On Tue, Jan 04, 2011 at 10:45:13PM +0900, Yoshiaki Tamura wrote:
> 2011/1/4 Michael S. Tsirkin <mst@xxxxxxxxxx>:
> > On Tue, Jan 04, 2011 at 09:20:53PM +0900, Yoshiaki Tamura wrote:
> >> 2011/1/4 Michael S. Tsirkin <mst@xxxxxxxxxx>:
> >> > On Tue, Jan 04, 2011 at 08:02:54PM +0900, Yoshiaki Tamura wrote:
> >> >> 2010/11/29 Stefan Hajnoczi <stefanha@xxxxxxxxx>:
> >> >> > On Thu, Nov 25, 2010 at 6:06 AM, Yoshiaki Tamura
> >> >> > <tamura.yoshiaki@xxxxxxxxxxxxx> wrote:
> >> >> >> event-tap controls when to start FT transaction, and provides proxy
> >> >> >> functions to called from net/block devices. While FT transaction, it
> >> >> >> queues up net/block requests, and flush them when the transaction gets
> >> >> >> completed.
> >> >> >>
> >> >> >> Signed-off-by: Yoshiaki Tamura <tamura.yoshiaki@xxxxxxxxxxxxx>
> >> >> >> Signed-off-by: OHMURA Kei <ohmura.kei@xxxxxxxxxxxxx>
> >> >> >> ---
> >> >> >> Makefile.target | 1 +
> >> >> >> block.h | 9 +
> >> >> >> event-tap.c | 794 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> >> >> event-tap.h | 34 +++
> >> >> >> net.h | 4 +
> >> >> >> net/queue.c | 1 +
> >> >> >> 6 files changed, 843 insertions(+), 0 deletions(-)
> >> >> >> create mode 100644 event-tap.c
> >> >> >> create mode 100644 event-tap.h
> >> >> >
> >> >> > event_tap_state is checked at the beginning of several functions. If
> >> >> > there is an unexpected state the function silently returns. Should
> >> >> > these checks really be assert() so there is an abort and backtrace if
> >> >> > the program ever reaches this state?
> >> >> >
> >> >> >> +typedef struct EventTapBlkReq {
> >> >> >> + char *device_name;
> >> >> >> + int num_reqs;
> >> >> >> + int num_cbs;
> >> >> >> + bool is_multiwrite;
> >> >> >
> >> >> > Is multiwrite logging necessary? If event tap is called from within
> >> >> > the block layer then multiwrite is turned into one or more
> >> >> > bdrv_aio_writev() calls.
> >> >> >
> >> >> >> +static void event_tap_replay(void *opaque, int running, int reason)
> >> >> >> +{
> >> >> >> + EventTapLog *log, *next;
> >> >> >> +
> >> >> >> + if (!running) {
> >> >> >> + return;
> >> >> >> + }
> >> >> >> +
> >> >> >> + if (event_tap_state != EVENT_TAP_LOAD) {
> >> >> >> + return;
> >> >> >> + }
> >> >> >> +
> >> >> >> + event_tap_state = EVENT_TAP_REPLAY;
> >> >> >> +
> >> >> >> + QTAILQ_FOREACH(log, &event_list, node) {
> >> >> >> + EventTapBlkReq *blk_req;
> >> >> >> +
> >> >> >> + /* event resume */
> >> >> >> + switch (log->mode & ~EVENT_TAP_TYPE_MASK) {
> >> >> >> + case EVENT_TAP_NET:
> >> >> >> + event_tap_net_flush(&log->net_req);
> >> >> >> + break;
> >> >> >> + case EVENT_TAP_BLK:
> >> >> >> + blk_req = &log->blk_req;
> >> >> >> + if ((log->mode & EVENT_TAP_TYPE_MASK) == EVENT_TAP_IOPORT) {
> >> >> >> + switch (log->ioport.index) {
> >> >> >> + case 0:
> >> >> >> + cpu_outb(log->ioport.address, log->ioport.data);
> >> >> >> + break;
> >> >> >> + case 1:
> >> >> >> + cpu_outw(log->ioport.address, log->ioport.data);
> >> >> >> + break;
> >> >> >> + case 2:
> >> >> >> + cpu_outl(log->ioport.address, log->ioport.data);
> >> >> >> + break;
> >> >> >> + }
> >> >> >> + } else {
> >> >> >> + /* EVENT_TAP_MMIO */
> >> >> >> + cpu_physical_memory_rw(log->mmio.address,
> >> >> >> + log->mmio.buf,
> >> >> >> + log->mmio.len, 1);
> >> >> >> + }
> >> >> >> + break;
> >> >> >
> >> >> > Why are net tx packets replayed at the net level but blk requests are
> >> >> > replayed at the pio/mmio level?
> >> >> >
> >> >> > I expected everything to replay either as pio/mmio or as net/block.
> >> >>
> >> >> Stefan,
> >> >>
> >> >> After doing some heavy load tests, I realized that we have to
> >> >> take a hybrid approach to replay for now. This is because when a
> >> >> device moves to the next state (e.g. virtio decreases inuse) is
> >> >> different between net and block. For example, virtio-net
> >> >> decreases inuse upon returning from the net layer,
> >> >> but virtio-blk
> >> >> does that inside of the callback.
> >> >
> >> > For TX, virtio-net calls virtqueue_push from virtio_net_tx_complete.
> >> > For RX, virtio-net calls virtqueue_flush from virtio_net_receive.
> >> > Both are invoked from a callback.
> >> >
> >> >> If we only use pio/mmio
> >> >> replay, even though event-tap tries to replay net requests, some
> >> >> get lost because the state has proceeded already.
> >> >
> >> > It seems that all you need to do to avoid this is to
> >> > delay the callback?
> >>
> >> Yeah, if it's possible. But if you take a look at virtio-net,
> >> you'll see that virtio_push is called immediately after calling
> >> qemu_sendv_packet
> >> while virtio-blk does that in the callback.
> >
> > This is only if the packet was sent immediately.
> > I was referring to the case where the packet is queued.
>
> I see. I usually don't see packets get queued in the net layer.
> What would be the effect to devices? Restraint sending packets?
Yes.
> >
> >> >
> >> >> This doesn't
> >> >> happen with block, because the state is still old enough to
> >> >> replay. Note that using hybrid approach won't cause duplicated
> >> >> requests on the secondary.
> >> >
> >> > An assumption devices make is that a buffer is unused once
> >> > completion callback was invoked. Does this violate that assumption?
> >>
> >> No, it shouldn't. In case of net with net layer replay, we copy
> >> the content of the requests, and in case of block, because we
> >> haven't called the callback yet, the requests remains fresh.
> >>
> >> Yoshi
> >>
> >
> > Yes, as long as you copy it should be fine. Maybe it's a good idea for
> > event-tap to queue all packets to avoid the copy and avoid the need to
> > replay at the net level.
>
> If queuing works fine for the devices, it seems to be a good
> idea. I think the ordering issue doesn't happen still.
>
> Yoshi
If you replay and both net and pio level, it becomes complex.
Maybe it's ok, but certainly harder to reason about.
> >
> >> >
> >> > --
> >> > MST
> >> >
> >> >
> >
> >
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
