diff --git a/xen/arch/x86/hvm/vmsi.c b/xen/arch/x86/hvm/vmsi.c
index adbac965f9..9999179837 100644
--- a/xen/arch/x86/hvm/vmsi.c
+++ b/xen/arch/x86/hvm/vmsi.c
@@ -283,8 +283,8 @@ static int msixtbl_write(struct vcpu *v, unsigned long address,
     unsigned long flags;
     struct irq_desc *desc;
 
-    if ( (len != 4 && len != 8) || (address & (len - 1)) )
-        return r;
+    if ( !IS_ALIGNED(address, len) )
+        return X86EMUL_OKAY;
 
     rcu_read_lock(&msixtbl_rcu_lock);
 
@@ -345,8 +345,7 @@ static int msixtbl_write(struct vcpu *v, unsigned long address,
 
 unlock:
     spin_unlock_irqrestore(&desc->lock, flags);
-    if ( len == 4 )
-        r = X86EMUL_OKAY;
+    r = X86EMUL_OKAY;
 
 out:
     rcu_read_unlock(&msixtbl_rcu_lock);
@@ -357,7 +356,17 @@ static int cf_check _msixtbl_write(
     const struct hvm_io_handler *handler, uint64_t address, uint32_t len,
     uint64_t val)
 {
-    return msixtbl_write(current, address, len, val);
+    /* Ignore invalid length or unaligned writes. */
+    if ( (len != 4 && len != 8) || !IS_ALIGNED(address, len) )
+        return X86EMUL_OKAY;
+
+    /*
+     * This function returns X86EMUL_UNHANDLEABLE even if write is properly
+     * handled, to propagate it to the device model (so it can keep its
+     * internal state in sync).
+     */
+    msixtbl_write(current, address, len, val);
+    return X86EMUL_UNHANDLEABLE;
 }
 
 static bool cf_check msixtbl_range(
diff --git a/xen/common/kernel.c b/xen/common/kernel.c
index 08dbaa2a05..b44b2439ca 100644
--- a/xen/common/kernel.c
+++ b/xen/common/kernel.c
@@ -637,6 +637,7 @@ long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
                 fi.submap |= (1U << XENFEAT_hvm_safe_pvclock) |
                              (1U << XENFEAT_hvm_callback_vector) |
                              (has_pirq(d) ? (1U << XENFEAT_hvm_pirqs) : 0);
+            fi.submap |= (1U << XENFEAT_dm_msix_all_writes);
 #endif
             if ( !paging_mode_translate(d) || is_domain_direct_mapped(d) )
                 fi.submap |= (1U << XENFEAT_direct_mapped);
diff --git a/xen/include/public/features.h b/xen/include/public/features.h
index 4437f25d25..8801930947 100644
--- a/xen/include/public/features.h
+++ b/xen/include/public/features.h
@@ -120,6 +120,14 @@
 #define XENFEAT_runstate_phys_area        18
 #define XENFEAT_vcpu_time_phys_area       19
 
+/*
+ * If set, Xen will passthrough all MSI-X vector ctrl writes to device model,
+ * not only those unmasking an entry. This allows device model to properly keep
+ * track of the MSI-X table without having to read it from the device behind
+ * Xen's backs. This information is relevant only for device models.
+ */
+#define XENFEAT_dm_msix_all_writes        20
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */