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

[Max Reitz <mreitz@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 <pl@kamp.de>
>>> ---
>>>   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