[PATCH 7/8] KVM: x86/xen: Use 32-bit locked bts for vcpu_info evtchn_pending_sel

[David Woodhouse]

From: David Woodhouse <dwmw@xxxxxxxxxxxx>

Replace test_and_set_bit() on vcpu_info->evtchn_pending_sel with an
explicit 'lock btsl' in kvm_xen_set_evtchn_fast(). The generic
test_and_set_bit() uses a 64-bit locked operation ('lock btsq') on
x86-64, which requires 8-byte alignment to avoid split-lock #AC
exceptions.

Since evtchn_pending_sel is at most 64 bits wide and port_word_bit
ranges 0-63, a 32-bit 'lock btsl' suffices for both native and compat
vcpu_info layouts, and only requires the 4-byte alignment that is
already guaranteed by the registration path.

This also eliminates the bogus cast of compat_vcpu_info's 32-bit
evtchn_pending_sel to 'unsigned long *' which was the original source
of the split-lock hazard.

Fixes: 14243b387137 ("KVM: x86/xen: Add KVM_IRQ_ROUTING_XEN_EVTCHN and event channel delivery")
Reported-by: sashiko-bot@xxxxxxxxxx
Assisted-by: Kiro:claude-opus-4.6-1m
Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
---
 arch/x86/kvm/xen.c | 42 ++++++++++++++++++++++++++++--------------
 1 file changed, 28 insertions(+), 14 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index ae0713718367..24e939ef5d64 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1811,7 +1811,7 @@ 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;
+	unsigned long *pending_bits, *mask_bits, vi_pending_sel_ofs;
 	struct kvm_vcpu *vcpu;
 	unsigned long flags;
 	int port_word_bit;
@@ -1844,11 +1844,18 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		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);
 	}
 
 	/*
@@ -1864,6 +1871,8 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		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);
@@ -1880,19 +1889,24 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 			goto out_rcu;
 		}
 
-		if (has_64bit_shinfo) {
-			struct vcpu_info *vcpu_info = gpc->khva;
-			if (!test_and_set_bit(port_word_bit, &vcpu_info->evtchn_pending_sel)) {
-				WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
-				kick_vcpu = true;
-			}
-		} else {
-			struct compat_vcpu_info *vcpu_info = gpc->khva;
-			if (!test_and_set_bit(port_word_bit,
-					      (unsigned long *)&vcpu_info->evtchn_pending_sel)) {
-				WRITE_ONCE(vcpu_info->evtchn_upcall_pending, 1);
-				kick_vcpu = true;
-			}
+		/*
+		 * Explicitly use btsl instead of test_and_set_bit() because
+		 * a 32-bit guest is not required to align to 64 bits, and
+		 * that might cause splitlock exceptions.
+		 */
+		asm volatile(LOCK_PREFIX "btsl %[bit], %[sel]"
+			     : [sel] "+m" (*(u32 *)(gpc->khva + vi_pending_sel_ofs)),
+			       "=@ccc" (old)
+			     : [bit] "Ir" (port_word_bit) : "memory");
+		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. */
-- 
2.54.0