Re: [PATCH 2/4] block: Split padded I/O vectors exceeding IOV_MAX

[Eric Blake <eblake@redhat.com>]

On Fri, Mar 17, 2023 at 06:50:17PM +0100, Hanna Czenczek wrote:
> When processing vectored guest requests that are not aligned to the
> storage request alignment, we pad them by adding head and/or tail
> buffers for a read-modify-write cycle.
> 

> 
> To do this, the use of qemu_iovec_init_extended() in bdrv_pad_request()
> is effectively replaced by the new function bdrv_create_padded_qiov(),
> which not only wraps the request IOV with padding head/tail, but also
> ensures that the resulting vector will not have more than IOV_MAX
> elements.  Putting that functionality into qemu_iovec_init_extended() is
> infeasible because it requires allocating a bounce buffer; doing so
> would require many more parameters (buffer alignment, how to initialize
> the buffer, and out parameters like the buffer, its length, and the
> original elements), which is not reasonable.
> 
> Conversely, it is not difficult to move qemu_iovec_init_extended()'s
> functionality into bdrv_create_padded_qiov() by using public
> qemu_iovec_* functions, so that is what this patch does.
> 
> Because bdrv_pad_request() was the only "serious" user of
> qemu_iovec_init_extended(), the next patch will remove the latter
> function, so the functionality is not implemented twice.
> 
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2141964
> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> ---
>  block/io.c | 153 +++++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 143 insertions(+), 10 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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