Re: [Qemu-devel] virtio scsi host draft specification, v2

["Michael S. Tsirkin" <mst@redhat.com>]

BTW, I think Cc the virtio mailing list
in the next version is a good idea.

On Wed, Jun 01, 2011 at 05:59:28PM +0200, Paolo Bonzini wrote:
> On 06/01/2011 04:36 PM, Michael S. Tsirkin wrote:
> >Ah, I think I understand now. Both sense and data have in
> >fields that might only be used partially?
> >In that case I think I agree: it's best to require the use of separate
> >buffers for them, in this way used len will give you useful information
> >and you won't need sense_len and data_len: just a flag to
> >mark the fact that there*is*  a sense buffer following.
> 
> If the device wants a sense buffer to be there always, that's
> sensible.  No flag needed here, then.  Also, sense is always "in",
> there is no out.
> 
> But I do not understand how the used len helps me.  If I read the
> spec correctly, the length will be the number of bytes written, but
> this will always point to after the last field.  If sense or data
> are written partially, this will not be written in the fields---in
> fact, virtio_blk does contain both sense_len and residual.  Its
> sense field is fixed size, which is probably why it doesn't contain
> something like datain_size (there is just one variable-size field).
> 
> Strictly speaking I wouldn't need dataout_size too, because I have
> only one variable-size read-only field, but I prefer to be
> future-proof.
> 
> I think the following is a good compromise:
> 
> 1) keep dataout_size, datain_size and sense_size;
> 
> 2) dataout, datain and sense shall each start a separate buffer, and
> sense shall be contained in a single buffer; it is permissible to
> put sense and the subsequent fields in the same buffer.  This will
> make it easy for the QEMU implementation to pick up its iovecs.
> 
> It will also let the device detect mistakes in filling the data
> sizes.

I am not sure whether the length info in the header is redundant. If so, I
think it's better not to duplicate it on the assumption that this will
let us detect bugs: the bugs will be in the header as likely as not.
If the overlap is not complete, some redundancy is not too bad.

> In practice I expect 3 descriptors will be used (one direct
> for read-only stuff up to data; one possibly indirect for data; one
> direct for sense and other write-only stuff).
> 
> >However some questions:
> >1. I think you don't need numdatain/numdataout: each
> >buffer can include in and out segments. Just tell device how many
> >buffers are there.
> 
> I don't understand.
> 
> Paolo

I think I didn't express myself clearly. Follows a somewhat
lengthy background to make sure we use the same terms.
Feel free to ignore until ----


There seems to be some confusion about the terminology: the term buffers
is confusing. I'm guilty of mixing terms too. Let's refer to the virtio
spec.  There are two kinds of entities there:

- descriptor: each descriptor points to a location
  in memory. It can be in or out. descriptors can be
  chained together.

- head: internally, points to a chain of descriptors, both in and out.
  * driver makes head available to device, thus adding some
    memory for out + some memory for in.
  * device uses the memory, and reports to driver how much
    in memory was used. The assumption is always that
    device consumes in memory from the beginning
    in a contigious way. That's why length is enough.
    That's also why it's called add_buf: conceptually
    it is a single buffer.

Drivers and devices never operate in terms of descriptors: these are
internal to the virtio ring transport.  Instead, they always operate in
term of heads. virtio interface does let you pass in
s/g entries but this is an artifact of linux.

At the moment many devices in qemu assume that drivers put some info in
specific descriptors. This is not a good idea, they should always
operate in terms of offsets from start of descriptor.  vhost does this
correctly BTW.  I posted a patch to fix that previously, need to dust it
up and merge.

----

Now to our problem:
As far as I can tell there are two input buffers in each request: sense
and data. Right?

If sense is fixed length, we can simply put it first, have device write
sense then data.  This does not seem too limiting, if you want a lot of
flexibility sense length can be in device config.  If we don't want to
limit ourselves to fixed length sense, we would have driver use two
heads for a request.  This is possible but one needs to be careful in
the driver to make sure there's enough space for both requests. Maybe
add_bufs API to add multiple bufs might be a good idea here.


-- 
MST