazon.com>
> 
> As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
> kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
> There is only actually blocking with PREEMPT_RT because the locks will
> turned into mutexes. There is no 'raw' version of rwlock_t that can be used
> to avoid that, so use read_trylock() and treat failure to lock the same as
> an invalid cache.

Are rwlocks fundamentally incapable of supporting a raw version?  Because that's
the only argument I see for adding a hack like this.
David Woodhouse Feb. 7, 2024, 4:21 a.m. UTC | #2 
On Tue, 2024-02-06 at 20:17 -0800, Sean Christopherson wrote:
> On Mon, Jan 15, 2024, Paul Durrant wrote:
> > From: Paul Durrant <pdurrant@amazon.com>
> > 
> > As described in [1] compiling with CONFIG_PROVE_RAW_LOCK_NESTING shows that
> > kvm_xen_set_evtchn_fast() is blocking on pfncache locks in IRQ context.
> > There is only actually blocking with PREEMPT_RT because the locks will
> > turned into mutexes. There is no 'raw' version of rwlock_t that can be used
> > to avoid that, so use read_trylock() and treat failure to lock the same as
> > an invalid cache.
> 
> Are rwlocks fundamentally incapable of supporting a raw version?  Because that's
> the only argument I see for adding a hack like this.

I don't know about "fundamentally incapable", but there's no point in
adding them just for this, because the write lock is very rarely going
to be held, so it's not an issue to be falling back to the slow path.
And when the write lock *was* held, something often changed so we may
well be going back to the slow path anyway.
diff mbox series

Patch

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 5ce02699f44c..9168a6ec88fd 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1673,10 +1673,13 @@  static int set_shinfo_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
 	unsigned long flags;
 	int rc = -EWOULDBLOCK;
 
-	read_lock_irqsave(&gpc->lock, flags);
-	if (!kvm_gpc_check(gpc, PAGE_SIZE))
+	local_irq_save(flags);
+	if (!read_trylock(&gpc->lock))
 		goto out;
 
+	if (!kvm_gpc_check(gpc, PAGE_SIZE))
+		goto out_unlock;
+
 	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
 		struct shared_info *shinfo = gpc->khva;
 
@@ -1698,8 +1701,10 @@  static int set_shinfo_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
 		rc = 1; /* It is newly raised */
 	}
 
+ out_unlock:
+	read_unlock(&gpc->lock);
  out:
-	read_unlock_irqrestore(&gpc->lock, flags);
+	local_irq_restore(flags);
 	return rc;
 }
 
@@ -1709,21 +1714,23 @@  static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
 	struct gfn_to_pfn_cache *gpc = &vcpu->arch.xen.vcpu_info_cache;
 	unsigned long flags;
 	bool kick_vcpu = false;
+	bool locked;
 
-	read_lock_irqsave(&gpc->lock, flags);
+	local_irq_save(flags);
+	locked = read_trylock(&gpc->lock);
 
 	/*
 	 * Try to deliver the event directly to the vcpu_info. If successful and
 	 * the guest is using upcall_vector delivery, send the MSI.
-	 * If the pfncache is invalid, set the shadow. In this case, or if the
-	 * guest is using another form of event delivery, the vCPU must be
-	 * kicked to complete the delivery.
+	 * If the pfncache lock is contended or the cache is invalid, set the
+	 * shadow. In this case, or if the guest is using another form of event
+	 * delivery, the vCPU must be kicked to complete the delivery.
 	 */
 	if (IS_ENABLED(CONFIG_64BIT) && kvm->arch.xen.long_mode) {
 		struct vcpu_info *vcpu_info = gpc->khva;
 		int port_word_bit = port / 64;
 
-		if (!kvm_gpc_check(gpc, sizeof(*vcpu_info))) {
+		if ((!locked || !kvm_gpc_check(gpc, sizeof(*vcpu_info)))) {
 			if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
 				kick_vcpu = true;
 			goto out;
@@ -1737,7 +1744,7 @@  static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
 		struct compat_vcpu_info *vcpu_info = gpc->khva;
 		int port_word_bit = port / 32;
 
-		if (!kvm_gpc_check(gpc, sizeof(*vcpu_info))) {
+		if ((!locked || !kvm_gpc_check(gpc, sizeof(*vcpu_info)))) {
 			if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
 				kick_vcpu = true;
 			goto out;
@@ -1756,7 +1763,10 @@  static bool set_vcpu_info_evtchn_pending(struct kvm_vcpu *vcpu, u32 port)
 	}
 
  out:
-	read_unlock_irqrestore(&gpc->lock, flags);
+	if (locked)
+		read_unlock(&gpc->lock);
+
+	local_irq_restore(flags);
 	return kick_vcpu;
 }