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

From: Stefan Hajnoczi
Subject: Re: [PATCH v4 0/5] vhost-user block device backend implementation
Date: Thu, 27 Feb 2020 07:41:14 +0000

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.


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