Hi On Thu, Feb 27, 2020 at 11:55 AM Kevin Wolf <kw...@redhat.com> wrote: > > Am 27.02.2020 um 11:28 hat Coiby Xu geschrieben: > > > > 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. > > The way your latest patches have it, with a separate read function, > works for me.
Done right, I am not against it, fwiw > You could probably change libvhost-user to reimplement the same > functionality, and it might be an improvement for other users of the > library, but it's also code duplication and doesn't provide more value > in the context of the vhost-user export in QEMU. > > The point that's really important to me is just that we never block when > we run inside QEMU because that would actually stall the guest. This > means busy waiting in a tight loop until read() returns enough bytes is > not acceptable in QEMU. In the context of vhost-user, local unix sockets with short messages (do we have >1k messages?), I am not sure if this is really a problem. And isn't it possible to run libvhost-user in its own thread for this series? > > Kevin > > > On Thu, Feb 27, 2020 at 6:02 PM Kevin Wolf <kw...@redhat.com> 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 <stefa...@redhat.com> > > > > 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 > > >
