Re: [Qemu-devel] [Qemu-block] [RFC PATCH] block: local qiov helper

[Kevin Wolf <kwolf@redhat.com>]

Am 29.01.2019 um 12:18 hat Vladimir Sementsov-Ogievskiy geschrieben:
> 29.01.2019 13:32, Stefan Hajnoczi wrote:
> > On Tue, Jan 29, 2019 at 6:24 PM Vladimir Sementsov-Ogievskiy
> > <vsementsov@virtuozzo.com> wrote:
> >> 29.01.2019 6:31, Stefan Hajnoczi wrote:
> >>> On Fri, Jan 25, 2019 at 07:46:01PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> >> Hmm. In this case we definitely will have tiny extra memory usage, but we gain beautiful
> >> readability.
> >>
> >> So, like this:
> >>
> >> diff --git a/include/qemu/iov.h b/include/qemu/iov.h
> >> index 5f433c7768..53de1b38bb 100644
> >> --- a/include/qemu/iov.h
> >> +++ b/include/qemu/iov.h
> >> @@ -134,9 +134,31 @@ typedef struct QEMUIOVector {
> >>        struct iovec *iov;
> >>        int niov;
> >>        int nalloc;
> >> -    size_t size;
> >> +    union {
> >> +        struct {
> >> +            void *__unused_iov_base;
> > 
> > This could be struct iovec *iov for nalloc > 0 users.  Must resist hackiness! :)
> 
> can't be iov, gcc complains:
> include/qemu/iov.h:139:27: error: duplicate member ‘iov’
>               struct iovec *iov;

I think Stefan meant moving the existing iov here. But then we would
have to make sure that it's never used for local qiovs, which would
probably involve touching a lot more code.

Your original suggestion to put nalloc there looks a bit more practical
if we want to save those bytes.

Kevin