Re: [PATCH v4 0/5] vhost-user block device backend implementation

[Coiby Xu]

> > we still need customized vu_message_read because libvhost-user assumes
> > we will always get a full-size VhostUserMsg and hasn't taken care of
> > this short read case. I will improve libvhost-user's vu_message_read
> > by making it keep reading from socket util getting enough bytes. I
> > assume short read is a rare case thus introduced performance penalty
> > would be negligible.

> In any case, please make sure that we use the QIOChannel functions
> called from a coroutine in QEMU so that it will never block, but the
> coroutine can just yield while it's waiting for more bytes.

But if I am not wrong, libvhost-user is supposed to be indepdent from
the main QEMU code. So it can't use the QIOChannel functions if we
simply modify exiting vu_message_read to address the short read issue.
In v3 & v4, I extended libvhost-user to allow vu_message_read to be
replaced by one which will depend on the main QEMU code. I'm not sure
which way is better.

On Thu, Feb 27, 2020 at 6:02 PM Kevin Wolf <address@hidden> wrote:
>
> Am 27.02.2020 um 10:53 hat Coiby Xu geschrieben:
> > Thank you for reminding me of this socket short read issue! It seems
> > we still need customized vu_message_read because libvhost-user assumes
> > we will always get a full-size VhostUserMsg and hasn't taken care of
> > this short read case. I will improve libvhost-user's vu_message_read
> > by making it keep reading from socket util getting enough bytes. I
> > assume short read is a rare case thus introduced performance penalty
> > would be negligible.
>
> In any case, please make sure that we use the QIOChannel functions
> called from a coroutine in QEMU so that it will never block, but the
> coroutine can just yield while it's waiting for more bytes.
>
> Kevin
>
> > On Thu, Feb 27, 2020 at 3:41 PM Stefan Hajnoczi <address@hidden> wrote:
> > >
> > > On Wed, Feb 26, 2020 at 11:18:41PM +0800, Coiby Xu wrote:
> > > > Hi Stefan,
> > > >
> > > > Thank you for reviewing my code!
> > > >
> > > > I tried to reach you on IRC. But somehow either you missed my message
> > > > or I missed your reply. So I will reply by email instead.
> > > >
> > > > If we use qio_channel_set_aio_fd_handler to monitor G_IO_IN event,
> > > > i.e. use vu_dispatch as the read handler, then we can re-use
> > > > vu_message_read. And "removing the blocking recv from libvhost-user"
> > > > isn't necessary because "the operation of poll() and ppoll() is not
> > > > affected by the O_NONBLOCK flag" despite that we use
> > > > qio_channel_set_blocking before calling qio_channel_set_aio_fd_handler
> > > > to make recv non-blocking.
> > >
> > > I'm not sure I understand.  poll() just says whether the file descriptor
> > > is readable.  It does not say whether enough bytes are readable :).  So
> > > our callback will be invoked if there is 1 byte ready, but when we try
> > > to read 20 bytes either it will block (without O_NONBLOCK) or return
> > > only 1 byte (with O_NONBLOCK).  Neither case is okay, so I expect that
> > > code changes will be necessary.
> > >
> > > But please go ahead and send the next revision and I'll take a look.
> > >
> > > Stefan
> >
> >
> >
> > --
> > Best regards,
> > Coiby
> >
>


-- 
Best regards,
Coiby