Re: [Qemu-devel] [PATCH 6/9] nbd: BLOCK_STATUS for standard get_block_status function: client part

[Eric Blake]

On 03/12/2018 08:13 AM, Vladimir Sementsov-Ogievskiy wrote:
> > We should probably validate that the length field is a multiple of 
> > min_block (if a server tells us that all operations must be 512-byte 
> > aligned, then reports an extent that is smaller than 512 bytes, we 
> > have no way to ask for the status of the second half of the sector). 
> > Probably also something that needs to be explicitly stated in the NBD 
> > spec. [1]
>
> related question: what server should reply on zero-length request? I'll add
>
> +     if (!bytes) {
> +         *pnum = 0;
> +         return 0;
> +     }
>
> to nbd_client_co_block_status, to prevent such situation, but looks like 
> spec lacks the information.

nbd.git commit ee926037 mentions that NBD_CMD_READ, _WRITE, _TRIM, and 
_WRITE_ZEROES have unspecified behavior for zero-length transactions; we 
should do the same for NBD_CMD_BLOCK_STATUS.  But in the meantime, 
handling it gracefully with a no-op reply (the way qemu.git commit 
ef8c887e handles 0-length structured read) is fine.

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org