[Qemu-devel] Re: [PATCH 0/4] Block DMA helpers (v2)

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
>>   The qemu host interface is at the ring element level, leaving the 
>> task of gathering ring elements to make a request (and gathering 
>> segments to make a scatter/gather list) to users.
>
> I don't think that's totally accurate.  Where the host and guest 
> interface differ is that instead of having the symmetric version of 
> this, namely:
>
> vdev->buf_added(vq, sg, out_num, in_num, data);
>
> Which then would require some sort of:
>
> put_buf(vq, data, len)
>
> We have a virtqueue_pop(elem) where elem contains all of the 
> information passed to add_buf.  We have a virtqueue_push() function 
> that's analogous to the above put_buf() and then we have a separate 
> virtio_notify() function which would be the symmetric form of kick().  
> The names are different but the basic functions are the same.
>
> Other than the difference of having split sg lists and using a 
> structure, I don't see how they are different.
>

You're right - I didn't dig deep enough.

We could replace

    struct iovec in_sg[VIRTQUEUE_MAX_SIZE];
    struct iovec out_sg[VIRTQUEUE_MAX_SIZE];

by QEMUSGList, which would be a minor improvement, but I thought things 
were very different for some reason.

>>
>> I think we should make the qemu virtio host interface talk at the 
>> request level, rather than the ring entry level.  If we do that, 
>> we'll get a cleaner, easier to use interface.
>
> Can you give an example?  I'm having a hard time seeing what you mean.

An example would be how the code is structured now (as opposed to how I 
thought it was structured).

-- 
error compiling committee.c: too many arguments to function