[Qemu-devel] [PATCH] virtio: Do not notify virtqueue if no element was pushed back.
Signed-off-by: Gal Hammer gham...@redhat.com --- hw/char/virtio-serial-bus.c | 6 +- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c index da417c7..0d38b4b 100644 --- a/hw/char/virtio-serial-bus.c +++ b/hw/char/virtio-serial-bus.c @@ -105,6 +105,7 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, VirtIODevice *vdev) { VirtIOSerialPortClass *vsc; +bool elem_pushed = false; assert(port); assert(virtio_queue_ready(vq)); @@ -145,9 +146,12 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, break; } virtqueue_push(vq, port-elem, 0); +elem_pushed = true; port-elem.out_num = 0; } -virtio_notify(vdev, vq); +if (elem_pushed) { +virtio_notify(vdev, vq); +} } static void flush_queued_data(VirtIOSerialPort *port) -- 1.8.1.4
Re: [Qemu-devel] [PATCH] virtio: Do not notify virtqueue if no element was pushed back.
On 08/05/13 10:18, Gal Hammer wrote: Signed-off-by: Gal Hammer gham...@redhat.com --- hw/char/virtio-serial-bus.c | 6 +- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c index da417c7..0d38b4b 100644 --- a/hw/char/virtio-serial-bus.c +++ b/hw/char/virtio-serial-bus.c @@ -105,6 +105,7 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, VirtIODevice *vdev) { VirtIOSerialPortClass *vsc; +bool elem_pushed = false; assert(port); assert(virtio_queue_ready(vq)); @@ -145,9 +146,12 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, break; } virtqueue_push(vq, port-elem, 0); +elem_pushed = true; port-elem.out_num = 0; } -virtio_notify(vdev, vq); +if (elem_pushed) { +virtio_notify(vdev, vq); +} } static void flush_queued_data(VirtIOSerialPort *port) I could be missing something, but it looks good to me. BTW the subject should say: virtio-serial: ..., not generic virtio: How did you catch this? Is this a performance problem, or did it expose a guest driver bug? (The guest driver should be prepared for spurious wakeups.) Reviewed-by: Laszlo Ersek ler...@redhat.com
Re: [Qemu-devel] [PATCH] virtio: Do not notify virtqueue if no element was pushed back.
On 05/08/2013 14:49, Laszlo Ersek wrote: On 08/05/13 10:18, Gal Hammer wrote: Signed-off-by: Gal Hammer gham...@redhat.com --- hw/char/virtio-serial-bus.c | 6 +- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/hw/char/virtio-serial-bus.c b/hw/char/virtio-serial-bus.c index da417c7..0d38b4b 100644 --- a/hw/char/virtio-serial-bus.c +++ b/hw/char/virtio-serial-bus.c @@ -105,6 +105,7 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, VirtIODevice *vdev) { VirtIOSerialPortClass *vsc; +bool elem_pushed = false; assert(port); assert(virtio_queue_ready(vq)); @@ -145,9 +146,12 @@ static void do_flush_queued_data(VirtIOSerialPort *port, VirtQueue *vq, break; } virtqueue_push(vq, port-elem, 0); +elem_pushed = true; port-elem.out_num = 0; } -virtio_notify(vdev, vq); +if (elem_pushed) { +virtio_notify(vdev, vq); +} } static void flush_queued_data(VirtIOSerialPort *port) I could be missing something, but it looks good to me. Thanks. BTW the subject should say: virtio-serial: ..., not generic virtio: OK. I can change that. How did you catch this? Is this a performance problem, or did it expose a guest driver bug? (The guest driver should be prepared for spurious wakeups.) I had a bug in the Windows' driver that caused a buffer duplication when the char device didn't read the whole message. It was found when working with SPICE client over WAN (i.e. slow connection) and trying to copypaste a large image. The driver was fixed. However I still think that this is needed although I'm not sure if there is a major performance penalty because of this redundant notification. Gal. Reviewed-by: Laszlo Ersek ler...@redhat.com