Re: [PATCH] migration: add FEATURE_SEEKABLE to QIOChannelBlock

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

On Tue, Apr 15, 2025 at 05:27:02PM +0530, Prasad Pandit wrote:
> On Tue, 15 Apr 2025 at 16:33, Daniel P. Berrangé <berrange@redhat.com> wrote:
> > Because that's what the QEMU API specification declares
> >  * Not all implementations will support this facility, so may report
> >  * an error. To avoid errors, the caller may check for the feature
> >  * flag QIO_CHANNEL_FEATURE_SEEKABLE prior to calling this method.
> >
> > and what the QEMU API impl defines
> >
> >       if (!qio_channel_has_feature(ioc, QIO_CHANNEL_FEATURE_SEEKABLE)) {
> >           error_setg_errno(errp, EINVAL, "Requested channel is not seekable");
> >           return -1;
> >       }
> 
> * ie. _FEATURE_SEEKABLE should be set iff the underlying
> channel/stream supports seek (random access) functionality, right?
> That is quite connected with the lseek(2) OR ->io_seek() and such
> support, no?

The current semantics of QIO_CHANNEL_FEATURE_SEEKABLE is that it indicates
whether qio_channel_{preadv/pwritev} are usable or not.

For plain files that may also indicate that lseek is possible. For the
block driver based channel that doesn't map as there's no concept of
current file marker for that.

With regards,
Daniel
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