Re: [Qemu-devel] [PATCH 2/5] block/vpc: simplify vpc_read

[Max Reitz]

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 <address@hidden>
> > > ---
> > >   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