[PATCH v13 18/21] KVM: x86/xen: don't block on pfncache locks in kvm_xen_set_evtchn_fast()

Paul Durrant
2024-02-15T15:44:33Z

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

v13:
- Patch title change.

v11:
- Amended the commit comment.

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

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 59073642c078..8650141b266e 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1678,10 +1678,13 @@ 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;

@@ -1703,8 +1706,10 @@ 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;
}

@@ -1714,21 +1719,23 @@ 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;
@@ -1742,7 +1749,7 @@ 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;
@@ -1761,7 +1768,10 @@ 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;
}

--
2.39.2