[PATCH 00/10] KVM: x86/xen: Don't drop event deliveries which race memslot updates

[PATCH 00/10] KVM: x86/xen: Don't drop event deliveries which race memslot updates

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

Sashiko analysis and torture testing of the upcoming conversion of the
gfn_to_pfn_cache from rwlock to atomic SRCU¹ highlighted a pre-existing
problem: refreshing a cache can fail while a memslot is briefly marked
KVM_MEMSLOT_INVALID during a memslot update. The Xen event channel and
timer delivery paths silently dropped events when they hit that window.

Return -EAGAIN from __kvm_gpc_refresh() to distinguish the transient
case from a genuinely unbacked GFN, and add a retry loop for the common
case callers which can just wait for the memslot update to complete, as
well as dealing with the corner cases which can't:

 - The Xen attribute ioctls wait, so userspace never sees the
   transient state.

 - Timer delivery leaves ->arch.xen.timer_pending set and retries from
   kvm_xen_inject_timer_irqs() on the next vCPU entry.

 - The guest's own EVTCHNOP_send hypercall runs under vcpu_run's
   kvm->srcu read section — the very reader the memslot update is
   waiting for — so it cannot wait. Leave the hypercall uncompleted
   without advancing RIP, so the guest re-executes it. This is how
   Xen itself restarts preempted hypercalls.

Patches 6-9 are pulled forward from the SRCU series because they stand
alone:

 - Three from Sean², extracting vcpu_info delivery into a separate
   helper and taking the shared_info and vcpu_info pages out of dirty
   tracking. Rebased: his series had first converted the GPC locks to
   read_trylock(), which this one doesn't carry.

 - Requesting the guest TLB flush from record_steal_time() itself, as
   a standalone cleanup in preparation for converting that to GPC.

The final patch adds a selftest racing a guest EVTCHNOP_send loop
against a tight memslot delete/recreate loop. Tested with that and
xen_shinfo_test; an instrumented run showed 10 of its 4000 sends
hitting the update window and re-executing.

Based on kvm/next (d4b7fb647204).

https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/share/gpc-eagain

¹ https://git.infradead.org/?p=users/dwmw2/linux.git;a=shortlog;h=refs/heads/gpc-srcu-v2
² https://lore.kernel.org/kvm/20260529165114.748639-1-seanjc@google.com/

David Woodhouse (7):
  KVM: pfncache: Return -EAGAIN for a lookup which hits an invalid memslot
  KVM: pfncache: Add a 'wait' argument to the refresh and activate functions
  KVM: x86/xen: Wait out memslot updates in the Xen attribute ioctls
  KVM: x86/xen: Don't lose the timer event when delivery fails transiently
  KVM: x86/xen: Retry event channel delivery which races a memslot update
  KVM: x86: Request the guest TLB flush from record_steal_time()
  KVM: selftests: Add Xen evtchn vs memslot-update race test

Sean Christopherson (3):
  KVM: x86/xen: Extract delivery of event to vCPU into a separate helper
  KVM: x86/xen: Explicitly tag "shared info" page as never being dirty tracked
  KVM: x86/xen: Don't dirty track "vCPU info" page

 arch/x86/kvm/irq.c                            |   2 +-
 arch/x86/kvm/msrs.c                           |   2 +-
 arch/x86/kvm/x86.c                            |  28 +-
 arch/x86/kvm/xen.c                            | 312 +++++++++++-------
 arch/x86/kvm/xen.h                            |   4 +-
 include/linux/kvm_host.h                      |  30 +-
 include/linux/kvm_types.h                     |   1 +
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../kvm/x86/xen_evtchn_memslot_race_test.c    | 178 ++++++++++
 .../selftests/kvm/x86/xen_shinfo_test.c       |  11 +-
 virt/kvm/pfncache.c                           | 136 ++++++--
 11 files changed, 532 insertions(+), 173 deletions(-)
 create mode 100644 tools/testing/selftests/kvm/x86/xen_evtchn_memslot_race_test.c

base-commit: d4b7fb647204f0c81dfeae2d1a708e4d858e0c94
-- 
2.43.0

[PATCH 01/10] KVM: pfncache: Return -EAGAIN for a lookup which hits an invalid memslot

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

A memslot being deleted or moved is present in the active array but
flagged KVM_MEMSLOT_INVALID — a transient state which exists only
within a single KVM_SET_USER_MEMORY_REGION call and is announced by
a further generation bump when it resolves. gfn_to_hva_memslot()
collapses that case and a genuinely unbacked gfn into one error, and
the pfncache refresh reported both as -EFAULT.

Distinguish them: return -EAGAIN when the slot exists but is
invalid, mirroring what RET_PF_RETRY does for ordinary guest faults
on an invalid slot. Existing users don't *need* to change; most
will simply resolve again at the next refresh. But the distinct
error code *allows* a caller to retry.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 virt/kvm/pfncache.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 3659686b97c2..17aaebd76db8 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -304,7 +304,22 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
 			gpc->uhva = gfn_to_hva_memslot(gpc->memslot, gfn);
 
 			if (kvm_is_error_hva(gpc->uhva)) {
-				ret = -EFAULT;
+				/*
+				 * A slot being deleted or moved is present
+				 * but flagged invalid; the lookup failure is
+				 * transient, resolved when the update
+				 * completes (or reverts) and announced by a
+				 * new memslot generation. Distinguish that
+				 * from a genuinely unbacked gfn so that the
+				 * caller may retry rather than report an
+				 * error, as RET_PF_RETRY does for ordinary
+				 * guest faults.
+				 */
+				if (gpc->memslot &&
+				    (gpc->memslot->flags & KVM_MEMSLOT_INVALID))
+					ret = -EAGAIN;
+				else
+					ret = -EFAULT;
 				goto out;
 			}
 
-- 
2.43.0

[PATCH 02/10] KVM: pfncache: Add a 'wait' argument to the refresh and activate functions

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

A caller which does not hold kvm->srcu may now pass wait=true to
kvm_gpc_refresh() or kvm_gpc_activate() to wait out a concurrent
memslot update instead of receiving -EAGAIN, looping with
cond_resched() until the lookup resolves or a fatal signal is
pending. (An HVA-based cache never consults the memslots, so
kvm_gpc_activate_hva() keeps its signature.)

The refresh now takes its own kvm->srcu read lock for each attempt, so
callers no longer need to hold it. Holding it with wait=true would
deadlock — the wait would block the memslot update from completing —
and is rejected with a lockdep assertion.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/msrs.c      |   2 +-
 arch/x86/kvm/x86.c       |   2 +-
 arch/x86/kvm/xen.c       |  28 +++++-----
 include/linux/kvm_host.h |  17 ++++--
 virt/kvm/pfncache.c      | 114 ++++++++++++++++++++++++++++-----------
 5 files changed, 112 insertions(+), 51 deletions(-)

diff --git a/arch/x86/kvm/msrs.c b/arch/x86/kvm/msrs.c
index dd3bb04878ca..4c0fbba6e7ae 100644
--- a/arch/x86/kvm/msrs.c
+++ b/arch/x86/kvm/msrs.c
@@ -1280,7 +1280,7 @@ static void kvm_write_system_time(struct kvm_vcpu *vcpu, gpa_t system_time,
 	/* we verify if the enable bit is set... */
 	if (system_time & 1)
 		kvm_gpc_activate(&vcpu->arch.pv_time, system_time & ~1ULL,
-				 sizeof(struct pvclock_vcpu_time_info));
+				 sizeof(struct pvclock_vcpu_time_info), false);
 	else
 		kvm_gpc_deactivate(&vcpu->arch.pv_time);
 
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 79468ddfe473..eadcaeee651a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1727,7 +1727,7 @@ static void kvm_setup_guest_pvclock(struct pvclock_vcpu_time_info *ref_hv_clock,
 	while (!kvm_gpc_check(gpc, offset + sizeof(*guest_hv_clock))) {
 		read_unlock_irqrestore(&gpc->lock, flags);
 
-		if (kvm_gpc_refresh(gpc, offset + sizeof(*guest_hv_clock)))
+		if (kvm_gpc_refresh(gpc, offset + sizeof(*guest_hv_clock), false))
 			return;
 
 		read_lock_irqsave(&gpc->lock, flags);
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6aa00f746cba..fae893cd5762 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -50,7 +50,7 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
 	while (!kvm_gpc_check(gpc, PAGE_SIZE)) {
 		read_unlock_irq(&gpc->lock);
 
-		ret = kvm_gpc_refresh(gpc, PAGE_SIZE);
+		ret = kvm_gpc_refresh(gpc, PAGE_SIZE, false);
 		if (ret)
 			goto out;
 
@@ -161,7 +161,7 @@ static int xen_get_guest_pvclock(struct kvm_vcpu *vcpu,
 	while (!kvm_gpc_check(gpc, offset + sizeof(*hv_clock))) {
 		read_unlock_irqrestore(&gpc->lock, flags);
 
-		r = kvm_gpc_refresh(gpc, offset + sizeof(*hv_clock));
+		r = kvm_gpc_refresh(gpc, offset + sizeof(*hv_clock), false);
 		if (r)
 			return r;
 
@@ -435,7 +435,7 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 		if (atomic)
 			return;
 
-		if (kvm_gpc_refresh(gpc1, user_len1))
+		if (kvm_gpc_refresh(gpc1, user_len1, false))
 			return;
 
 		read_lock_irqsave(&gpc1->lock, flags);
@@ -488,7 +488,7 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
 			 * 64-bit mode, the second GPC won't have been set up.
 			 */
 			if (kvm_gpc_activate(gpc2, gpc1->gpa + user_len1,
-					     user_len2))
+					     user_len2, false))
 				return;
 
 			/*
@@ -669,7 +669,7 @@ void kvm_xen_inject_pending_events(struct kvm_vcpu *v)
 	while (!kvm_gpc_check(gpc, sizeof(struct vcpu_info))) {
 		read_unlock_irqrestore(&gpc->lock, flags);
 
-		if (kvm_gpc_refresh(gpc, sizeof(struct vcpu_info)))
+		if (kvm_gpc_refresh(gpc, sizeof(struct vcpu_info), false))
 			return;
 
 		read_lock_irqsave(&gpc->lock, flags);
@@ -757,7 +757,7 @@ int __kvm_xen_has_interrupt(struct kvm_vcpu *v)
 		if (in_atomic() || !task_is_running(current))
 			return 1;
 
-		if (kvm_gpc_refresh(gpc, sizeof(struct vcpu_info))) {
+		if (kvm_gpc_refresh(gpc, sizeof(struct vcpu_info), false)) {
 			/*
 			 * If this failed, userspace has screwed up the
 			 * vcpu_info mapping. No interrupts for you.
@@ -813,7 +813,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 				r = 0;
 			} else {
 				r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
-						     gfn_to_gpa(gfn), PAGE_SIZE);
+						     gfn_to_gpa(gfn), PAGE_SIZE, false);
 			}
 		} else {
 			void __user * hva = u64_to_user_ptr(data->u.shared_info.hva);
@@ -825,7 +825,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 				r = 0;
 			} else {
 				r = kvm_gpc_activate_hva(&kvm->arch.xen.shinfo_cache,
-							 (unsigned long)hva, PAGE_SIZE);
+							 (unsigned long)hva, PAGE_SIZE, false);
 			}
 		}
 
@@ -960,7 +960,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 
 			r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
-					     data->u.gpa, sizeof(struct vcpu_info));
+					     data->u.gpa, sizeof(struct vcpu_info), false);
 		} else {
 			if (data->u.hva == 0) {
 				kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
@@ -973,7 +973,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 
 			r = kvm_gpc_activate_hva(&vcpu->arch.xen.vcpu_info_cache,
-						 data->u.hva, sizeof(struct vcpu_info));
+						 data->u.hva, sizeof(struct vcpu_info), false);
 		}
 
 		if (!r)
@@ -990,7 +990,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 
 		r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_time_info_cache,
 				     data->u.gpa,
-				     sizeof(struct pvclock_vcpu_time_info));
+				     sizeof(struct pvclock_vcpu_time_info), false);
 		if (!r)
 			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 		break;
@@ -1023,7 +1023,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		/* How much fits in the (first) page? */
 		sz1 = PAGE_SIZE - (data->u.gpa & ~PAGE_MASK);
 		r = kvm_gpc_activate(&vcpu->arch.xen.runstate_cache,
-				     data->u.gpa, sz1);
+				     data->u.gpa, sz1, false);
 		if (r)
 			goto deactivate_out;
 
@@ -1034,7 +1034,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 			sz2 = sz - sz1;
 			BUG_ON((data->u.gpa + sz1) & ~PAGE_MASK);
 			r = kvm_gpc_activate(&vcpu->arch.xen.runstate2_cache,
-					     data->u.gpa + sz1, sz2);
+					     data->u.gpa + sz1, sz2, false);
 			if (r)
 				goto deactivate_out;
 		}
@@ -2011,7 +2011,7 @@ static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 			break;
 
 		idx = srcu_read_lock(&kvm->srcu);
-		rc = kvm_gpc_refresh(gpc, PAGE_SIZE);
+		rc = kvm_gpc_refresh(gpc, PAGE_SIZE, false);
 		srcu_read_unlock(&kvm->srcu, idx);
 	} while(!rc);
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3dd04605f2e5..88c1ee37550c 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1474,16 +1474,23 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm);
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @gpa:	   guest physical address to map.
  * @len:	   sanity check; the range being access must fit a single page.
+ * @wait:	   wait out a concurrent memslot update instead of returning
+ *		   -EAGAIN. May not be used while this task holds any kvm->srcu
+ *		   read section (lockdep-asserted): the update's
+ *		   synchronize_srcu() would deadlock against the wait.
  *
  * @return:	   0 for success.
  *		   -EINVAL for a mapping which would cross a page boundary.
  *		   -EFAULT for an untranslatable guest physical address.
+ *		   -EAGAIN if the memslot is being updated (only with
+ *		   wait == false, or on a fatal signal while waiting).
  *
  * This primes a gfn_to_pfn_cache and links it into the @gpc->kvm's list for
  * invalidations to be processed.  Callers are required to use kvm_gpc_check()
  * to ensure that the cache is valid before accessing the target page.
  */
-int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len);
+int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len,
+		     bool wait);
 
 /**
  * kvm_gpc_activate_hva - prepare a cached kernel mapping and HPA for a given HVA.
@@ -1499,7 +1506,8 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
  * The semantics of this function are the same as those of kvm_gpc_activate(). It
  * merely bypasses a layer of address translation.
  */
-int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long hva, unsigned long len);
+int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long hva,
+			 unsigned long len);
 
 /**
  * kvm_gpc_check - check validity of a gfn_to_pfn_cache.
@@ -1524,10 +1532,13 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len);
  *
  * @gpc:	   struct gfn_to_pfn_cache object.
  * @len:	   sanity check; the range being access must fit a single page.
+ * @wait:	   as kvm_gpc_activate().
  *
  * @return:	   0 for success.
  *		   -EINVAL for a mapping which would cross a page boundary.
  *		   -EFAULT for an untranslatable guest physical address.
+ *		   -EAGAIN if the memslot is being updated (only with
+ *		   wait == false, or on a fatal signal while waiting).
  *
  * This will attempt to refresh a gfn_to_pfn_cache. Note that a successful
  * return from this function does not mean the page can be immediately
@@ -1535,7 +1546,7 @@ bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len);
  * still lock and check the cache status, as this function does not return
  * with the lock still held to permit access.
  */
-int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len);
+int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len, bool wait);
 
 /**
  * kvm_gpc_deactivate - deactivate and unlink a gfn_to_pfn_cache.
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 17aaebd76db8..8504e26fa111 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -378,23 +378,57 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned l
 	return ret;
 }
 
-int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len)
+/*
+ * One refresh attempt, under the caller's refresh_lock. Takes a kvm->srcu
+ * read section for the attempt, as the memslot lookups consult the active
+ * memslots.
+ */
+static int gpc_refresh_locked(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
+			      unsigned long uhva)
+{
+	int idx, ret;
+
+	idx = srcu_read_lock(&gpc->kvm->srcu);
+	ret = __kvm_gpc_refresh(gpc, gpa, uhva);
+	srcu_read_unlock(&gpc->kvm->srcu, idx);
+
+	return ret;
+}
+
+int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len, bool wait)
 {
 	unsigned long uhva;
+	int ret;
 
-	guard(mutex)(&gpc->refresh_lock);
+	if (wait)
+		lockdep_assert(!srcu_read_lock_held(&gpc->kvm->srcu));
 
-	if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len))
-		return -EINVAL;
+	for (;;) {
+		scoped_guard(mutex, &gpc->refresh_lock) {
+			if (!kvm_gpc_is_valid_len(gpc->gpa, gpc->uhva, len))
+				return -EINVAL;
 
-	/*
-	 * If the GPA is valid then ignore the HVA, as a cache can be GPA-based
-	 * or HVA-based, not both.  For GPA-based caches, the HVA will be
-	 * recomputed during refresh if necessary.
-	 */
-	uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva : KVM_HVA_ERR_BAD;
+			/*
+			 * If the GPA is valid then ignore the HVA, as a cache
+			 * can be GPA-based or HVA-based, not both.  For
+			 * GPA-based caches, the HVA will be recomputed during
+			 * refresh if necessary.
+			 */
+			uhva = kvm_is_error_gpa(gpc->gpa) ? gpc->uhva : KVM_HVA_ERR_BAD;
+
+			ret = gpc_refresh_locked(gpc, gpc->gpa, uhva);
+		}
+
+		if (ret != -EAGAIN || !wait || fatal_signal_pending(current))
+			return ret;
 
-	return __kvm_gpc_refresh(gpc, gpc->gpa, uhva);
+		/*
+		 * Wait with refresh_lock dropped: a kvm->srcu reader blocked
+		 * on it must be able to proceed and quiesce, or the memslot
+		 * update being waited for could never complete.
+		 */
+		cond_resched();
+	}
 }
 
 void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
@@ -410,36 +444,50 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
 }
 
 static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
-			      unsigned long len)
+			      unsigned long len, bool wait)
 {
 	struct kvm *kvm = gpc->kvm;
 
 	if (!kvm_gpc_is_valid_len(gpa, uhva, len))
 		return -EINVAL;
 
-	guard(mutex)(&gpc->refresh_lock);
+	if (wait)
+		lockdep_assert(!srcu_read_lock_held(&kvm->srcu));
 
-	if (!gpc->active) {
-		if (KVM_BUG_ON(gpc->valid, kvm))
-			return -EIO;
+	for (;;) {
+		int ret;
 
-		spin_lock(&kvm->gpc_lock);
-		list_add(&gpc->list, &kvm->gpc_list);
-		spin_unlock(&kvm->gpc_lock);
+		scoped_guard(mutex, &gpc->refresh_lock) {
+			if (!gpc->active) {
+				if (KVM_BUG_ON(gpc->valid, kvm))
+					return -EIO;
 
-		/*
-		 * Activate the cache after adding it to the list, a concurrent
-		 * refresh must not establish a mapping until the cache is
-		 * reachable by mmu_notifier events.
-		 */
-		write_lock_irq(&gpc->lock);
-		gpc->active = true;
-		write_unlock_irq(&gpc->lock);
+				spin_lock(&kvm->gpc_lock);
+				list_add(&gpc->list, &kvm->gpc_list);
+				spin_unlock(&kvm->gpc_lock);
+
+				/*
+				 * Activate the cache after adding it to the
+				 * list, a concurrent refresh must not
+				 * establish a mapping until the cache is
+				 * reachable by mmu_notifier events.
+				 */
+				write_lock_irq(&gpc->lock);
+				gpc->active = true;
+				write_unlock_irq(&gpc->lock);
+			}
+			ret = gpc_refresh_locked(gpc, gpa, uhva);
+		}
+
+		if (ret != -EAGAIN || !wait || fatal_signal_pending(current))
+			return ret;
+
+		cond_resched();
 	}
-	return __kvm_gpc_refresh(gpc, gpa, uhva);
 }
 
-int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
+int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len,
+		     bool wait)
 {
 	/*
 	 * Explicitly disallow INVALID_GPA so that the magic value can be used
@@ -448,15 +496,17 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
 	if (WARN_ON_ONCE(kvm_is_error_gpa(gpa)))
 		return -EINVAL;
 
-	return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len);
+	return __kvm_gpc_activate(gpc, gpa, KVM_HVA_ERR_BAD, len, wait);
 }
 
-int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
+int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva,
+			 unsigned long len)
 {
 	if (!access_ok((void __user *)uhva, len))
 		return -EINVAL;
 
-	return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
+	/* An HVA-based cache never consults the memslots, so never waits. */
+	return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len, false);
 }
 
 void kvm_gpc_deactivate(struct gfn_to_pfn_cache *gpc)
-- 
2.43.0

[PATCH 03/10] KVM: x86/xen: Wait out memslot updates in the Xen attribute ioctls

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

Activating a shared_info, vcpu_info, pvclock or runstate cache can
fail if it races a memslot update: previously with -EFAULT, now with
-EAGAIN. These ioctls may sleep, so pass wait and let the activation
ride the update out; userspace never sees the transient state.

kvm_xen_shared_info_init() gains the same flag for its refresh:
true from the attribute ioctls, false from the hypercall page MSR
write, which runs in vCPU context under kvm->srcu and keeps its
existing failure behaviour. That one could retry by restarting the
WRMSR, but that is painful to plumb for an error which only matters
to legacy userspace (the non-_HVA shared_info).

The kvm->srcu read sections around the activation calls go away;
they existed to satisfy the old refresh contract, and holding one
across a waiting activation would deadlock against the memslot
update. The runstate updates in kvm_xen_vcpu_set_attr() still read
guest state through the caches, so they keep narrow read sections
of their own.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/xen.c | 40 ++++++++++++++++++----------------------
 1 file changed, 18 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index fae893cd5762..76708b896026 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -36,7 +36,7 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r);
 
 DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_enabled, HZ);
 
-static int kvm_xen_shared_info_init(struct kvm *kvm)
+static int kvm_xen_shared_info_init(struct kvm *kvm, bool wait)
 {
 	struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
 	struct pvclock_wall_clock *wc;
@@ -44,13 +44,12 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
 	u32 wc_version;
 	u64 wall_nsec;
 	int ret = 0;
-	int idx = srcu_read_lock(&kvm->srcu);
 
 	read_lock_irq(&gpc->lock);
 	while (!kvm_gpc_check(gpc, PAGE_SIZE)) {
 		read_unlock_irq(&gpc->lock);
 
-		ret = kvm_gpc_refresh(gpc, PAGE_SIZE, false);
+		ret = kvm_gpc_refresh(gpc, PAGE_SIZE, wait);
 		if (ret)
 			goto out;
 
@@ -100,7 +99,6 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
 	read_unlock_irq(&gpc->lock);
 
 out:
-	srcu_read_unlock(&kvm->srcu, idx);
 	return ret;
 }
 
@@ -792,19 +790,15 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 			 * to make the call anyway.
 			 */
 			r = kvm->arch.xen.shinfo_cache.active ?
-				kvm_xen_shared_info_init(kvm) : 0;
+				kvm_xen_shared_info_init(kvm, true) : 0;
 			mutex_unlock(&kvm->arch.xen.xen_lock);
 		}
 		break;
 
 	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
 	case KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA: {
-		int idx;
-
 		mutex_lock(&kvm->arch.xen.xen_lock);
 
-		idx = srcu_read_lock(&kvm->srcu);
-
 		if (data->type == KVM_XEN_ATTR_TYPE_SHARED_INFO) {
 			gfn_t gfn = data->u.shared_info.gfn;
 
@@ -813,7 +807,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 				r = 0;
 			} else {
 				r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
-						     gfn_to_gpa(gfn), PAGE_SIZE, false);
+						     gfn_to_gpa(gfn), PAGE_SIZE, true);
 			}
 		} else {
 			void __user * hva = u64_to_user_ptr(data->u.shared_info.hva);
@@ -825,14 +819,12 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 				r = 0;
 			} else {
 				r = kvm_gpc_activate_hva(&kvm->arch.xen.shinfo_cache,
-							 (unsigned long)hva, PAGE_SIZE, false);
+							 (unsigned long)hva, PAGE_SIZE);
 			}
 		}
 
-		srcu_read_unlock(&kvm->srcu, idx);
-
 		if (!r && kvm->arch.xen.shinfo_cache.active)
-			r = kvm_xen_shared_info_init(kvm);
+			r = kvm_xen_shared_info_init(kvm, true);
 
 		mutex_unlock(&kvm->arch.xen.xen_lock);
 		break;
@@ -937,7 +929,6 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 	int idx, r = -ENOENT;
 
 	mutex_lock(&vcpu->kvm->arch.xen.xen_lock);
-	idx = srcu_read_lock(&vcpu->kvm->srcu);
 
 	switch (data->type) {
 	case KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO:
@@ -960,7 +951,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 
 			r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
-					     data->u.gpa, sizeof(struct vcpu_info), false);
+					     data->u.gpa, sizeof(struct vcpu_info), true);
 		} else {
 			if (data->u.hva == 0) {
 				kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
@@ -973,7 +964,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 				break;
 
 			r = kvm_gpc_activate_hva(&vcpu->arch.xen.vcpu_info_cache,
-						 data->u.hva, sizeof(struct vcpu_info), false);
+						 data->u.hva, sizeof(struct vcpu_info));
 		}
 
 		if (!r)
@@ -990,7 +981,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 
 		r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_time_info_cache,
 				     data->u.gpa,
-				     sizeof(struct pvclock_vcpu_time_info), false);
+				     sizeof(struct pvclock_vcpu_time_info), true);
 		if (!r)
 			kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
 		break;
@@ -1023,7 +1014,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		/* How much fits in the (first) page? */
 		sz1 = PAGE_SIZE - (data->u.gpa & ~PAGE_MASK);
 		r = kvm_gpc_activate(&vcpu->arch.xen.runstate_cache,
-				     data->u.gpa, sz1, false);
+				     data->u.gpa, sz1, true);
 		if (r)
 			goto deactivate_out;
 
@@ -1034,12 +1025,14 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 			sz2 = sz - sz1;
 			BUG_ON((data->u.gpa + sz1) & ~PAGE_MASK);
 			r = kvm_gpc_activate(&vcpu->arch.xen.runstate2_cache,
-					     data->u.gpa + sz1, sz2, false);
+					     data->u.gpa + sz1, sz2, true);
 			if (r)
 				goto deactivate_out;
 		}
 
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
 		kvm_xen_update_runstate_guest(vcpu, false);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		break;
 	}
 	case KVM_XEN_VCPU_ATTR_TYPE_RUNSTATE_CURRENT:
@@ -1052,7 +1045,9 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 			break;
 		}
 
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
 		kvm_xen_update_runstate(vcpu, data->u.runstate.state);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		r = 0;
 		break;
 
@@ -1132,10 +1127,12 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		vcpu->arch.xen.runstate_times[RUNSTATE_offline] +=
 			data->u.runstate.time_offline;
 
+		idx = srcu_read_lock(&vcpu->kvm->srcu);
 		if (data->u.runstate.state <= RUNSTATE_offline)
 			kvm_xen_update_runstate(vcpu, data->u.runstate.state);
 		else if (vcpu->arch.xen.runstate_cache.active)
 			kvm_xen_update_runstate_guest(vcpu, false);
+		srcu_read_unlock(&vcpu->kvm->srcu, idx);
 		r = 0;
 		break;
 
@@ -1181,7 +1178,6 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		break;
 	}
 
-	srcu_read_unlock(&vcpu->kvm->srcu, idx);
 	mutex_unlock(&vcpu->kvm->arch.xen.xen_lock);
 	return r;
 }
@@ -1329,7 +1325,7 @@ int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 		 * correct place.
 		 */
 		if (kvm->arch.xen.shinfo_cache.active &&
-		    kvm_xen_shared_info_init(kvm))
+		    kvm_xen_shared_info_init(kvm, false))
 			r = 1;
 	}
 	mutex_unlock(&kvm->arch.xen.xen_lock);
-- 
2.43.0

[PATCH 04/10] KVM: x86/xen: Don't lose the timer event when delivery fails transiently

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

kvm_xen_inject_timer_irqs() can drop events if kvm_xen_set_evtchn()
fails transiently, as it can when a memslot is in the process of
being updated. Defer the delivery in that case by leaving
->arch.xen.timer_pending set, so that it gets retried. The same
applies when the shared_info cache has been deactivated (-EINVAL).
If delivery fails with -EFAULT, the shared_info gfn is genuinely
unbacked and can never succeed: drop the event and complete the
timer rather than spin on it.

True Xen does not have this problem: the shared_info page is owned
by Xen and always exists, even when not mapped into the guest. The
equivalent for a KVM/Xen VMM is KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA,
which has no memslot dependency and is the recommended mode; this
fixes the legacy GFN mode, which xen_shinfo_test exercises.

Don't re-arm the hrtimer in the KVM_XEN_VCPU_GET_ATTR(TIMER) flush
while a delivery is still pending; a re-fired callback racing the
retry would deliver twice.

The drop is vanishingly rare upstream, but hangs roughly half of all
xen_shinfo_test runs once the following atomic SRCU conversion
widens the window; hence fixing it first.

Fixes: 536395260582 ("KVM: x86/xen: handle PV timers oneshot mode")
Assisted-by: LLM
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/xen.c                            | 36 ++++++++++++++-----
 .../selftests/kvm/x86/xen_shinfo_test.c       | 11 +++---
 2 files changed, 34 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 76708b896026..aea6572163d0 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -106,13 +106,23 @@ void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
 {
 	if (atomic_read(&vcpu->arch.xen.timer_pending) > 0) {
 		struct kvm_xen_evtchn e;
+		int rc;
 
 		e.vcpu_id = vcpu->vcpu_id;
 		e.vcpu_idx = vcpu->vcpu_idx;
 		e.port = vcpu->arch.xen.timer_virq;
 		e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
 
-		kvm_xen_set_evtchn(&e, vcpu->kvm);
+		rc = kvm_xen_set_evtchn(&e, vcpu->kvm);
+
+		/*
+		 * Deliberately leave timer_pending set if the memslot is
+		 * being updated (-EAGAIN) or the shared_info cache has
+		 * been deactivated (-EINVAL); delivery is retried on the
+		 * next entry pass.
+		 */
+		if (rc == -EAGAIN || rc == -EINVAL)
+			return;
 
 		vcpu->arch.xen.timer_expires = 0;
 		atomic_set(&vcpu->arch.xen.timer_pending, 0);
@@ -1263,12 +1273,13 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 
 	case KVM_XEN_VCPU_ATTR_TYPE_TIMER:
 		/*
-		 * Ensure a consistent snapshot of state is captured, with a
-		 * timer either being pending, or the event channel delivered
-		 * to the corresponding bit in the shared_info. Not still
-		 * lurking in the timer_pending flag for deferred delivery.
-		 * Purely as an optimisation, if the timer_expires field is
-		 * zero, that means the timer isn't active (or even in the
+		 * Ensure a consistent snapshot of state is captured: a timer
+		 * which already fired has either been delivered to the
+		 * corresponding bit in the shared_info, or is reported as
+		 * still owed (expires_ns != 0 with the pending flag set, if
+		 * the shared_info page was transiently unavailable). Purely
+		 * as an optimisation, if the timer_expires field is zero,
+		 * that means the timer isn't active (or even in the
 		 * timer_pending flag) and there is no need to cancel it.
 		 */
 		if (vcpu->arch.xen.timer_expires) {
@@ -1287,8 +1298,17 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		 * That's fine, as the guest won't even have had a chance
 		 * to run and handle the interrupt. Asserting an already
 		 * pending event channel is idempotent.
+		 *
+		 * Don't re-arm if the flush above failed transiently and
+		 * left the timer pending: delivery is owned by the retry
+		 * on the guest entry path, and an immediate hrtimer fire
+		 * would race it — the callback's timer_pending suppression
+		 * check is not serialized against the entry path, so a
+		 * late callback could re-assert a port the entry path had
+		 * already delivered and the guest already consumed.
 		 */
-		if (vcpu->arch.xen.timer_expires)
+		if (vcpu->arch.xen.timer_expires &&
+		    !atomic_read(&vcpu->arch.xen.timer_pending))
 			hrtimer_start_expires(&vcpu->arch.xen.timer,
 					      HRTIMER_MODE_ABS_HARD);
 
diff --git a/tools/testing/selftests/kvm/x86/xen_shinfo_test.c b/tools/testing/selftests/kvm/x86/xen_shinfo_test.c
index aaa414c677ad..3b11d0ee253f 100644
--- a/tools/testing/selftests/kvm/x86/xen_shinfo_test.c
+++ b/tools/testing/selftests/kvm/x86/xen_shinfo_test.c
@@ -356,11 +356,12 @@ static void guest_code(void)
 wait_for_timer:
 	/*
 	 * Poll for a timer wake event while the worker thread is mucking with
-	 * the shared info.  KVM XEN drops timer IRQs if the shared info is
-	 * invalid when the timer expires.  Arbitrarily poll 100 times before
-	 * giving up and asking the VMM to re-arm the timer.  100 polls should
-	 * consume enough time to beat on KVM without taking too long if the
-	 * timer IRQ is dropped due to an invalid event channel.
+	 * the shared info.  KVM is now supposed to retry a timer IRQ whose
+	 * delivery hit an invalid shared info rather than drop it, but older
+	 * kernels drop it on the floor.  Arbitrarily poll 100 times before
+	 * giving up and asking the VMM to re-arm the timer, so the test still
+	 * completes (and documents the VMM-side recovery protocol) on
+	 * unfixed kernels.
 	 */
 	for (i = 0; i < 100 && !guest_saw_irq; i++)
 		__xen_hypercall(__HYPERVISOR_sched_op, SCHEDOP_poll, &p);
-- 
2.43.0

[PATCH 05/10] KVM: x86/xen: Retry event channel delivery which races a memslot update

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

Event channel delivery now returns -EAGAIN when it races a memslot
update. It always failed; now the failure is distinguishable, and
instead of dropping the event each caller reacts according to its
context:

 - kvm_xen_hvm_evtchn_send(): the KVM_XEN_HVM_EVTCHN_SEND ioctl.
   May sleep: passes wait == true, and kvm_gpc_refresh() rides the
   update out.

 - evtchn_set_fn(): irqfd routing, from the irqfd workqueue after
   the atomic fast path failed. May also wait.

 - kvm_xen_hcall_evtchn_send(): the guest's own EVTCHNOP_send, in
   vcpu_run's exit handling under the vCPU's kvm->srcu read section
   — the very reader the memslot update is waiting out, so waiting
   there deadlocks, and any guest can trigger it. Instead, leave the
   hypercall uncompleted, with RIP not advanced, and the guest
   re-executes it; each attempt cycles the srcu read section on the
   way back in, letting the update complete. This is how Xen itself
   restarts preempted hypercalls, and the pending delivery is pure
   guest state, so there is nothing extra to migrate.

 - kvm_xen_inject_timer_irqs(): the timer retry added by the
   previous commit — timer_pending *is* its pending state, so it
   keeps that and retries on the next entry pass. (Its other caller,
   the KVM_XEN_VCPU_GET_ATTR timer flush, must not wait under
   xen_lock — a vCPU writing the hypercall page MSR blocks on that
   lock inside its kvm->srcu section — and doesn't need to: an
   undelivered event is reported as still owed in the snapshot.)

Reported-by: sashiko-bot@kernel.org
Closes: https://lore.kernel.org/all/20260920215341.5ECE91F000FF@smtp.kernel.org
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/irq.c |  2 +-
 arch/x86/kvm/xen.c | 65 ++++++++++++++++++++++++++++++----------------
 arch/x86/kvm/xen.h |  4 +--
 3 files changed, 46 insertions(+), 25 deletions(-)

diff --git a/arch/x86/kvm/irq.c b/arch/x86/kvm/irq.c
index 871977288272..4e986eb4c3c3 100644
--- a/arch/x86/kvm/irq.c
+++ b/arch/x86/kvm/irq.c
@@ -171,7 +171,7 @@ void kvm_inject_pending_timer_irqs(struct kvm_vcpu *vcpu)
 	if (lapic_in_kernel(vcpu))
 		kvm_inject_apic_timer_irqs(vcpu);
 	if (kvm_xen_timer_enabled(vcpu))
-		kvm_xen_inject_timer_irqs(vcpu);
+		kvm_xen_inject_timer_irqs(vcpu, false);
 }
 
 void __kvm_migrate_timers(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index aea6572163d0..ad7708be6c04 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -30,9 +30,10 @@
 #include "cpuid.h"
 #include "trace.h"
 
-static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm);
+static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm,
+			      bool wait);
 static int kvm_xen_setattr_evtchn(struct kvm *kvm, struct kvm_xen_hvm_attr *data);
-static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r);
+static int kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r);
 
 DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_enabled, HZ);
 
@@ -102,7 +103,7 @@ static int kvm_xen_shared_info_init(struct kvm *kvm, bool wait)
 	return ret;
 }
 
-void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
+void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu, bool wait)
 {
 	if (atomic_read(&vcpu->arch.xen.timer_pending) > 0) {
 		struct kvm_xen_evtchn e;
@@ -113,7 +114,7 @@ void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
 		e.port = vcpu->arch.xen.timer_virq;
 		e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
 
-		rc = kvm_xen_set_evtchn(&e, vcpu->kvm);
+		rc = kvm_xen_set_evtchn(&e, vcpu->kvm, wait);
 
 		/*
 		 * Deliberately leave timer_pending set if the memslot is
@@ -661,10 +662,11 @@ void kvm_xen_inject_vcpu_vector(struct kvm_vcpu *v)
  */
 void kvm_xen_inject_pending_events(struct kvm_vcpu *v)
 {
-	unsigned long evtchn_pending_sel = READ_ONCE(v->arch.xen.evtchn_pending_sel);
+	unsigned long evtchn_pending_sel;
 	struct gfn_to_pfn_cache *gpc = &v->arch.xen.vcpu_info_cache;
 	unsigned long flags;
 
+	evtchn_pending_sel = READ_ONCE(v->arch.xen.evtchn_pending_sel);
 	if (!evtchn_pending_sel)
 		return;
 
@@ -1284,7 +1286,7 @@ int kvm_xen_vcpu_get_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
 		 */
 		if (vcpu->arch.xen.timer_expires) {
 			hrtimer_cancel(&vcpu->arch.xen.timer);
-			kvm_xen_inject_timer_irqs(vcpu);
+			kvm_xen_inject_timer_irqs(vcpu, false);
 		}
 
 		data->u.timer.port = vcpu->arch.xen.timer_virq;
@@ -1791,8 +1793,21 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 		}
 		break;
 	case __HYPERVISOR_event_channel_op:
-		if (params[0] == EVTCHNOP_send)
-			handled = kvm_xen_hcall_evtchn_send(vcpu, params[1], &r);
+		if (params[0] == EVTCHNOP_send) {
+			int ret = kvm_xen_hcall_evtchn_send(vcpu, params[1], &r);
+
+			/*
+			 * Delivery raced a memslot update; waiting for it
+			 * here would deadlock under vcpu_run's kvm->srcu
+			 * read section. Leave the hypercall uncompleted —
+			 * RIP not advanced — so the guest re-executes it.
+			 * Each attempt cycles the srcu read section on the
+			 * way back in, letting the update complete.
+			 */
+			if (ret == -EAGAIN)
+				return 1;
+			handled = !ret;
+		}
 		break;
 	case __HYPERVISOR_sched_op:
 		handled = kvm_xen_hcall_sched_op(vcpu, is_64bit, params[0],
@@ -1983,7 +1998,8 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 	return rc;
 }
 
-static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
+static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm,
+			      bool wait)
 {
 	bool mm_borrowed = false;
 	int rc;
@@ -2020,16 +2036,13 @@ static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 	 */
 	do {
 		struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
-		int idx;
 
 		rc = kvm_xen_set_evtchn_fast(xe, kvm);
 		if (rc != -EWOULDBLOCK)
 			break;
 
-		idx = srcu_read_lock(&kvm->srcu);
-		rc = kvm_gpc_refresh(gpc, PAGE_SIZE, false);
-		srcu_read_unlock(&kvm->srcu, idx);
-	} while(!rc);
+		rc = kvm_gpc_refresh(gpc, PAGE_SIZE, wait);
+	} while (!rc);
 
 	if (mm_borrowed)
 		kthread_unuse_mm(kvm->mm);
@@ -2044,7 +2057,7 @@ static int evtchn_set_fn(struct kvm_kernel_irq_routing_entry *e, struct kvm *kvm
 	if (!level)
 		return -EINVAL;
 
-	return kvm_xen_set_evtchn(&e->xen_evtchn, kvm);
+	return kvm_xen_set_evtchn(&e->xen_evtchn, kvm, true);
 }
 
 /*
@@ -2118,7 +2131,7 @@ int kvm_xen_hvm_evtchn_send(struct kvm *kvm, struct kvm_irq_routing_xen_evtchn *
 	e.vcpu_idx = -1;
 	e.priority = uxe->priority;
 
-	ret = kvm_xen_set_evtchn(&e, kvm);
+	ret = kvm_xen_set_evtchn(&e, kvm, true);
 
 	/*
 	 * None of that 'return 1 if it actually got delivered' nonsense.
@@ -2347,7 +2360,11 @@ static int kvm_xen_setattr_evtchn(struct kvm *kvm, struct kvm_xen_hvm_attr *data
 	return kvm_xen_eventfd_assign(kvm, data);
 }
 
-static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
+/*
+ * Returns 0 with *r set if handled, -EAGAIN if the hypercall should be
+ * re-executed by the guest, or -ENOENT to hand it to userspace.
+ */
+static int kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
 {
 	struct evtchnfd *evtchnfd;
 	struct evtchn_send send;
@@ -2357,7 +2374,7 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
 	BUILD_BUG_ON(sizeof(send) != 4);
 	if (kvm_read_guest_virt(vcpu, param, &send, sizeof(send), &e)) {
 		*r = -EFAULT;
-		return true;
+		return 0;
 	}
 
 	/*
@@ -2366,18 +2383,22 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
 	 */
 	evtchnfd = xa_load(&vcpu->kvm->arch.xen.evtchn_ports, send.port);
 	if (!evtchnfd)
-		return false;
+		return -ENOENT;
 
 	if (evtchnfd->deliver.port.port) {
-		int ret = kvm_xen_set_evtchn(&evtchnfd->deliver.port, vcpu->kvm);
+		int ret = kvm_xen_set_evtchn(&evtchnfd->deliver.port,
+					     vcpu->kvm, false);
+
+		if (ret == -EAGAIN)
+			return ret;
 		if (ret < 0 && ret != -ENOTCONN)
-			return false;
+			return -ENOENT;
 	} else {
 		eventfd_signal(evtchnfd->deliver.eventfd.ctx);
 	}
 
 	*r = 0;
-	return true;
+	return 0;
 }
 
 void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu)
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index 9d04e350bdb1..f6202510bbd6 100644
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -100,7 +100,7 @@ static inline int kvm_xen_has_pending_timer(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu);
+void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu, bool wait);
 #else
 static inline int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 {
@@ -161,7 +161,7 @@ static inline int kvm_xen_has_pending_timer(struct kvm_vcpu *vcpu)
 	return 0;
 }
 
-static inline void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
+static inline void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu, bool wait)
 {
 }
 
-- 
2.43.0

[PATCH 06/10] KVM: x86/xen: Extract delivery of event to vCPU into a separate helper

[David Woodhouse]

From: Sean Christopherson <seanjc@google.com>

Move the marking of an event as pending in the target vCPU's vcpu_info
into a separate helper, __kvm_xen_set_evtchn_fast(), invoked after the
shared_info processing is complete and its RCU read-side critical
section has been exited. This makes both halves easier to read.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
[dwmw2: rebased onto the -EAGAIN series. Sean's series first
 converted the GPC locks to read_trylock(), which this series does
 not carry, so the helper takes the plain read lock and the
 original's trylock failure path does not apply. The caller's
 kvm->srcu section extends across the helper call, since
 kvm_gpc_check() on the vcpu_info cache consults the memslot
 generation and the irqfd path enters holding only irq_srcu; in
 Sean's series that was covered by a guard(srcu) spanning the whole
 function.]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/xen.c | 158 ++++++++++++++++++++++++++++-----------------
 1 file changed, 97 insertions(+), 61 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index ad7708be6c04..a931458f0c71 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1862,6 +1862,89 @@ static void kvm_xen_check_poller(struct kvm_vcpu *vcpu, int port)
 	}
 }
 
+static void __kvm_xen_set_evtchn_fast(struct kvm_vcpu *vcpu, int port_word_bit,
+				      bool has_64bit_shinfo)
+{
+	struct gfn_to_pfn_cache *gpc = &vcpu->arch.xen.vcpu_info_cache;
+	unsigned long vi_pending_sel_ofs;
+	bool inject_upcall = false;
+	bool kick_vcpu = false;
+	unsigned long flags;
+	bool old;
+
+	vi_pending_sel_ofs = has_64bit_shinfo ?
+		offsetof(struct vcpu_info, evtchn_pending_sel) :
+		offsetof(struct compat_vcpu_info, evtchn_pending_sel);
+
+	/* Now switch to the vCPU's vcpu_info to set the index and pending_sel */
+	read_lock_irqsave(&gpc->lock, flags);
+	if (!kvm_gpc_check(gpc, sizeof(struct vcpu_info))) {
+		/*
+		 * Could not access the vcpu_info. Set the bit in-kernel and
+		 * prod the vCPU to deliver it for itself. Note that an MSI is
+		 * *not* an acceptable substitute here even if the vCPU has an
+		 * upcall vector: the pending bit only exists in
+		 * evtchn_pending_sel so far, and the vCPU has to be forced
+		 * out of the guest so that kvm_xen_inject_pending_events()
+		 * can refresh the cache and propagate it to the guest.
+		 */
+		if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
+			kick_vcpu = true;
+		goto out_unlock;
+	}
+	inject_upcall = true;
+
+	/*
+	 * Explicitly use a 32-bit btsl instead of test_and_set_bit(), which
+	 * would use btsq on x86-64. The vcpu_info is guest-controlled and only
+	 * required to be 32-bit aligned, so a 64-bit access could generate a
+	 * split-lock #AC.
+	 *
+	 * Note, this does not apply to the test_and_set_bit() on pending_bits
+	 * in the caller: that is in the per-VM shared_info, which is page
+	 * aligned, so the access is guaranteed to be 64-bit aligned.
+	 */
+	old = GEN_BINARY_RMWcc(LOCK_PREFIX "btsl",
+			       *(u32 *)(gpc->khva + vi_pending_sel_ofs),
+			       c, "Ir", port_word_bit);
+	if (!old) {
+		struct vcpu_info *vi = gpc->khva;
+
+		/* No need for compat handling */
+		BUILD_BUG_ON(offsetof(struct vcpu_info, evtchn_upcall_pending) !=
+			     offsetof(struct compat_vcpu_info, evtchn_upcall_pending));
+
+		WRITE_ONCE(vi->evtchn_upcall_pending, 1);
+		kick_vcpu = true;
+	}
+
+out_unlock:
+	read_unlock_irqrestore(&gpc->lock, flags);
+
+	/*
+	 * Deliver the upcall or kick the vCPU only after dropping the GPC
+	 * read lock. Both paths end up in kvm_vcpu_kick(), and the MSI
+	 * delivery also walks the APIC map and takes APIC locks; none of
+	 * that wants to be nested inside the GPC read lock, which a
+	 * concurrent invalidation may be waiting to take for write.
+	 */
+	if (!kick_vcpu)
+		return;
+
+	/*
+	 * For the per-vCPU lapic vector, deliver it as MSI — but only if the
+	 * vcpu_info was actually updated above. If it wasn't, the vCPU must
+	 * be kicked instead (see above).
+	 */
+	if (inject_upcall && vcpu->arch.xen.upcall_vector) {
+		kvm_xen_inject_vcpu_vector(vcpu);
+		return;
+	}
+
+	kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
+	kvm_vcpu_kick(vcpu);
+}
+
 /*
  * The return value from this function is propagated to kvm_set_irq() API,
  * so it returns:
@@ -1876,11 +1959,10 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 {
 	struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
 	bool has_64bit_shinfo = kvm_xen_has_64bit_shinfo(kvm);
-	unsigned long *pending_bits, *mask_bits, vi_pending_sel_ofs;
+	unsigned long *pending_bits, *mask_bits;
 	struct kvm_vcpu *vcpu;
 	unsigned long flags;
 	int port_word_bit;
-	bool kick_vcpu = false;
 	int vcpu_idx, idx, rc;
 
 	vcpu_idx = READ_ONCE(xe->vcpu_idx);
@@ -1902,23 +1984,19 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 
 	read_lock_irqsave(&gpc->lock, flags);
 	if (!kvm_gpc_check(gpc, PAGE_SIZE))
-		goto out_rcu;
+		goto out_unlock;
 
 	if (has_64bit_shinfo) {
 		struct shared_info *shinfo = gpc->khva;
 		pending_bits = (unsigned long *)&shinfo->evtchn_pending;
 		mask_bits = (unsigned long *)&shinfo->evtchn_mask;
 		port_word_bit = xe->port / 64;
-
-		vi_pending_sel_ofs = offsetof(struct vcpu_info, evtchn_pending_sel);
 	} else {
 		struct compat_shared_info *shinfo = gpc->khva;
 		pending_bits = (unsigned long *)&shinfo->evtchn_pending;
 		mask_bits = (unsigned long *)&shinfo->evtchn_mask;
 		port_word_bit = xe->port / 32;
 
-		vi_pending_sel_ofs = offsetof(struct compat_vcpu_info, evtchn_pending_sel);
-
 		/* test_and_set_bit() needs 64-bit alignment, but that's OK */
 		BUILD_BUG_ON(offsetof(struct compat_shared_info, evtchn_pending) & 7);
 	}
@@ -1934,66 +2012,24 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		rc = 0; /* It was already raised */
 	} else if (test_bit(xe->port, mask_bits)) {
 		rc = -ENOTCONN; /* Masked */
-		kvm_xen_check_poller(vcpu, xe->port);
 	} else {
-		bool old;
-
 		rc = 1; /* Delivered to the bitmap in shared_info. */
-		/* Now switch to the vCPU's vcpu_info to set the index and pending_sel */
-		read_unlock_irqrestore(&gpc->lock, flags);
-		gpc = &vcpu->arch.xen.vcpu_info_cache;
-
-		read_lock_irqsave(&gpc->lock, flags);
-		if (!kvm_gpc_check(gpc, sizeof(struct vcpu_info))) {
-			/*
-			 * Could not access the vcpu_info. Set the bit in-kernel
-			 * and prod the vCPU to deliver it for itself.
-			 */
-			if (!test_and_set_bit(port_word_bit, &vcpu->arch.xen.evtchn_pending_sel))
-				kick_vcpu = true;
-			goto out_rcu;
-		}
-
-		/*
-		 * Explicitly use a 32-bit btsl instead of test_and_set_bit(),
-		 * which would use btsq on x86-64. The vcpu_info is guest-
-		 * controlled and only required to be 32-bit aligned, so a
-		 * 64-bit access could generate a split-lock #AC.
-		 *
-		 * Note, this does not apply to the test_and_set_bit() on
-		 * pending_bits above: that is in the per-VM shared_info, which
-		 * is page aligned, so the access is guaranteed to be 64-bit
-		 * aligned.
-		 */
-		old = GEN_BINARY_RMWcc(LOCK_PREFIX "btsl",
-				       *(u32 *)(gpc->khva + vi_pending_sel_ofs),
-				       c, "Ir", port_word_bit);
-		if (!old) {
-			struct vcpu_info *vi = gpc->khva;
-
-			/* No need for compat handling */
-			BUILD_BUG_ON(offsetof(struct vcpu_info, evtchn_upcall_pending) !=
-				     offsetof(struct compat_vcpu_info, evtchn_upcall_pending));
-
-			WRITE_ONCE(vi->evtchn_upcall_pending, 1);
-			kick_vcpu = true;
-		}
-
-		/* For the per-vCPU lapic vector, deliver it as MSI. */
-		if (kick_vcpu && vcpu->arch.xen.upcall_vector) {
-			kvm_xen_inject_vcpu_vector(vcpu);
-			kick_vcpu = false;
-		}
 	}
 
- out_rcu:
+ out_unlock:
 	read_unlock_irqrestore(&gpc->lock, flags);
-	srcu_read_unlock(&kvm->srcu, idx);
 
-	if (kick_vcpu) {
-		kvm_make_request(KVM_REQ_UNBLOCK, vcpu);
-		kvm_vcpu_kick(vcpu);
-	}
+	/*
+	 * Both of these can kick the vCPU, so keep them outside the GPC
+	 * read lock; an invalidation may be waiting to take it for write.
+	 */
+	if (rc == -ENOTCONN)
+		kvm_xen_check_poller(vcpu, xe->port);
+	else if (rc == 1)
+		__kvm_xen_set_evtchn_fast(vcpu, port_word_bit,
+					  has_64bit_shinfo);
+
+	srcu_read_unlock(&kvm->srcu, idx);
 
 	return rc;
 }
-- 
2.43.0

[PATCH 07/10] KVM: x86/xen: Explicitly tag "shared info" page as never being dirty tracked

[David Woodhouse]

From: Sean Christopherson <seanjc@google.com>

Explicitly mark the Xen shared info page as never being dirty tracked so
that higher-level gpc APIs can be added to automatically take care of
things like dirty tracking, without reintroducing the bug fixed by
commit 55749769fe60 ("KVM: x86: Fix wall clock writes in Xen shared_info
not to mark page dirty").  And because the code _looks_ buggy.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/xen.c        |  2 +-
 include/linux/kvm_host.h  | 10 ++++++++--
 include/linux/kvm_types.h |  1 +
 virt/kvm/pfncache.c       |  4 +++-
 4 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index a931458f0c71..f8162914d6c6 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -2469,7 +2469,7 @@ void kvm_xen_init_vm(struct kvm *kvm)
 {
 	mutex_init(&kvm->arch.xen.xen_lock);
 	xa_init(&kvm->arch.xen.evtchn_ports);
-	kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm);
+	__kvm_gpc_init(&kvm->arch.xen.shinfo_cache, kvm, true);
 }
 
 void kvm_xen_destroy_vm(struct kvm *kvm)
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 88c1ee37550c..1abd3811b953 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1465,7 +1465,13 @@ int kvm_vcpu_write_guest(struct kvm_vcpu *vcpu, gpa_t gpa, const void *data,
  * immutable attributes.  Note, the cache must be zero-allocated (or zeroed by
  * the caller before init).
  */
-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm);
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+		    bool never_dirty);
+
+static inline void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+{
+	__kvm_gpc_init(gpc, kvm, false);
+}
 
 /**
  * kvm_gpc_activate - prepare a cached kernel mapping and HPA for a given guest
@@ -1987,7 +1993,7 @@ static inline void kvm_gpc_mark_dirty_in_slot(struct gfn_to_pfn_cache *gpc)
 {
 	lockdep_assert_held(&gpc->lock);
 
-	if (!gpc->memslot)
+	if (!gpc->memslot || gpc->never_dirty)
 		return;
 
 	mark_page_dirty_in_slot(gpc->kvm, gpc->memslot, gpa_to_gfn(gpc->gpa));
diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
index a568d8e6f4e8..e850adc3f47e 100644
--- a/include/linux/kvm_types.h
+++ b/include/linux/kvm_types.h
@@ -94,6 +94,7 @@ struct gfn_to_pfn_cache {
 	kvm_pfn_t pfn;
 	bool active;
 	bool valid;
+	bool never_dirty;
 };
 
 #ifdef KVM_ARCH_NR_OBJS_PER_MEMORY_CACHE
diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
index 8504e26fa111..0e2c9cfa8056 100644
--- a/virt/kvm/pfncache.c
+++ b/virt/kvm/pfncache.c
@@ -431,7 +431,8 @@ int kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, unsigned long len, bool wait)
 	}
 }
 
-void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
+void __kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm,
+		    bool never_dirty)
 {
 	rwlock_init(&gpc->lock);
 	mutex_init(&gpc->refresh_lock);
@@ -441,6 +442,7 @@ void kvm_gpc_init(struct gfn_to_pfn_cache *gpc, struct kvm *kvm)
 	gpc->gpa = INVALID_GPA;
 	gpc->uhva = KVM_HVA_ERR_BAD;
 	gpc->active = gpc->valid = false;
+	gpc->never_dirty = never_dirty;
 }
 
 static int __kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long uhva,
-- 
2.43.0

[PATCH 08/10] KVM: x86/xen: Don't dirty track "vCPU info" page

[David Woodhouse]

From: Sean Christopherson <seanjc@google.com>

Give the Xen per-vCPU info page the same treatment as the per-VM shared
info page, and never mark it dirty, as KVM clearly relies on userspace to
assume the page is always dirty.  While the page is marked dirty on writes
via kvm_xen_inject_pending_events(), it's not marked dirty when written by
__kvm_xen_set_evtchn_fast().

Furthermore, as was the case with the shared info page, writes in the event
channel fastpath may be done without an active vCPU, e.g. when called via
timer callback or irqfd injection.  I.e. attempting to fix the fastpath
would run afoul of the same issue that was fixed by
commit 55749769fe60 ("KVM: x86: Fix wall clock writes in Xen shared_info
not to mark page dirty").

This also covers the pvclock writes kvm_guest_time_update() makes
through the same cache; userspace assuming the page is always dirty
covers those equally.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/xen.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index f8162914d6c6..ef0d59ebc6a7 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -715,7 +715,6 @@ void kvm_xen_inject_pending_events(struct kvm_vcpu *v)
 		WRITE_ONCE(vi->evtchn_upcall_pending, 1);
 	}
 
-	kvm_gpc_mark_dirty_in_slot(gpc);
 	read_unlock_irqrestore(&gpc->lock, flags);
 
 	/* For the per-vCPU lapic vector, deliver it as MSI. */
@@ -2448,7 +2447,7 @@ void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu)
 
 	kvm_gpc_init(&vcpu->arch.xen.runstate_cache, vcpu->kvm);
 	kvm_gpc_init(&vcpu->arch.xen.runstate2_cache, vcpu->kvm);
-	kvm_gpc_init(&vcpu->arch.xen.vcpu_info_cache, vcpu->kvm);
+	__kvm_gpc_init(&vcpu->arch.xen.vcpu_info_cache, vcpu->kvm, true);
 	kvm_gpc_init(&vcpu->arch.xen.vcpu_time_info_cache, vcpu->kvm);
 }
 
-- 
2.43.0

[PATCH 09/10] KVM: x86: Request the guest TLB flush from record_steal_time()

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

record_steal_time() performs a TLB flush on the guest's behalf when the
guest sets KVM_VCPU_FLUSH_TLB. With shadow paging,
kvm_vcpu_flush_tlb_guest() ends up in mmu_sync_children(), which can
reschedule via cond_resched_rwlock_write(). That is why the flush
currently sits between a user_access_end() and a fresh
user_access_begin(): it cannot be done while the guest page is mapped
for access.

Raise KVM_REQ_TLB_FLUSH_GUEST instead of flushing inline, and move the
processing of KVM_REQ_STEAL_UPDATE ahead of where the TLB flush requests
are serviced in vcpu_enter_guest() so that the flush still happens before
the vCPU enters the guest. There is no requirement for it to complete any
earlier than that.

kvm_make_request() on the vCPU which is running is nothing more than a
barrier and a set_bit(), so unlike the flush itself it can be done from
any context — which lets the surrounding access be simplified later.

Note that the xchg() must stay where it is: it atomically consumes any
KVM_VCPU_FLUSH_TLB which the guest has set, as required by commit
b043138246a4 ("x86/KVM: Make sure KVM_VCPU_FLUSH_TLB flag is not missed").

Reported-by: Sashiko AI review <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260530061932.7849D1F00893@smtp.kernel.org
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 arch/x86/kvm/x86.c | 26 +++++++++++++++++++++++---
 1 file changed, 23 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index eadcaeee651a..20119d07b477 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2106,8 +2106,22 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 
 		trace_kvm_pv_tlb_flush(vcpu->vcpu_id,
 				       st_preempted & KVM_VCPU_FLUSH_TLB);
+		/*
+		 * Request the flush instead of performing it inline. With
+		 * shadow paging kvm_vcpu_flush_tlb_guest() can reschedule (via
+		 * mmu_sync_children()), which is why the user access above has
+		 * to be closed before it. Requesting the flush is just a bit
+		 * set on the running vCPU, and the request is serviced before
+		 * the vCPU enters the guest; nothing requires the flush to
+		 * have completed any earlier than that.
+		 *
+		 * Note the xchg above must remain as-is: it atomically
+		 * consumes any KVM_VCPU_FLUSH_TLB the guest set, as required
+		 * by commit b043138246a4 ("x86/KVM: Make sure
+		 * KVM_VCPU_FLUSH_TLB flag is not missed").
+		 */
 		if (st_preempted & KVM_VCPU_FLUSH_TLB)
-			kvm_vcpu_flush_tlb_guest(vcpu);
+			kvm_make_request(KVM_REQ_TLB_FLUSH_GUEST, vcpu);
 
 		if (!user_access_begin(st, sizeof(*st)))
 			goto dirty;
@@ -8089,6 +8103,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			if (unlikely(r))
 				goto out;
 		}
+		/*
+		 * Process the steal time update before the TLB flush requests
+		 * are serviced below; record_steal_time() requests
+		 * KVM_REQ_TLB_FLUSH_GUEST on the guest's behalf rather than
+		 * performing the flush itself.
+		 */
+		if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
+			record_steal_time(vcpu);
 		if (kvm_check_request(KVM_REQ_MMU_SYNC, vcpu))
 			kvm_mmu_sync_roots(vcpu);
 		if (kvm_check_request(KVM_REQ_LOAD_MMU_PGD, vcpu))
@@ -8138,8 +8160,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 			r = 1;
 			goto out;
 		}
-		if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
-			record_steal_time(vcpu);
 		if (kvm_check_request(KVM_REQ_PMU, vcpu))
 			kvm_pmu_handle_event(vcpu);
 		if (kvm_check_request(KVM_REQ_PMI, vcpu))
-- 
2.43.0

[PATCH 10/10] KVM: selftests: Add Xen evtchn vs memslot-update race test

[David Woodhouse]

From: David Woodhouse <dwmw@amazon.co.uk>

Race a guest EVTCHNOP_send loop against a host thread which deletes
and recreates the shared_info memslot. Delivery which hits the update
window now returns -EAGAIN and the hypercall is re-executed; if KVM
instead waited in the vCPU's exit-handling context — under vcpu_run's
kvm->srcu read section — it would deadlock against
kvm_swap_active_memslots() waiting out that reader, and this test
livelocks within seconds.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Assisted-by: LLM
---
 tools/testing/selftests/kvm/Makefile.kvm      |   1 +
 .../kvm/x86/xen_evtchn_memslot_race_test.c    | 178 ++++++++++++++++++
 2 files changed, 179 insertions(+)
 create mode 100644 tools/testing/selftests/kvm/x86/xen_evtchn_memslot_race_test.c

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 96bab7002d39..e7e17d19d6cb 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -144,6 +144,7 @@ TEST_GEN_PROGS_x86 += x86/xss_msr_test
 TEST_GEN_PROGS_x86 += x86/debug_regs
 TEST_GEN_PROGS_x86 += x86/tsc_msrs_test
 TEST_GEN_PROGS_x86 += x86/vmx_pmu_caps_test
+TEST_GEN_PROGS_x86 += x86/xen_evtchn_memslot_race_test
 TEST_GEN_PROGS_x86 += x86/xen_shinfo_test
 TEST_GEN_PROGS_x86 += x86/xen_vmcall_test
 TEST_GEN_PROGS_x86 += x86/sev_dbg_test
diff --git a/tools/testing/selftests/kvm/x86/xen_evtchn_memslot_race_test.c b/tools/testing/selftests/kvm/x86/xen_evtchn_memslot_race_test.c
new file mode 100644
index 000000000000..2b6974d3d464
--- /dev/null
+++ b/tools/testing/selftests/kvm/x86/xen_evtchn_memslot_race_test.c
@@ -0,0 +1,178 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * xen_evtchn_memslot_race_test
+ *
+ * A guest EVTCHNOP_send whose delivery races a DELETE/re-CREATE of the
+ * memslot holding the shared_info page. The delivery path runs in the
+ * vCPU's exit-handling context, under vcpu_run's kvm->srcu read
+ * section; if it waits there for the memslot update to complete, it
+ * deadlocks against kvm_swap_active_memslots()'s
+ * synchronize_srcu_expedited() waiting out that very read section.
+ * Regression test for that deadlock: KVM must defer the delivery to
+ * the next guest entry rather than spin in exit context.
+ *
+ * The guest sends events in a tight loop; a host thread deletes and
+ * recreates the shared_info slot in a tight loop. On a broken host
+ * this livelocks within a second or two (the test then dies by
+ * timeout); on a fixed host every send eventually delivers and the
+ * test completes N iterations.
+ */
+#include <pthread.h>
+#include <stdatomic.h>
+
+#include "test_util.h"
+#include "kvm_util.h"
+#include "processor.h"
+
+#include <stdint.h>
+
+#define SHINFO_REGION_GPA	0xc0000000ULL
+#define SHINFO_REGION_SLOT	10
+
+#define EVTCHN_PORT		15
+#define TEST_ITERATIONS		4000
+
+#define EVTCHNOP_send		4
+#define __HYPERVISOR_event_channel_op	32
+
+#define XEN_HYPERCALL_MSR	0x40000000
+
+#define EVTCHNSTAT_ipi		5
+
+struct evtchn_send {
+	uint32_t port;
+};
+
+static void guest_code(void)
+{
+	struct evtchn_send s = { .port = EVTCHN_PORT };
+	int i;
+
+	for (i = 0; i < TEST_ITERATIONS; i++) {
+		/*
+		 * The delivery may legitimately fail while the shared_info
+		 * slot is mid-churn (between DELETE and re-CREATE there is
+		 * no slot at all, which is -EFAULT, not a transient). The
+		 * failure mode under test is the vCPU never coming back at
+		 * all (deadlock -> timeout), not the hypercall rc.
+		 */
+		__xen_hypercall(__HYPERVISOR_event_channel_op, EVTCHNOP_send, &s);
+		GUEST_SYNC(i);
+	}
+
+	GUEST_DONE();
+}
+
+static atomic_bool churn_stop;
+
+static void *memslot_churn(void *arg)
+{
+	struct kvm_vm *vm = arg;
+
+	while (!atomic_load(&churn_stop)) {
+		vm_mem_region_delete(vm, SHINFO_REGION_SLOT);
+		vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
+					    SHINFO_REGION_GPA,
+					    SHINFO_REGION_SLOT, 2, 0);
+	}
+	return NULL;
+}
+
+int main(int argc, char *argv[])
+{
+	struct kvm_vcpu *vcpu;
+	struct kvm_vm *vm;
+	pthread_t thread;
+	struct ucall uc;
+	int done = 0;
+	int r;
+
+	int xen_caps = kvm_check_cap(KVM_CAP_XEN_HVM);
+
+	TEST_REQUIRE(xen_caps & KVM_XEN_HVM_CONFIG_SHARED_INFO);
+	TEST_REQUIRE(xen_caps & KVM_XEN_HVM_CONFIG_EVTCHN_SEND);
+
+	vm = vm_create_with_one_vcpu(&vcpu, guest_code);
+
+	vm_userspace_mem_region_add(vm, VM_MEM_SRC_ANONYMOUS,
+				    SHINFO_REGION_GPA, SHINFO_REGION_SLOT, 2, 0);
+
+	struct kvm_xen_hvm_config hvmc = {
+		.flags = KVM_XEN_HVM_CONFIG_INTERCEPT_HCALL |
+			 KVM_XEN_HVM_CONFIG_EVTCHN_SEND,
+		.msr = XEN_HYPERCALL_MSR,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_CONFIG, &hvmc);
+
+	struct kvm_xen_hvm_attr lm = {
+		.type = KVM_XEN_ATTR_TYPE_LONG_MODE,
+		.u.long_mode = 1,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &lm);
+
+	struct kvm_xen_hvm_attr ha = {
+		.type = KVM_XEN_ATTR_TYPE_SHARED_INFO,
+		.u.shared_info.gfn = SHINFO_REGION_GPA / PAGE_SIZE,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &ha);
+
+	struct kvm_xen_vcpu_attr vi = {
+		.type = KVM_XEN_VCPU_ATTR_TYPE_VCPU_INFO,
+		.u.gpa = SHINFO_REGION_GPA + 0x40,
+	};
+	vcpu_ioctl(vcpu, KVM_XEN_VCPU_SET_ATTR, &vi);
+
+	/* Loop the port back to this vCPU as an IPI. */
+	struct kvm_xen_hvm_attr evt = {
+		.type = KVM_XEN_ATTR_TYPE_EVTCHN,
+		.u.evtchn.send_port = EVTCHN_PORT,
+		.u.evtchn.type = EVTCHNSTAT_ipi,
+		.u.evtchn.flags = 0,
+		.u.evtchn.deliver.port.port = EVTCHN_PORT,
+		.u.evtchn.deliver.port.vcpu = vcpu->id,
+		.u.evtchn.deliver.port.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt);
+
+	r = pthread_create(&thread, NULL, memslot_churn, vm);
+	TEST_ASSERT(!r, "pthread_create() failed: %d", r);
+
+	while (!done) {
+		vcpu_run(vcpu);
+
+		/*
+		 * In the window where the shared_info slot is fully deleted
+		 * (between the churn thread's DELETE and re-CREATE), the
+		 * delivery fails with -EFAULT and the hypercall bounces to
+		 * userspace as KVM_EXIT_XEN. Complete it with an error
+		 * result and keep going; the failure mode under test is
+		 * the vCPU never coming back (deadlock -> timeout), not
+		 * the hypercall rc.
+		 */
+		if (vcpu->run->exit_reason == KVM_EXIT_XEN &&
+		    vcpu->run->xen.type == KVM_EXIT_XEN_HCALL) {
+			vcpu->run->xen.u.hcall.result = -EFAULT;
+			continue;
+		}
+
+		TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);
+
+		switch (get_ucall(vcpu, &uc)) {
+		case UCALL_SYNC:
+			break;
+		case UCALL_DONE:
+			done = 1;
+			break;
+		case UCALL_ABORT:
+			REPORT_GUEST_ASSERT(uc);
+		default:
+			TEST_FAIL("Unexpected ucall %lu", uc.cmd);
+		}
+	}
+
+	atomic_store(&churn_stop, true);
+	pthread_join(thread, NULL);
+
+	kvm_vm_free(vm);
+	return 0;
+}
-- 
2.43.0

Re: [PATCH 03/10] KVM: x86/xen: Wait out memslot updates in the Xen attribute ioctls

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] kvm: UAF and lockdep splat in kvm_gpc_check() due to dropped SRCU lock
- [High] kvm: x86/xen: 3-way AB-BA-CA deadlock between srcu, xen_lock, and slots_lock
--

commit 34df3b53ec91de034b47e7637dd9880ba9d6c924
Author: David Woodhouse <dwmw@amazon.co.uk>

KVM: x86/xen: Wait out memslot updates in the Xen attribute ioctls

This patch updates Xen attribute ioctls in KVM to handle memslot updates
by waiting them out (passing wait=true to kvm_gpc_refresh and
kvm_gpc_activate), instead of returning -EFAULT. It also drops the
srcu_read_lock sections around activation calls.

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index fae893cd5762c..76708b8960265 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -36,7 +36,7 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r);
>  
>  DEFINE_STATIC_KEY_DEFERRED_FALSE(kvm_xen_enabled, HZ);
>  
> -static int kvm_xen_shared_info_init(struct kvm *kvm)
> +static int kvm_xen_shared_info_init(struct kvm *kvm, bool wait)
>  {
>  	struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
>  	struct pvclock_wall_clock *wc;
> @@ -44,13 +44,12 @@ static int kvm_xen_shared_info_init(struct kvm *kvm)
>  	u32 wc_version;
>  	u64 wall_nsec;
>  	int ret = 0;
> -	int idx = srcu_read_lock(&kvm->srcu);
>  
>  	read_lock_irq(&gpc->lock);
>  	while (!kvm_gpc_check(gpc, PAGE_SIZE)) {
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[Severity: High]
Does this code trigger a use-after-free and RCU lockdep splat?

Because the srcu_read_lock() was removed above, this loop calls
kvm_gpc_check() without SRCU protection. 

If we look at virt/kvm/pfncache.c:kvm_gpc_check():

bool kvm_gpc_check(struct gfn_to_pfn_cache *gpc, unsigned long len)
{
	struct kvm_memslots *slots = kvm_memslots(gpc->kvm);
...

This dereferences the memslots array and requires SRCU protection. Without it,
a concurrent memslot update can free the memslots structure before 
slots->generation is accessed.

[ ... ]

> @@ -792,19 +790,15 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
>  			 * to make the call anyway.
>  			 */
>  			r = kvm->arch.xen.shinfo_cache.active ?
> -				kvm_xen_shared_info_init(kvm) : 0;
> +				kvm_xen_shared_info_init(kvm, true) : 0;
>  			mutex_unlock(&kvm->arch.xen.xen_lock);
>  		}
>  		break;
>  
>  	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
>  	case KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA: {
> -		int idx;
> -
>  		mutex_lock(&kvm->arch.xen.xen_lock);
>  
> -		idx = srcu_read_lock(&kvm->srcu);
> -
>  		if (data->type == KVM_XEN_ATTR_TYPE_SHARED_INFO) {
>  			gfn_t gfn = data->u.shared_info.gfn;
>  
> @@ -813,7 +807,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
>  				r = 0;
>  			} else {
>  				r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
> -						     gfn_to_gpa(gfn), PAGE_SIZE, false);
> +						     gfn_to_gpa(gfn), PAGE_SIZE, true);

[Severity: High]
Does this code introduce a 3-way AB-BA-CA deadlock between srcu, xen_lock, and
slots_lock?

With wait=true, the thread executes KVM_XEN_HVM_SET_ATTR, acquires xen_lock,
and calls kvm_gpc_activate() which spins in a loop calling cond_resched()
while waiting for a memslot update to complete.

Meanwhile, a concurrent vCPU thread exiting after a WRMSR to the hypercall page
MSR might enter kvm_xen_write_hypercall_page() while holding the srcu lock:

arch/x86/kvm/xen.c:kvm_xen_write_hypercall_page() {
    ...
	mutex_lock(&kvm->arch.xen.xen_lock);

This vCPU thread blocks waiting for xen_lock, which is held by the ioctl thread
above.

A third thread performing the memslot update (holding slots_lock) calls
synchronize_srcu(&kvm->srcu), which blocks waiting for the vCPU thread to
release its srcu lock.

This creates an unrecoverable 3-way cycle where the ioctl thread waits for the
memslot update, the memslot update waits for the vCPU thread, and the vCPU
thread waits for the ioctl thread's xen_lock.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261013204824.454208-1-dwmw2@infradead.org?part=3

Re: [PATCH 04/10] KVM: x86/xen: Don't lose the timer event when delivery fails transiently

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kvm: x86/xen: infinite vcpu loop when shared_info deactivated
--

commit 51f8b9d346d917ad2de67baf6af85e344406d8e0
Author: David Woodhouse <dwmw@amazon.co.uk>

KVM: x86/xen: Don't lose the timer event when delivery fails transiently

This commit modifies KVM's Xen timer injection to handle transient failures
from kvm_xen_set_evtchn(). If the event channel set fails with -EAGAIN
or -EINVAL, it leaves the timer_pending flag set so the delivery can be
retried on the next entry pass.

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -106,13 +106,23 @@ void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
>  {
>  	if (atomic_read(&vcpu->arch.xen.timer_pending) > 0) {
>  		struct kvm_xen_evtchn e;
> +		int rc;
>  
>  		e.vcpu_id = vcpu->vcpu_id;
>  		e.vcpu_idx = vcpu->vcpu_idx;
>  		e.port = vcpu->arch.xen.timer_virq;
>  		e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
>  
> -		kvm_xen_set_evtchn(&e, vcpu->kvm);
> +		rc = kvm_xen_set_evtchn(&e, vcpu->kvm);
> +
> +		/*
> +		 * Deliberately leave timer_pending set if the memslot is
> +		 * being updated (-EAGAIN) or the shared_info cache has
> +		 * been deactivated (-EINVAL); delivery is retried on the
> +		 * next entry pass.
> +		 */
> +		if (rc == -EAGAIN || rc == -EINVAL)
> +			return;

[Severity: High]
Does this early return cause an infinite loop in the vCPU run loop if the
shared_info cache is deactivated by userspace and returns -EINVAL?

When the vCPU attempts to halt, kvm_vcpu_check_block() evaluates
kvm_cpu_has_pending_timer(), which returns true because timer_pending is
still > 0. This aborts the block, immediately returning to vcpu_block().

However, vcpu_block() then checks kvm_arch_vcpu_runnable(), which does not
consider timer_pending, so it evaluates to false. This causes vcpu_block()
to return 1, forcing the vcpu_run() loop to spin indefinitely without
breaking, sleeping, or entering the guest:

virt/kvm/kvm_main.c:kvm_vcpu_check_block() {
    ...
    if (kvm_cpu_has_pending_timer(vcpu))
        goto out;
    ...
}

arch/x86/kvm/x86.c:vcpu_block() {
    ...
    if (!kvm_arch_vcpu_runnable(vcpu))
        return 1;
    ...
}

arch/x86/kvm/x86.c:vcpu_run() {
    for (;;) {
        if (kvm_vcpu_running(vcpu)) {
            r = vcpu_enter_guest(vcpu);
        } else {
            r = vcpu_block(vcpu);
        }

        if (r <= 0)
            break;
        ...
    }
}

With r being 1, the loop unconditionally retries, hits -EINVAL again, and
spins monopolizing the host CPU until the cache is reactivated. Could we
avoid this stall by either dropping the event or handling the sleep abort
differently when shared_info is unavailable?

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261013204824.454208-1-dwmw2@infradead.org?part=4

Re: [PATCH 05/10] KVM: x86/xen: Retry event channel delivery which races a memslot update

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] kvm: infinite loop in vcpu_run due to Xen timer delivery failure
- [High] kvm: ABBA deadlock between xen_lock and kvm->srcu in KVM_XEN_HVM_SET_ATTR
--

commit b27b9db29f6c5d0bc3a62af7e5baefbb8e0f7c51
Author: David Woodhouse <dwmw@amazon.co.uk>

KVM: x86/xen: Retry event channel delivery which races a memslot update

Event channel delivery now returns -EAGAIN when it races a memslot
update. It always failed; now the failure is distinguishable, and
instead of dropping the event each caller reacts according to its
context.

Closes: https://lore.kernel.org/all/20260920215341.5ECE91F000FF@smtp.kernel.org

> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index aea6572163d0c..ad7708be6c049 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -113,7 +114,7 @@ void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu)
>  		e.port = vcpu->arch.xen.timer_virq;
>  		e.priority = KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL;
>  
> -		rc = kvm_xen_set_evtchn(&e, vcpu->kvm);
> +		rc = kvm_xen_set_evtchn(&e, vcpu->kvm, wait);
>  
>  		/*
>  		 * Deliberately leave timer_pending set if the memslot is

[Severity: High]
Does this code cause an infinite loop in vcpu_run() due to the Xen timer
delivery failure?

If rc is -EAGAIN or -EINVAL, timer_pending remains set. A set
timer_pending causes block attempts to instantly abort:

virt/kvm/kvm_main.c:kvm_vcpu_check_block() {
    ...
	if (kvm_cpu_has_pending_timer(vcpu))
		goto out;
	if (signal_pending(current))
		goto out;
    ...
}

Because the block is aborted and kvm_vcpu_has_events() evaluates to false
(as it does not check timer_pending), the vCPU's mp_state remains halted.

This forces vcpu_run() into an infinite retry loop on the host core
without entering the guest:

arch/x86/kvm/x86.c:vcpu_run() {
    ...
		if (kvm_vcpu_running(vcpu)) {
			r = vcpu_enter_guest(vcpu);
		} else {
			r = vcpu_block(vcpu);
		}

		if (r <= 0)
			break;

		kvm_clear_request(KVM_REQ_UNBLOCK, vcpu);
		...
		if (kvm_cpu_has_pending_timer(vcpu))
			kvm_inject_pending_timer_irqs(vcpu);
    ...
}

[ ... ]

> @@ -2020,16 +2036,13 @@ static int kvm_xen_set_evtchn(struct kvm_xen_evtchn *xe, struct kvm *kvm)
>  	 */
>  	do {
>  		struct gfn_to_pfn_cache *gpc = &kvm->arch.xen.shinfo_cache;
> -		int idx;
>  
>  		rc = kvm_xen_set_evtchn_fast(xe, kvm);
>  		if (rc != -EWOULDBLOCK)
>  			break;
>  
> -		idx = srcu_read_lock(&kvm->srcu);
> -		rc = kvm_gpc_refresh(gpc, PAGE_SIZE, false);
> -		srcu_read_unlock(&kvm->srcu, idx);
> -	} while(!rc);
> +		rc = kvm_gpc_refresh(gpc, PAGE_SIZE, wait);
> +	} while (!rc);
>  
>  	if (mm_borrowed)
>  		kthread_unuse_mm(kvm->mm);

[Severity: High]
Can this cause a deadlock between xen_lock and kvm->srcu?

The SET_ATTR ioctl for LONG_MODE acquires xen_lock and calls
kvm_xen_shared_info_init() with wait=true:

arch/x86/kvm/xen.c:kvm_xen_hvm_set_attr() {
    mutex_lock(&kvm->arch.xen.xen_lock);
    ...
    r = kvm->arch.xen.shinfo_cache.active ?
        kvm_xen_shared_info_init(kvm, true) : 0;
    mutex_unlock(&kvm->arch.xen.xen_lock);
}

This causes it to sleep waiting for memslot updates while holding
xen_lock.

Meanwhile, a vCPU running in guest mode (holding kvm->srcu read lock)
can block trying to acquire xen_lock in kvm_xen_write_hypercall_page():

arch/x86/kvm/xen.c:kvm_xen_write_hypercall_page() {
    mutex_lock(&kvm->arch.xen.xen_lock);
}

This prevents synchronize_srcu() in the memslot update from completing,
resulting in an ABBA deadlock.

The SHARED_INFO case similarly acquires xen_lock and explicitly loops in
kvm_gpc_activate() and kvm_gpc_refresh() with wait=true while holding
the lock.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261013204824.454208-1-dwmw2@infradead.org?part=5

Re: [PATCH 03/10] KVM: x86/xen: Wait out memslot updates in the Xen attribute ioctls

[David Woodhouse]

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On Tue, 2026-10-13 at 21:04 +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
> - [High] kvm: UAF and lockdep splat in kvm_gpc_check() due to dropped SRCU lock
> - [High] kvm: x86/xen: 3-way AB-BA-CA deadlock between srcu, xen_lock, and slots_lock
> --

Meh, I thought I'd dealt with all the Sashiko feedback locally.

Will redo it and post v2 anon.

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