Re: [PATCH] migration: add FEATURE_SEEKABLE to QIOChannelBlock

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

Hello Prasad,

On Friday, April 04, 2025 10:19 CEST, Prasad Pandit <ppandit@redhat.com> wrote:
> * IIUC, when _FEATURE_SEEKABLE is set, the channel I/O sequence
> eventually makes underlying preadv(2)/pwritev(2) calls, which use
> lseek(2) to adjust the stream r/w pointer with the given offset,
> before doing the r/w operation.
Almost. Unlike  lseek(2), pread(2) and co. do not have side effects on
the fd's offset.
From the man page:
> The pread() and pwrite() system calls are especially useful in
> multithreaded applications.  They allow multiple threads to
> perform I/O on the same file descriptor without being affected by
> changes to the file offset by other threads.

> * QIOChannelBlock has '.io_seek' routine corresponding to the lseek(2)
> call. It is not clear how setting QIO_CHANNEL_FEATURE_SEEKABLE helps
> for QIOChannelBlock streams.
It is a first step in making QIOChannelBlock compatible with
mapped-ram feature that requires it since mapped-ram uses
io_preadv and io_pwritev methods.

Best,
Marco