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    SubjectRe: [PATCH 6/6] vhost_net: don't poll on -EFAULT
    On Mon, Apr 16, 2012 at 04:28:10PM +0800, Jason Wang wrote:
    > On 04/16/2012 03:16 PM, Michael S. Tsirkin wrote:
    > >On Mon, Apr 16, 2012 at 02:08:33PM +0800, Jason Wang wrote:
    > >>Currently, we restart tx polling unconditionally when sendmsg()
    > >>fails. This would cause unnecessary wakeups of vhost wokers as it's
    > >>only needed when the socket send buffer were exceeded.
    > >Why is this a problem?
    >
    > This issue is when guest driver is able to hit the -EFAULT, vhost
    > discard the the descriptor and restart the polling. This would wake
    > vhost thread and repeat the loop again which waste cpu.

    Does same thing happen if we get an error from copy from user?

    > Another possible solution is don't discard the descriptor.
    > >
    > >>Fix this by
    > >>restart the tx polling only when sendmsg() returns value other than
    > >>-EFAULT.
    > >>
    > >>Signed-off-by: Jason Wang<jasowang@redhat.com>
    > >>---
    > >> drivers/vhost/net.c | 3 ++-
    > >> 1 files changed, 2 insertions(+), 1 deletions(-)
    > >>
    > >>diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
    > >>index 29abd65..035fa95 100644
    > >>--- a/drivers/vhost/net.c
    > >>+++ b/drivers/vhost/net.c
    > >>@@ -262,7 +262,8 @@ static void handle_tx(struct vhost_net *net)
    > >> UIO_MAXIOV;
    > >> }
    > >> vhost_discard_vq_desc(vq, 1);
    > >>- tx_poll_start(net, sock);
    > >>+ if (err != -EFAULT)
    > >>+ tx_poll_start(net, sock);
    > >> break;
    > >> }
    > >> if (err != len)
    > >--
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