Re: [virtio-dev] Clarifications for the new DISCARD and WRITE ZEROES commands

[Jakub Jermar <jakub.jermar@kernkonzept.com>]

On 08/15/2018 05:47 PM, Paolo Bonzini wrote:
> On 15/08/2018 17:12, Jakub Jermar wrote:
>> In the absence of such a conforming patch, I would kindly ask someone to
>> clarify the relationship between the virtio_blk_discard_write_zeroes
>> structures and the virtio descriptor. [1] seems to indicate that there
>> is always one struct virtio_blk_discard_write_zeroes per descriptor, but
>> the wording is not very clear. If the containing descriptor must be a
>> multiple of 512 in size (because of u8 data[][512]),
> 
> The descriptors are intended to be a multiple of 16 bytes in size for
> discard and write zeroes.
> 
> In fact, even for reads/writes the descriptors are a multiple of the
> logical block size, not of 512.  That ought to be corrected as well.
> 
>> there is no way to
>> figure out how many of these structs it contains. So there must be only
>> one and the only way to add a segment is to chain descriptors.
> 
> No, it does not matter how descriptors are chained.  Data is always
> treated as if it was just one descriptor.
> 
>> Unless
>> the descriptor size is somehow allowed to be a multiple of sizeof(struct
>> virtio_blk_discard_write_zeroes) in which case it could be determined
>> and there may be several of these structures per descriptor.
>>
>> Also, why is it that there is discard_sector_alignment but no
>> write_zeroes_sector_alignment?
> 
> You can use discard_zeroes_alignment too if unmap=1 in the descriptor's
> flags.  If unmap=0, it doesn't matter.

Thank you, that was quite helpful.

Jakub