Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-22 Thread Michael S. Tsirkin 
On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote:
 On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
  Il 19/09/2014 09:10, Jason Wang ha scritto:
   
  -if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
  +if (vq-urgent || !vhost_has_feature(vq, 
  VIRTIO_RING_F_EVENT_IDX)) {
  So the urgent descriptor only work when event index was not enabled?
  This seems suboptimal, we may still want to benefit from event index
  even if urgent descriptor is used. Looks like we need return true here
  when vq-urgent is true?
  Its ||, not .
 
  Without event index, all descriptors are treated as urgent.
 
  Paolo
 
 
 The problem is if vq-urgent is true, the patch checks
 VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
 virtqueue_enable_cb() regardless of event index feature and cleared
 unconditionally in virtqueue_disable_cb().

The reverse actually, right?

 So virtqueue_enable_cb() was
 used to not only publish a new event index but also enable the urgent
 descriptor. And virtqueue_disable_cb() disabled all interrupts including
 the urgent descriptor. Guest won't get urgent interrupts by just adding
 virtqueue_add_outbuf_urgent() since what it needs is to enable and
 disable interrupt for !urgent descriptor.

Right, we want a new API that advances event index but does not
set VRING_AVAIL_F_NO_INTERRUPT.
IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx
interrupts, to avoid interrupt storms.

 Btw, not sure urgent is a suitable name, since interrupt is often slow
 in kvm guest. And in fact virtio-net will probably use urgent
 descriptor for those packets (e.g stream packets who can be delayed a
 little bit to batch more bytes from userspace) who was not urgent
 compared to other packets.
 

Yes but we are asking for an interrupt before event index is reached
because something is waiting for the packet to be transmitted.
I couldn't come up with a better name.

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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-22 Thread Jason Wang 
On 09/22/2014 02:55 PM, Michael S. Tsirkin wrote:
 On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote:
 On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
 Il 19/09/2014 09:10, Jason Wang ha scritto:
  
 -if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 +if (vq-urgent || !vhost_has_feature(vq, 
 VIRTIO_RING_F_EVENT_IDX)) {
 So the urgent descriptor only work when event index was not enabled?
 This seems suboptimal, we may still want to benefit from event index
 even if urgent descriptor is used. Looks like we need return true here
 when vq-urgent is true?
 Its ||, not .

 Without event index, all descriptors are treated as urgent.

 Paolo

 The problem is if vq-urgent is true, the patch checks
 VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
 virtqueue_enable_cb() regardless of event index feature and cleared
 unconditionally in virtqueue_disable_cb().
 The reverse actually, right?

Ah, right.

 So virtqueue_enable_cb() was
 used to not only publish a new event index but also enable the urgent
 descriptor. And virtqueue_disable_cb() disabled all interrupts including
 the urgent descriptor. Guest won't get urgent interrupts by just adding
 virtqueue_add_outbuf_urgent() since what it needs is to enable and
 disable interrupt for !urgent descriptor.
 Right, we want a new API that advances event index but does not
 set VRING_AVAIL_F_NO_INTERRUPT.
 IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx
 interrupts, to avoid interrupt storms.

I see, so urgent descriptor needs to be disabled in this case. But vhost
parts need a little big changes, we could not just check
VRING_AVAIL_F_NO_INTERRUPT when vq-urgent is true. If event index is
enabled, we still need to check used event to make sure the current tx
delayed interrupt works.

But just re-using VRING_AVAIL_F_NO_INTERRUPT for urgent descriptor may
not work in some case. I see codes of virtqueue_get_buf() that may
breaks this:

/* If we expect an interrupt for the next entry, tell
host
   

 * by writing event index and flush out the write
before  
   

 * the read in the next get_buf call. */
if (!(vq-vring.avail-flags  VRING_AVAIL_F_NO_INTERRUPT)) {
vring_used_event(vq-vring) = vq-last_used_idx;
virtio_mb(vq-weak_barriers);
}

Consider if only urgent descriptor is enabled, this will publish used
event which in fact enable lots of unnecessary interrupt. In fact I
don't quite understand how the above lines is used. Virtio-net stop the
queue before enable the tx interrupt in start_xmit(), so the above lines
will not run at all.

 Btw, not sure urgent is a suitable name, since interrupt is often slow
 in kvm guest. And in fact virtio-net will probably use urgent
 descriptor for those packets (e.g stream packets who can be delayed a
 little bit to batch more bytes from userspace) who was not urgent
 compared to other packets.

 Yes but we are asking for an interrupt before event index is reached
 because something is waiting for the packet to be transmitted.
 I couldn't come up with a better name.


Ok.

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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-22 Thread Michael S. Tsirkin 
On Mon, Sep 22, 2014 at 05:55:23PM +0800, Jason Wang wrote:
 On 09/22/2014 02:55 PM, Michael S. Tsirkin wrote:
  On Mon, Sep 22, 2014 at 11:30:23AM +0800, Jason Wang wrote:
  On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
  Il 19/09/2014 09:10, Jason Wang ha scritto:
   
  -  if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
  +  if (vq-urgent || !vhost_has_feature(vq, 
  VIRTIO_RING_F_EVENT_IDX)) {
  So the urgent descriptor only work when event index was not enabled?
  This seems suboptimal, we may still want to benefit from event index
  even if urgent descriptor is used. Looks like we need return true here
  when vq-urgent is true?
  Its ||, not .
 
  Without event index, all descriptors are treated as urgent.
 
  Paolo
 
  The problem is if vq-urgent is true, the patch checks
  VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
  virtqueue_enable_cb() regardless of event index feature and cleared
  unconditionally in virtqueue_disable_cb().
  The reverse actually, right?
 
 Ah, right.
 
  So virtqueue_enable_cb() was
  used to not only publish a new event index but also enable the urgent
  descriptor. And virtqueue_disable_cb() disabled all interrupts including
  the urgent descriptor. Guest won't get urgent interrupts by just adding
  virtqueue_add_outbuf_urgent() since what it needs is to enable and
  disable interrupt for !urgent descriptor.
  Right, we want a new API that advances event index but does not
  set VRING_AVAIL_F_NO_INTERRUPT.
  IMO still want to set VRING_AVAIL_F_NO_INTERRUPT when handling tx
  interrupts, to avoid interrupt storms.
 
 I see, so urgent descriptor needs to be disabled in this case. But vhost
 parts need a little big changes, we could not just check
 VRING_AVAIL_F_NO_INTERRUPT when vq-urgent is true. If event index is
 enabled, we still need to check used event to make sure the current tx
 delayed interrupt works.
 
 But just re-using VRING_AVAIL_F_NO_INTERRUPT for urgent descriptor may
 not work in some case. I see codes of virtqueue_get_buf() that may
 breaks this:
 
 /* If we expect an interrupt for the next entry, tell
 host  
  
 
  * by writing event index and flush out the write
 before
  
 
  * the read in the next get_buf call. */
 if (!(vq-vring.avail-flags  VRING_AVAIL_F_NO_INTERRUPT)) {
 vring_used_event(vq-vring) = vq-last_used_idx;
 virtio_mb(vq-weak_barriers);
 }


Right, we need to change this path to use a private flag.

 Consider if only urgent descriptor is enabled, this will publish used
 event which in fact enable lots of unnecessary interrupt. In fact I
 don't quite understand how the above lines is used. Virtio-net stop the
 queue before enable the tx interrupt in start_xmit(), so the above lines
 will not run at all.

True - this code is for drivers which process VQ without
disabling interrupts first. No rule says this is not allowed.

  Btw, not sure urgent is a suitable name, since interrupt is often slow
  in kvm guest. And in fact virtio-net will probably use urgent
  descriptor for those packets (e.g stream packets who can be delayed a
  little bit to batch more bytes from userspace) who was not urgent
  compared to other packets.
 
  Yes but we are asking for an interrupt before event index is reached
  because something is waiting for the packet to be transmitted.
  I couldn't come up with a better name.
 
 
 Ok.
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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-21 Thread Jason Wang 
On 09/20/2014 06:00 PM, Paolo Bonzini wrote:
 Il 19/09/2014 09:10, Jason Wang ha scritto:
  
 -  if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 +  if (vq-urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 So the urgent descriptor only work when event index was not enabled?
 This seems suboptimal, we may still want to benefit from event index
 even if urgent descriptor is used. Looks like we need return true here
 when vq-urgent is true?
 Its ||, not .

 Without event index, all descriptors are treated as urgent.

 Paolo


The problem is if vq-urgent is true, the patch checks
VRING_AVAIL_F_NO_INTERRUPT bit. This bit were set unconditionally in
virtqueue_enable_cb() regardless of event index feature and cleared
unconditionally in virtqueue_disable_cb(). So virtqueue_enable_cb() was
used to not only publish a new event index but also enable the urgent
descriptor. And virtqueue_disable_cb() disabled all interrupts including
the urgent descriptor. Guest won't get urgent interrupts by just adding
virtqueue_add_outbuf_urgent() since what it needs is to enable and
disable interrupt for !urgent descriptor.

Btw, not sure urgent is a suitable name, since interrupt is often slow
in kvm guest. And in fact virtio-net will probably use urgent
descriptor for those packets (e.g stream packets who can be delayed a
little bit to batch more bytes from userspace) who was not urgent
compared to other packets.


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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-20 Thread Paolo Bonzini 
Il 19/09/2014 09:10, Jason Wang ha scritto:
   
  -  if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
  +  if (vq-urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 So the urgent descriptor only work when event index was not enabled?
 This seems suboptimal, we may still want to benefit from event index
 even if urgent descriptor is used. Looks like we need return true here
 when vq-urgent is true?

Its ||, not .

Without event index, all descriptors are treated as urgent.

Paolo

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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-19 Thread Jason Wang 
On 07/01/2014 06:49 PM, Michael S. Tsirkin wrote:
 Signed-off-by: Michael S. Tsirkin m...@redhat.com
 ---
  drivers/vhost/vhost.h | 19 +--
  drivers/vhost/net.c   | 30 +-
  drivers/vhost/scsi.c  | 23 +++
  drivers/vhost/test.c  |  5 +++--
  drivers/vhost/vhost.c | 23 ---
  5 files changed, 68 insertions(+), 32 deletions(-)

 diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
 index 3eda654..61ca542 100644
 --- a/drivers/vhost/vhost.h
 +++ b/drivers/vhost/vhost.h
[...]
  EXPORT_SYMBOL_GPL(vhost_add_used_n);
 @@ -1433,12 +1439,13 @@ static bool vhost_notify(struct vhost_dev *dev, 
 struct vhost_virtqueue *vq)
   unlikely(vq-avail_idx == vq-last_avail_idx))
   return true;
  
 - if (!vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {
 + if (vq-urgent || !vhost_has_feature(vq, VIRTIO_RING_F_EVENT_IDX)) {

So the urgent descriptor only work when event index was not enabled?
This seems suboptimal, we may still want to benefit from event index
even if urgent descriptor is used. Looks like we need return true here
when vq-urgent is true?

Another question is whether or not we need to do this a new flag.
Technically we can do it purely in guest side through event index, this
can help to eliminate both the changes in host and a new feature bit.
   __u16 flags;
   if (__get_user(flags, vq-avail-flags)) {
   vq_err(vq, Failed to get flags);
   return true;
   }
 + vq-urgent = false;
   return !(flags  VRING_AVAIL_F_NO_INTERRUPT);
   }
   old = vq-signalled_used;
 @@ -1468,9 +1475,10 @@ EXPORT_SYMBOL_GPL(vhost_signal);
  /* And here's the combo meal deal.  Supersize me! */
  void vhost_add_used_and_signal(struct vhost_dev *dev,
  struct vhost_virtqueue *vq,
 +bool urgent,
  unsigned int head, int len)
  {
 - vhost_add_used(vq, head, len);
 + vhost_add_used(vq, urgent, head, len);
   vhost_signal(dev, vq);
  }
  EXPORT_SYMBOL_GPL(vhost_add_used_and_signal);
 @@ -1478,9 +1486,10 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal);
  /* multi-buffer version of vhost_add_used_and_signal */
  void vhost_add_used_and_signal_n(struct vhost_dev *dev,
struct vhost_virtqueue *vq,
 +  bool urgent,
struct vring_used_elem *heads, unsigned count)
  {
 - vhost_add_used_n(vq, heads, count);
 + vhost_add_used_n(vq, urgent, heads, count);
   vhost_signal(dev, vq);
  }
  EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);

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Re: [PATCH RFC 2/2] vhost: support urgent descriptors

2014-09-19 Thread Jason Wang 
On 07/01/2014 06:49 PM, Michael S. Tsirkin wrote:
 Signed-off-by: Michael S. Tsirkin m...@redhat.com
 ---
  drivers/vhost/vhost.h | 19 +--
  drivers/vhost/net.c   | 30 +-
  drivers/vhost/scsi.c  | 23 +++
  drivers/vhost/test.c  |  5 +++--
  drivers/vhost/vhost.c | 23 ---
  5 files changed, 68 insertions(+), 32 deletions(-)

 diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
 index 3eda654..61ca542 100644
 --- a/drivers/vhost/vhost.h
 +++ b/drivers/vhost/vhost.h
 @@ -96,6 +96,9 @@ struct vhost_virtqueue {
   /* Last used index value we have signalled on */
   bool signalled_used_valid;
  
 + /* Urgent descriptor was used */
 + bool urgent;
 +
   /* Log writes to used structure. */
   bool log_used;
   u64 log_addr;
 @@ -138,20 +141,24 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, 
 void __user *argp);
  int vhost_vq_access_ok(struct vhost_virtqueue *vq);
  int vhost_log_access_ok(struct vhost_dev *);
  
 -int vhost_get_vq_desc(struct vhost_virtqueue *,
 +int vhost_get_vq_desc(struct vhost_virtqueue *, bool *urgent,
 struct iovec iov[], unsigned int iov_count,
 unsigned int *out_num, unsigned int *in_num,
 struct vhost_log *log, unsigned int *log_num);
  void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
  
  int vhost_init_used(struct vhost_virtqueue *);
 -int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
 -int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
 -  unsigned count);
 -void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue *,
 +int vhost_add_used(struct vhost_virtqueue *, bool urgent,
 +unsigned int head, int len);
 +int vhost_add_used_n(struct vhost_virtqueue *, bool urgent,
 +  struct vring_used_elem *heads, unsigned count);
 +void vhost_add_used_and_signal(struct vhost_dev *,
 +struct vhost_virtqueue *,
 +bool urgent,
  unsigned int id, int len);
  void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue 
 *,
 -struct vring_used_elem *heads, unsigned count);
 +  bool urgent,
 +  struct vring_used_elem *heads, unsigned count);
  void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
  void vhost_disable_notify(struct vhost_dev *, struct vhost_virtqueue *);
  bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
 diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
 index 8dae2f7..5f0567f 100644
 --- a/drivers/vhost/net.c
 +++ b/drivers/vhost/net.c
 @@ -48,9 +48,11 @@ MODULE_PARM_DESC(experimental_zcopytx, Enable Zero Copy 
 TX;
   * status internally; used for zerocopy tx only.
   */
  /* Lower device DMA failed */
 -#define VHOST_DMA_FAILED_LEN 3
 +#define VHOST_DMA_FAILED_LEN 4
  /* Lower device DMA done */
 -#define VHOST_DMA_DONE_LEN   2
 +#define VHOST_DMA_DONE_LEN   3
 +/* Lower device DMA in progress, urgent bit set */
 +#define VHOST_DMA_URGENT 2
  /* Lower device DMA in progress */
  #define VHOST_DMA_IN_PROGRESS1
  /* Buffer unused */
 @@ -284,11 +286,13 @@ static void vhost_zerocopy_signal_used(struct vhost_net 
 *net,
   container_of(vq, struct vhost_net_virtqueue, vq);
   int i, add;
   int j = 0;
 + bool urgent = false;
  
   for (i = nvq-done_idx; i != nvq-upend_idx; i = (i + 1) % UIO_MAXIOV) {
   if (vq-heads[i].len == VHOST_DMA_FAILED_LEN)
   vhost_net_tx_err(net);
   if (VHOST_DMA_IS_DONE(vq-heads[i].len)) {
 + urgent = urgent || vq-heads[i].len == VHOST_DMA_URGENT;

A problem is len was either VHOST_DMA_DONE_LEN or VHOST_DMA_FAILED_LEN
here. Looks like we need another new VHOST_DMA_DONE_LEN_URGENT and
change the len to this value if it was an urgent descriptor in zerocopy
callback.
   vq-heads[i].len = VHOST_DMA_CLEAR_LEN;
   ++j;
   } else
 @@ -296,7 +300,7 @@ static void vhost_zerocopy_signal_used(struct vhost_net 
 *net,
   }
   while (j) {
   add = min(UIO_MAXIOV - nvq-done_idx, j);
 - vhost_add_used_and_signal_n(vq-dev, vq,
 + vhost_add_used_and_signal_n(vq-dev, vq, urgent,
   vq-heads[nvq-done_idx], add);
   nvq-done_idx = (nvq-done_idx + add) % UIO_MAXIOV;
   j -= add;
 @@ -363,6 +367,7 @@ static void handle_tx(struct vhost_net *net)
   zcopy = nvq-ubufs;
  
   for (;;) {
 + bool urgent;
   /* Release DMAs done buffers first */
   if (zcopy)
   vhost_zerocopy_signal_used(net, vq);
 @@ -374,7 +379,7 @@ static void

[PATCH RFC 2/2] vhost: support urgent descriptors

2014-07-01 Thread Michael S. Tsirkin 
Signed-off-by: Michael S. Tsirkin m...@redhat.com
---
 drivers/vhost/vhost.h | 19 +--
 drivers/vhost/net.c   | 30 +-
 drivers/vhost/scsi.c  | 23 +++
 drivers/vhost/test.c  |  5 +++--
 drivers/vhost/vhost.c | 23 ---
 5 files changed, 68 insertions(+), 32 deletions(-)

diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index 3eda654..61ca542 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -96,6 +96,9 @@ struct vhost_virtqueue {
/* Last used index value we have signalled on */
bool signalled_used_valid;
 
+   /* Urgent descriptor was used */
+   bool urgent;
+
/* Log writes to used structure. */
bool log_used;
u64 log_addr;
@@ -138,20 +141,24 @@ long vhost_vring_ioctl(struct vhost_dev *d, int ioctl, 
void __user *argp);
 int vhost_vq_access_ok(struct vhost_virtqueue *vq);
 int vhost_log_access_ok(struct vhost_dev *);
 
-int vhost_get_vq_desc(struct vhost_virtqueue *,
+int vhost_get_vq_desc(struct vhost_virtqueue *, bool *urgent,
  struct iovec iov[], unsigned int iov_count,
  unsigned int *out_num, unsigned int *in_num,
  struct vhost_log *log, unsigned int *log_num);
 void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
 
 int vhost_init_used(struct vhost_virtqueue *);
-int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
-int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
-unsigned count);
-void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue *,
+int vhost_add_used(struct vhost_virtqueue *, bool urgent,
+  unsigned int head, int len);
+int vhost_add_used_n(struct vhost_virtqueue *, bool urgent,
+struct vring_used_elem *heads, unsigned count);
+void vhost_add_used_and_signal(struct vhost_dev *,
+  struct vhost_virtqueue *,
+  bool urgent,
   unsigned int id, int len);
 void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue *,
-  struct vring_used_elem *heads, unsigned count);
+bool urgent,
+struct vring_used_elem *heads, unsigned count);
 void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
 void vhost_disable_notify(struct vhost_dev *, struct vhost_virtqueue *);
 bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 8dae2f7..5f0567f 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -48,9 +48,11 @@ MODULE_PARM_DESC(experimental_zcopytx, Enable Zero Copy TX;
  * status internally; used for zerocopy tx only.
  */
 /* Lower device DMA failed */
-#define VHOST_DMA_FAILED_LEN   3
+#define VHOST_DMA_FAILED_LEN   4
 /* Lower device DMA done */
-#define VHOST_DMA_DONE_LEN 2
+#define VHOST_DMA_DONE_LEN 3
+/* Lower device DMA in progress, urgent bit set */
+#define VHOST_DMA_URGENT   2
 /* Lower device DMA in progress */
 #define VHOST_DMA_IN_PROGRESS  1
 /* Buffer unused */
@@ -284,11 +286,13 @@ static void vhost_zerocopy_signal_used(struct vhost_net 
*net,
container_of(vq, struct vhost_net_virtqueue, vq);
int i, add;
int j = 0;
+   bool urgent = false;
 
for (i = nvq-done_idx; i != nvq-upend_idx; i = (i + 1) % UIO_MAXIOV) {
if (vq-heads[i].len == VHOST_DMA_FAILED_LEN)
vhost_net_tx_err(net);
if (VHOST_DMA_IS_DONE(vq-heads[i].len)) {
+   urgent = urgent || vq-heads[i].len == VHOST_DMA_URGENT;
vq-heads[i].len = VHOST_DMA_CLEAR_LEN;
++j;
} else
@@ -296,7 +300,7 @@ static void vhost_zerocopy_signal_used(struct vhost_net 
*net,
}
while (j) {
add = min(UIO_MAXIOV - nvq-done_idx, j);
-   vhost_add_used_and_signal_n(vq-dev, vq,
+   vhost_add_used_and_signal_n(vq-dev, vq, urgent,
vq-heads[nvq-done_idx], add);
nvq-done_idx = (nvq-done_idx + add) % UIO_MAXIOV;
j -= add;
@@ -363,6 +367,7 @@ static void handle_tx(struct vhost_net *net)
zcopy = nvq-ubufs;
 
for (;;) {
+   bool urgent;
/* Release DMAs done buffers first */
if (zcopy)
vhost_zerocopy_signal_used(net, vq);
@@ -374,7 +379,7 @@ static void handle_tx(struct vhost_net *net)
  % UIO_MAXIOV == nvq-done_idx))
break;
 
-   head = vhost_get_vq_desc(vq, vq-iov,
+   head = vhost_get_vq_desc(vq, urgent, vq-iov,