Re: [Qemu-devel] Re: [PATCH 09/21] Introduce event-tap.

[Yoshiaki Tamura]

2011/1/4 Michael S. Tsirkin <address@hidden>:
> On Tue, Jan 04, 2011 at 10:45:13PM +0900, Yoshiaki Tamura wrote:
>> 2011/1/4 Michael S. Tsirkin <address@hidden>:
>> > On Tue, Jan 04, 2011 at 09:20:53PM +0900, Yoshiaki Tamura wrote:
>> >> 2011/1/4 Michael S. Tsirkin <address@hidden>:
>> >> > On Tue, Jan 04, 2011 at 08:02:54PM +0900, Yoshiaki Tamura wrote:
>> >> >> 2010/11/29 Stefan Hajnoczi <address@hidden>:
>> >> >> > On Thu, Nov 25, 2010 at 6:06 AM, Yoshiaki Tamura
>> >> >> > <address@hidden> 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 <address@hidden>
>> >> >> >> Signed-off-by: OHMURA Kei <address@hidden>
>> >> >> >> ---
>> >> >> >>  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.

Michael,

It seems queuing at event-tap like in net layer works for devices
that use qemu_send_packet_async as you suggested.  But for those
that use qemu_send_packet, we still need to copy the contents
just like net layer queuing does, and net level replay should be
kept to handle it.
Thanks,

Yoshi

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>> >
>> >> >
>> >> > --
>> >> > MST
>> >> >
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