Re: [PATCH] migration: add FEATURE_SEEKABLE to QIOChannelBlock

["Marco Cavenati" <Marco.Cavenati@eurecom.fr>]

On Friday, April 04, 2025 12:14 CEST, Prasad Pandit <ppandit@redhat.com> wrote:

> * If the r/w pointer adjustment (lseek(2)) is not required, then why
> set the  '*_FEATURE_SEEKABLE'  flag?

The QIO_CHANNEL_FEATURE_SEEKABLE flag is set to indicate that
the channel supports seekable operations. This flag is more about
signaling capability rather than dictating the use of the specific 
lseek(2) function.

> * The qio_channel_block_preadv/pwritev functions defined above, which
> shall be called via  '->io_preadv' and '->io_pwritev' methods, appear
> to call bdrv_readv/writev_vmstate() functions. Do those functions need
> to adjust (lseek(2)) the stream r/w pointers?

In this case, I don't think any lseek(2) is involved, instead some flavor
of pread(2) is used, which, according to the man page, requires that
> The file referenced by fd must be capable of seeking.
because pread(2) internally manages seeking without modifying the
file descriptor's offset.

Let me split the question here:
* Do those functions need to seek into the channel?
Yes
* Do those functions lseek(2) the stream r/w pointers?
No, they do not use lseek(2). The seeking is managed internally by the
pread(2) and co. functions, which perform I/O operations at
specified offsets without altering the file descriptor's position.

I hope this clarifies :)

Best,
Marco