Re: [PATCH v5 1/6] QIOChannel: Add io_writev_zerocopy & io_flush_zerocopy callbacks

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Nov 12, 2021 at 10:13:01AM +0000, Daniel P. Berrangé wrote:
> On Fri, Nov 12, 2021 at 02:10:36AM -0300, Leonardo Bras wrote:
> > Adds io_writev_zerocopy and io_flush_zerocopy as optional callback to QIOChannelClass,
> > allowing the implementation of zerocopy writes by subclasses.
> > 
> > How to use them:
> > - Write data using qio_channel_writev_zerocopy(),
> > - Wait write completion with qio_channel_flush_zerocopy().
> > 
> > Notes:
> > As some zerocopy implementations work asynchronously, it's
> > recommended to keep the write buffer untouched until the return of
> > qio_channel_flush_zerocopy(), to avoid the risk of sending an updated
> > buffer instead of the one at the write.
> > 
> > As the new callbacks are optional, if a subclass does not implement them, then:
> > - io_writev_zerocopy will return -1,
> > - io_flush_zerocopy will return 0 without changing anything.
> > 
> > Also, some functions like qio_channel_writev_full_all() were adapted to
> > receive a flag parameter. That allows shared code between zerocopy and
> > non-zerocopy writev.
> > 
> > Signed-off-by: Leonardo Bras <leobras@redhat.com>
> > ---
> >  include/io/channel.h | 93 ++++++++++++++++++++++++++++++++++++++------
> >  io/channel.c         | 65 +++++++++++++++++++++++++------
> >  2 files changed, 135 insertions(+), 23 deletions(-)
> > 
> > diff --git a/include/io/channel.h b/include/io/channel.h
> > index 88988979f8..a19c09bb84 100644
> > --- a/include/io/channel.h
> > +++ b/include/io/channel.h

> > +/**
> > + * qio_channel_writev_zerocopy:
> > + * @ioc: the channel object
> > + * @iov: the array of memory regions to write data from
> > + * @niov: the length of the @iov array
> > + * @errp: pointer to a NULL-initialized error object
> > + *
> > + * Behaves like qio_channel_writev_full_all_flags, but may write
> 
> qio_channel_writev
> 
> > + * data asynchronously while avoiding unnecessary data copy.
> > + * This function may return before any data is actually written,
> > + * but should queue every buffer for writing.
> 
> Callers mustn't rely on "should" docs - they must rely on the
> return value indicating how many bytes were accepted.

Also mention that this requires locked memory and can/will fail if
insufficient locked memory is available.


Regards,
Daniel
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