virtio: console: Don't block entire guest if host doesn't read data

[net-next-2.6.git] / drivers / char / virtio_console.c

diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
index 942a9826bd23ed64b83095fcdeb65ed059c9ce02..0f69c5ec0ecd48a2af0045b02f9a6894b368a6b6 100644 (file)
--- a/drivers/char/virtio_console.c
+++ b/drivers/char/virtio_console.c
@@ -459,9 +459,12 @@ static ssize_t send_buf(struct port *port, void *in_buf, size_t in_count,
 
        /*
         * Wait till the host acknowledges it pushed out the data we
-        * sent.  This is done for ports in blocking mode or for data
-        * from the hvc_console; the tty operations are performed with
-        * spinlocks held so we can't sleep here.
+        * sent.  This is done for data from the hvc_console; the tty
+        * operations are performed with spinlocks held so we can't
+        * sleep here.  An alternative would be to copy the data to a
+        * buffer and relax the spinning requirement.  The downside is
+        * we need to kmalloc a GFP_ATOMIC buffer each time the
+        * console driver writes something out.
         */
        while (!virtqueue_get_buf(out_vq, &len))
                cpu_relax();
@@ -596,6 +599,10 @@ static ssize_t port_fops_write(struct file *filp, const char __user *ubuf,
        ssize_t ret;
        bool nonblock;
 
+       /* Userspace could be out to fool us */
+       if (!count)
+               return 0;
+
        port = filp->private_data;
 
        nonblock = filp->f_flags & O_NONBLOCK;
@@ -622,6 +629,14 @@ static ssize_t port_fops_write(struct file *filp, const char __user *ubuf,
                goto free_buf;
        }
 
+       /*
+        * We now ask send_buf() to not spin for generic ports -- we
+        * can re-use the same code path that non-blocking file
+        * descriptors take for blocking file descriptors since the
+        * wait is already done and we're certain the write will go
+        * through to the host.
+        */
+       nonblock = true;
        ret = send_buf(port, buf, count, nonblock);
 
        if (nonblock && ret > 0)
@@ -642,7 +657,7 @@ static unsigned int port_fops_poll(struct file *filp, poll_table *wait)
        poll_wait(filp, &port->waitqueue, wait);
 
        ret = 0;
-       if (port->inbuf)
+       if (!will_read_block(port))
                ret |= POLLIN | POLLRDNORM;
        if (!will_write_block(port))
                ret |= POLLOUT;