Re: [Qemu-devel] [PATCH v1 07/22] migration: introduce a new QEMUFile impl based on QIOChannel

[Dr. David Alan Gilbert]

* Daniel P. Berrange (address@hidden) wrote:
> On Tue, Feb 02, 2016 at 05:06:24PM +0000, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrange (address@hidden) wrote:
> > > Introduce a new QEMUFile implementation that is based on
> > > the QIOChannel objects. This impl is different from existing
> > > impls in that there is no file descriptor that can be made
> > > available, as some channels may be based on higher level
> > > protocols such as TLS.
> > > 
> > > Although the QIOChannel based implementation can trivially
> > > provide a bi-directional stream, initially we have separate
> > > functions for opening input & output directions to fit with
> > > the expectation of the current QEMUFile interface.
> > > 
> > > Signed-off-by: Daniel P. Berrange <address@hidden>
> > > ---
> > >  include/migration/qemu-file.h |   4 +
> > >  migration/Makefile.objs       |   1 +
> > >  migration/qemu-file-channel.c | 201 
> > > ++++++++++++++++++++++++++++++++++++++++++
> > >  3 files changed, 206 insertions(+)
> > >  create mode 100644 migration/qemu-file-channel.c
> 
> > > +static ssize_t channel_writev_buffer(void *opaque,
> > > +                                     struct iovec *iov,
> > > +                                     int iovcnt,
> > > +                                     int64_t pos)
> > > +{
> > > +    QIOChannel *ioc = QIO_CHANNEL(opaque);
> > > +    ssize_t done = 0;
> > > +    ssize_t want = iov_size(iov, iovcnt);
> > > +    struct iovec oldiov = { NULL, 0 };
> > > +
> > > +    while (done < want) {
> > > +        ssize_t len;
> > > +        struct iovec *cur = iov;
> > > +        int curcnt = iovcnt;
> > > +
> > > +        channel_skip_iov(done, &cur, &curcnt, &oldiov);
> > > +
> > > +        len = qio_channel_writev(ioc, cur, curcnt, NULL);
> > > +        if (oldiov.iov_base) {
> > > +            /* Restore original caller's info in @iov */
> > > +            cur[0].iov_base = oldiov.iov_base;
> > > +            cur[0].iov_len = oldiov.iov_len;
> > > +            oldiov.iov_base = NULL;
> > > +            oldiov.iov_len = 0;
> > > +        }
> > > +        if (len == QIO_CHANNEL_ERR_BLOCK) {
> > > +            qio_channel_wait(ioc, G_IO_OUT);
> > > +            continue;
> > > +        }
> > > +        if (len < 0) {
> > > +            /* XXX handle Error objects */
> > > +            return -EIO;
> > 
> > It's best to return 'len' rather than lose what little
> > error information you had (similarly below).
> 
> In this case 'len' is in fact just '-1', as all error
> info is in the Error ** parameter, but the QEMUFile API
> contract requires an errno value. So we don't have much
> choice but to return a fixed EIO - returning 'len' would
> also be a fixed errno - whichever errno corresponds to
> the value -1.

Oh, I see - that's a shame that they didn't pass the
error along, but yeh if they're just returning -1 that makes
sense.

> I'd like to switch QEMUFile over to use Error **errp
> parameters for error reporting, so that we can make
> detailed error info available throughout the migration
> I/O code. That ought to wait until after this series
> is done though, to avoid complicating it yet more.

OK, you could use a local_error I guess;  I just worry
about not having any error information at all.

Dave

> 
> > 
> > > +        }
> > > +
> > > +        done += len;
> > > +    }
> > > +    return done;
> > > +}
> > > +
> > > +
> > > +static ssize_t channel_get_buffer(void *opaque,
> > > +                                  uint8_t *buf,
> > > +                                  int64_t pos,
> > > +                                  size_t size)
> > > +{
> > > +    QIOChannel *ioc = QIO_CHANNEL(opaque);
> > > +    ssize_t ret;
> > > +
> > > + reread:
> > > +    ret = qio_channel_read(ioc, (char *)buf, size, NULL);
> > > +    if (ret < 0) {
> > > +        if (ret == QIO_CHANNEL_ERR_BLOCK) {
> > > +            qio_channel_yield(ioc, G_IO_IN);
> > > +            goto reread;
> > > +        } else {
> > > +            /* XXX handle Error * object */
> > > +            return -EIO;
> > > +        }
> > > +    }
> > > +    return ret;
> > 
> > 
> > I'd prefer a loop to a goto; generally the only places we
> > use goto is an error exit.
> > 
> >   do {
> >        ret = qio_channel_read(ioc, (char *)buf, size, NULL);
> >        if (ret == QIO_CHANNEL_ERR_BLOCK) {
> >            qio_channel_yield(ioc, G_IO_IN);
> >        }
> >   } while (ret == QIO_CHANNEL_ERR_BLOCK);
> > 
> >   return ret;
> > 
> > and IMHO the loop is clearer.
> 
> Ok, will change that.
> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / address@hidden / Manchester, UK