From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:36354) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YQGBZ-0008Dw-Dq for qemu-devel@nongnu.org; Tue, 24 Feb 2015 09:10:11 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1YQGBT-0002u3-PK for qemu-devel@nongnu.org; Tue, 24 Feb 2015 09:10:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:56724) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1YQGBT-0002td-HJ for qemu-devel@nongnu.org; Tue, 24 Feb 2015 09:09:59 -0500 Message-ID: <54EC862B.1050602@redhat.com> Date: Tue, 24 Feb 2015 09:09:47 -0500 From: Max Reitz MIME-Version: 1.0 References: <1424701661-21241-1-git-send-email-pl@kamp.de> <1424701661-21241-3-git-send-email-pl@kamp.de> <54EB7177.9040801@redhat.com> <54EC1DB1.6000308@kamp.de> In-Reply-To: <54EC1DB1.6000308@kamp.de> Content-Type: text/plain; charset=iso-8859-15; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH 2/5] block/vpc: simplify vpc_read List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Lieven , qemu-devel@nongnu.org Cc: kwolf@redhat.com, carnold@suse.com, jcody@redhat.com, stefanha@redhat.com On 2015-02-24 at 01:44, Peter Lieven wrote: > Am 23.02.2015 um 19:29 schrieb Max Reitz: >> On 2015-02-23 at 09:27, Peter Lieven wrote: >>> Signed-off-by: Peter Lieven >>> --- >>> block/vpc.c | 116 >>> +++++++++++++++++++++++++++-------------------------------- >>> 1 file changed, 52 insertions(+), 64 deletions(-) >>> >>> diff --git a/block/vpc.c b/block/vpc.c >>> index 326c2bb..4e5ba85 100644 >>> --- a/block/vpc.c >>> +++ b/block/vpc.c >>> @@ -497,40 +497,70 @@ static int vpc_get_info(BlockDriverState *bs, >>> BlockDriverInfo *bdi) >>> return 0; >>> } >>> -static int vpc_read(BlockDriverState *bs, int64_t sector_num, >>> - uint8_t *buf, int nb_sectors) >>> +static int64_t coroutine_fn >>> vpc_co_get_block_status(BlockDriverState *bs, >>> + int64_t sector_num, int nb_sectors, int *pnum) >> >> How about just putting the function header here? If you really have >> to move vpc_co_get_block_status() up, I'd rather like it to be in a >> separate patch. >> >> Second, while apparently vpc_read() is actually called in a >> coroutine, that is pretty hard to know. Most importantly, it's not >> marked as a coroutine_fn. Therefore I don't think it's a good idea to >> call vpc_co_get_block_status() directly; I'd vote for either using >> bdrv_get_block_status(), or moving the content of >> vpc_co_get_block_status() to a non-coroutine_fn (it doesn't contain >> an coroutine-related function calls, so this is fine) and then making >> vpc_co_get_block_status() a wrapper around that, or just dropping >> this patch. >> >> The latter I'm proposing because I don't really see what this patch >> improves. The previous vpc_read() function was pretty >> straightforward, too, and I don't think it was unbearably longer. >> >> One could argue that the coroutine_fn stuff doesn't really matter in >> this situation because it doesn't actually do anything right now and >> vpc_co_get_block_status() does not call any other coroutine_fn >> functions in turn; however, it is a semantic contract established by >> include/block/coroutine.h and as far as I remember, Stefan did >> eventually want to have something to error out on compile-time if a >> non-coroutine_fn function calls a coroutine_fn. I don't like breaking >> this contract even if it's not bad in this specific case. >> >> Considering you probably think bdrv_get_block_status() to be too much >> overhead (it will fall down to the protocol layer on VHD_FIXED) and >> you probably find making vpc_read() shorter justified (again, which I >> don't necessarily), I think moving the contents of >> vpc_co_get_block_status() to a non-coroutine_fn might be the best way >> to go. >> >>> { >>> BDRVVPCState *s = bs->opaque; >>> - int ret; >>> - int64_t offset; >>> - int64_t sectors, sectors_per_block; >>> - VHDFooter *footer = (VHDFooter *) s->footer_buf; >>> + VHDFooter *footer = (VHDFooter*) s->footer_buf; >>> + int64_t start, offset; >>> + bool allocated; >>> + int n; >>> if (be32_to_cpu(footer->type) == VHD_FIXED) { >>> - return bdrv_read(bs->file, sector_num, buf, nb_sectors); >>> + *pnum = nb_sectors; >>> + return BDRV_BLOCK_RAW | BDRV_BLOCK_OFFSET_VALID | >>> BDRV_BLOCK_DATA | >>> + (sector_num << BDRV_SECTOR_BITS); >>> } >>> - while (nb_sectors > 0) { >>> - offset = get_sector_offset(bs, sector_num, 0); >>> - sectors_per_block = s->block_size >> BDRV_SECTOR_BITS; >>> - sectors = sectors_per_block - (sector_num % >>> sectors_per_block); >>> - if (sectors > nb_sectors) { >>> - sectors = nb_sectors; >>> + offset = get_sector_offset(bs, sector_num, 0); >>> + start = offset; >>> + allocated = (offset != -1); >>> + *pnum = 0; >>> + >>> + do { >>> + /* All sectors in a block are contiguous (without using the >>> bitmap) */ >>> + n = ROUND_UP(sector_num + 1, s->block_size / BDRV_SECTOR_SIZE) >>> + - sector_num; >>> + n = MIN(n, nb_sectors); >>> + >>> + *pnum += n; >>> + sector_num += n; >>> + nb_sectors -= n; >>> + >>> + if (allocated) { >>> + return BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID | start; >>> } >>> + if (nb_sectors == 0) { >>> + break; >>> + } >>> + offset = get_sector_offset(bs, sector_num, 0); >>> + } while (offset == -1); >>> - if (offset == -1) { >>> - memset(buf, 0, sectors * BDRV_SECTOR_SIZE); >>> - } else { >>> - ret = bdrv_pread(bs->file, offset, buf, >>> - sectors * BDRV_SECTOR_SIZE); >>> - if (ret != sectors * BDRV_SECTOR_SIZE) { >>> + return 0; >>> +} >>> + >>> +static int vpc_read(BlockDriverState *bs, int64_t sector_num, >>> + uint8_t *buf, int nb_sectors) >>> +{ >>> + int ret, n; >>> + int64_t ret2; >>> + >>> + while (nb_sectors > 0) { >>> + ret2 = vpc_co_get_block_status(bs, sector_num, nb_sectors, >>> &n); >>> + >> >> Superfluous whitespace here. >> >>> + if (ret2 & BDRV_BLOCK_OFFSET_VALID) { >>> + ret = bdrv_pread(bs->file, ret2 & BDRV_SECTOR_MASK, buf, >>> + n * BDRV_SECTOR_SIZE); >>> + if (ret != n * BDRV_SECTOR_SIZE) { >>> return -1; >> >> Please make that "return ret" (and possibly "if (ret < 0)", if you >> want to). > > I took this from the orignal vpc_read function. Maybe the author also > wanted > to catch short reads. There are no short reads (any more), though. ;-) > I tend to drop the whole patch anyway. I was tempted to use that new > vpc_co_get_block_status > function somehow because I saw that part of its logic is in vpc_read > as well. > > If it should stay maybe it would be an option to inline vpc_read in > vpc_co_read (and the same for write)? Oh, I totally missed the vpc_co_read(). That would make sense, because you actually can call vpc_co_get_block_status() from there, or you just add the coroutine_fn specifier to vpc_read(). It's up to you, I'm fine with all of dropping, inlining, and making vpc_read() a coroutine_fn. Max