The difference between these two functions:
- virtio_notify() uses the interrupt code path (MSI or classic IRQs)
- virtio_notify_irqfd() uses guest notifiers (irqfds)

virtio_notify() can only be called with the BQL held because the
interrupt code path requires the BQL. Device models use
virtio_notify_irqfd() from IOThreads since the BQL is not held.

The two functions can be unified by pushing down the if
(qemu_in_iothread()) check from virtio-blk and virtio-scsi into core
virtio code. This is in preparation for the next commit that will add
irqfd support to virtio_notify_config() and where it's unattractive to
introduce another irqfd-only API for device model callers.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/hw/virtio/virtio.h |  1 -
 hw/block/virtio-blk.c      |  6 +-----
 hw/scsi/virtio-scsi.c      |  6 +-----
 hw/virtio/virtio.c         | 28 +++++++++++++---------------
 hw/virtio/trace-events     |  1 -
 5 files changed, 15 insertions(+), 27 deletions(-)

diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
index c594764f23..8435f5e8fc 100644
--- a/include/hw/virtio/virtio.h
+++ b/include/hw/virtio/virtio.h
@@ -290,7 +290,6 @@ int virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
                               unsigned int *out_bytes, unsigned max_in_bytes,
                               unsigned max_out_bytes);
 
-void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq);
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq);
 
 int virtio_save(VirtIODevice *vdev, QEMUFile *f);
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 9bab2716c1..64efce4846 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -62,11 +62,7 @@ void virtio_blk_req_complete(VirtIOBlockReq *req, unsigned char status)
     iov_discard_undo(&req->inhdr_undo);
     iov_discard_undo(&req->outhdr_undo);
     virtqueue_push(req->vq, &req->elem, req->in_len);
-    if (qemu_in_iothread()) {
-        virtio_notify_irqfd(vdev, req->vq);
-    } else {
-        virtio_notify(vdev, req->vq);
-    }
+    virtio_notify(vdev, req->vq);
 }
 
 static int virtio_blk_handle_rw_error(VirtIOBlockReq *req, int error,
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 34ae14f7bf..d817fc42b4 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -116,11 +116,7 @@ static void virtio_scsi_complete_req(VirtIOSCSIReq *req, QemuMutex *vq_lock)
     }
 
     virtqueue_push(vq, &req->elem, req->qsgl.size + req->resp_iov.size);
-    if (s->dataplane_started && !s->dataplane_fenced) {
-        virtio_notify_irqfd(vdev, vq);
-    } else {
-        virtio_notify(vdev, vq);
-    }
+    virtio_notify(vdev, vq);
 
     if (vq_lock) {
         qemu_mutex_unlock(vq_lock);
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index 9a81ad912e..13830debfd 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -31,6 +31,7 @@
 #include "hw/qdev-properties.h"
 #include "hw/virtio/virtio-access.h"
 #include "system/dma.h"
+#include "system/iothread.h"
 #include "system/runstate.h"
 #include "virtio-qmp.h"
 
@@ -2654,16 +2655,8 @@ static void virtio_notify_irqfd_deferred_fn(void *opaque)
     event_notifier_set(notifier);
 }
 
-void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
+static void virtio_irq(VirtQueue *vq)
 {
-    WITH_RCU_READ_LOCK_GUARD() {
-        if (!virtio_should_notify(vdev, vq)) {
-            return;
-        }
-    }
-
-    trace_virtio_notify_irqfd(vdev, vq);
-
     /*
      * virtio spec 1.0 says ISR bit 0 should be ignored with MSI, but
      * windows drivers included in virtio-win 1.8.0 (circa 2015) are
@@ -2680,13 +2673,18 @@ void virtio_notify_irqfd(VirtIODevice *vdev, VirtQueue *vq)
      * to an atomic operation.
      */
     virtio_set_isr(vq->vdev, 0x1);
-    defer_call(virtio_notify_irqfd_deferred_fn, &vq->guest_notifier);
-}
 
-static void virtio_irq(VirtQueue *vq)
-{
-    virtio_set_isr(vq->vdev, 0x1);
-    virtio_notify_vector(vq->vdev, vq->vector);
+    /*
+     * The interrupt code path requires the Big QEMU Lock (BQL), so use the
+     * notifier instead when in an IOThread. This assumes that device models
+     * have already called ->set_guest_notifiers() sometime before calling this
+     * function.
+     */
+    if (qemu_in_iothread()) {
+        defer_call(virtio_notify_irqfd_deferred_fn, &vq->guest_notifier);
+    } else {
+        virtio_notify_vector(vq->vdev, vq->vector);
+    }
 }
 
 void virtio_notify(VirtIODevice *vdev, VirtQueue *vq)
diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
index 76f0d458b2..658cc365e7 100644
--- a/hw/virtio/trace-events
+++ b/hw/virtio/trace-events
@@ -75,7 +75,6 @@ virtqueue_flush(void *vq, unsigned int count) "vq %p count %u"
 virtqueue_pop(void *vq, void *elem, unsigned int in_num, unsigned int out_num) "vq %p elem %p in_num %u out_num %u"
 virtio_queue_notify(void *vdev, int n, void *vq) "vdev %p n %d vq %p"
 virtio_notify_irqfd_deferred_fn(void *vdev, void *vq) "vdev %p vq %p"
-virtio_notify_irqfd(void *vdev, void *vq) "vdev %p vq %p"
 virtio_notify(void *vdev, void *vq) "vdev %p vq %p"
 virtio_set_status(void *vdev, uint8_t val) "vdev %p val %u"
 
-- 
2.51.0

On 23/9/25 00:01, Stefan Hajnoczi wrote:
> The difference between these two functions:
> - virtio_notify() uses the interrupt code path (MSI or classic IRQs)
> - virtio_notify_irqfd() uses guest notifiers (irqfds)
> 
> virtio_notify() can only be called with the BQL held because the
> interrupt code path requires the BQL. Device models use
> virtio_notify_irqfd() from IOThreads since the BQL is not held.
> 
> The two functions can be unified by pushing down the if
> (qemu_in_iothread()) check from virtio-blk and virtio-scsi into core
> virtio code. This is in preparation for the next commit that will add
> irqfd support to virtio_notify_config() and where it's unattractive to
> introduce another irqfd-only API for device model callers.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>   include/hw/virtio/virtio.h |  1 -
>   hw/block/virtio-blk.c      |  6 +-----
>   hw/scsi/virtio-scsi.c      |  6 +-----
>   hw/virtio/virtio.c         | 28 +++++++++++++---------------
>   hw/virtio/trace-events     |  1 -
>   5 files changed, 15 insertions(+), 27 deletions(-)

Nice cleanup.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>