On Mon, Jan 18, 2010 at 11:34:59AM +0100, Markus Armbruster wrote: > "Michael S. Tsirkin" <m...@redhat.com> writes: > > > On Sun, Jan 17, 2010 at 04:48:13PM +0200, Naphtali Sprei wrote: > >> Instead of using the field 'readonly' of the BlockDriverState struct for > >> passing the request, > >> pass the request in the flags parameter to the function. > >> > >> Signed-off-by: Naphtali Sprei <nsp...@redhat.com> > > > > Many changes seem to be about passing 0 to bdrv_open. This is not what > > the changelog says the patch does. Better split unrelated changes to a > > separate patch. > > > > One of the things you seem to do is get rid of BDRV_O_RDONLY. Why is > > this an improvement? Symbolic name like BDRV_O_RDONLY seems better than > > 0. > > BDRV_O_RDWR is a flag, just like BDRV_SNAPSHOT. We don't have > BDRV_DONT_SNAPSHOT, either.
Well, this just mirros the file access macros: we have RDONLY, WRONLY and RDRW. I assume this similarity is just historical? > The old code can't quite devide whether BDRV_O_RDWR is a flag, or > whether to use bits BDRV_O_ACCESS for an access mode, with possible > values BDRV_O_RDONLY and BDRV_O_RDWR. I asked Naphtali to clean this > up, and recommended to go with flag rather than access mode: > > In my opinion, any benefit in readability you might hope gain by > having BDRV_O_RDONLY is outweighed by the tortuous bit twiddling you > need to keep knowledge of its encoding out of its users. > > http://lists.nongnu.org/archive/html/qemu-devel/2009-12/msg02504.html > > [...] > >> @@ -985,6 +986,7 @@ static int img_snapshot(int argc, char **argv) > >> return 0; > >> } > >> action = SNAPSHOT_LIST; > >> + bdrv_oflags &= ~BDRV_O_RDWR; /* no need for RW */ > > > > bdrv_oflags = BDRV_O_RDONLY would be clearer, and no need > > for comment then? > > BDRV_O_RDWR is a flag, and this is the clean way to clear it. > > "bdrv_oflags = BDRV_O_RDONLY" assumes that everything but the access > mode in bdrv_oflags is clear. Tolerable, because the correctness > argument is fairly local, but the clean way to do it would be > > bdrv_oflags = (bdrv_oflags & ~ BDRV_O_ACCESS) | BDRV_O_RDONLY; > > That's what I meant by "tortuous bit twiddling". > > [...] Thinking about it, /* no need for RW */ comment can just go. But other places in code just do flags = 0 maybe they should all do &= ~BDRV_O_RDWR? I don't really have an opinion here but I do think this patch needs a better commit log (all it says "pass the request in the flags parameter to the function") and be split up: patch 1 - get rid of BDRV_O_RDONLY/BDRV_O_ACCESS patch 2 - pass the request in the flags parameter to the function patch 3 - any other fixups As it is, sometimes e.g. BDRV_O_RDWR is replaced with 0 sometimes as well, and it's hard to see why. -- MST
