[PATCH v2 4/4] kvm tools: Fix virtio console hangs by removing IRQ injection for tx path

[Asias He <asias.hejun@gmail.com>]

As virtio spec says:

"""
 Because this is high importance and low bandwidth, the current Linux
 implementation polls for the buffer to be used, rather than waiting
 for an interrupt, simplifying the implementation signicantly.
"""

drivers/char/virtio_console.c
 send_buf() {
 ...
	/* Tell Host to go! */
	virtqueue_kick(out_vq);
 ...
        while (!virtqueue_get_buf(out_vq, &len))
                cpu_relax();
 ...
 }

The console hangs can simply be reproduced by yes command which
gives tremendous console IOs and IRQs.

[   16.786440] irq 4: nobody cared (try booting with the "irqpoll" option)
[   16.786440] Pid: 1437, comm: yes Tainted: G        W 2.6.39-rc6+ #56
[   16.786440] Call Trace:
[   16.786440]  [<c16578eb>] __report_bad_irq+0x30/0x89
[   16.786440]  [<c10980e6>] note_interrupt+0x118/0x17a
[   16.786440]  [<c1096e7d>] handle_irq_event_percpu+0x168/0x179
[   16.786440]  [<c1096eba>] handle_irq_event+0x2c/0x46
[   16.786440]  [<c1098516>] ? unmask_irq+0x1e/0x1e
[   16.786440]  [<c1098566>] handle_level_irq+0x50/0x6e
[   16.786440]  <IRQ>  [<c102fa69>] ? do_IRQ+0x35/0x7f
[   16.786440]  [<c1665ea9>] ? common_interrupt+0x29/0x30
[   16.786440]  [<c16610d6>] ? _raw_spin_unlock_irqrestore+0x7/0x28
[   16.786440]  [<c1364f65>] ? hvc_write+0x88/0x9e
[   16.786440]  [<c1355500>] ? do_output_char+0x88/0x18a
[   16.786440]  [<c1355631>] ? process_output+0x2f/0x42
[   16.786440]  [<c1355af6>] ? n_tty_write+0x211/0x2dc
[   16.786440]  [<c1059d77>] ? try_to_wake_up+0x226/0x226
[   16.786440]  [<c13534a4>] ? tty_write+0x15e/0x1d1
[   16.786440]  [<c12c1644>] ? security_file_permission+0x22/0x26
[   16.786440]  [<c13558e5>] ? process_echoes+0x241/0x241
[   16.786440]  [<c10dd9d2>] ? vfs_write+0x84/0xd7
[   16.786440]  [<c1353346>] ? tty_write_lock+0x3d/0x3d
[   16.786440]  [<c10ddb92>] ? sys_write+0x3b/0x5d
[   16.786440]  [<c166594c>] ? sysenter_do_call+0x12/0x22
[   16.786440] handlers:
[   16.786440] [<c1351397>] (vp_interrupt+0x0/0x3a)
[   16.786440] Disabling IRQ #4

Signed-off-by: Asias He <asias.hejun@gmail.com>
---
 tools/kvm/virtio/console.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/tools/kvm/virtio/console.c b/tools/kvm/virtio/console.c
index f5449ba..f9031cb 100644
--- a/tools/kvm/virtio/console.c
+++ b/tools/kvm/virtio/console.c
@@ -166,13 +166,18 @@ static void virtio_console_handle_callback(struct kvm *self, void *param)
 
 	vq = param;
 
+	/*
+	 * The current Linux implementation polls for the buffer
+	 * to be used, rather than waiting for an interrupt.
+	 * So there is no need to inject an interrupt for the tx path.
+	 */
+
 	while (virt_queue__available(vq)) {
 		head = virt_queue__get_iov(vq, iov, &out, &in, self);
 		len = term_putc_iov(CONSOLE_VIRTIO, iov, out);
 		virt_queue__set_used_elem(vq, head, len);
 	}
 
-	virt_queue__trigger_irq(vq, virtio_console_pci_device.irq_line, &cdev.isr, self);
 }
 
 static bool virtio_console_pci_io_out(struct kvm *self, u16 port, void *data, int size, u32 count)
-- 
1.7.5.1