Re: [PATCH v2 07/14] vhost: Shadow virtqueue buffers forwarding

[Jason Wang]

在 2022/3/3 上午2:23, Eugenio Perez Martin 写道:
> > > +
> > > +static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,
> > > +                                VirtQueueElement *elem,
> > > +                                unsigned *head)
> > > +{
> > > +    unsigned avail_idx;
> > > +    vring_avail_t *avail = svq->vring.avail;
> > > +
> > > +    *head = svq->free_head;
> > > +
> > > +    /* We need some descriptors here */
> > > +    if (unlikely(!elem->out_num && !elem->in_num)) {
> > > +        qemu_log_mask(LOG_GUEST_ERROR,
> > > +            "Guest provided element with no descriptors");
> > > +        return false;
> > > +    }
> > > +
> > > +    vhost_vring_write_descs(svq, elem->out_sg, elem->out_num,
> > > +                            elem->in_num > 0, false);
> > > +    vhost_vring_write_descs(svq, elem->in_sg, elem->in_num, false, true);
> > I wonder instead of passing in/out separately and using the hint like
> > more_descs, is it better to simply pass the elem to
> > vhost_vrign_write_descs() then we know which one is the last that
> > doesn't depend on more_descs.
> I'm not sure I follow this.
>
> The purpose of vhost_vring_write_descs is to abstract the writing of a
> batch of descriptors, its chaining, etc. It accepts the write
> parameter just for the write flag. If we make elem as a parameter, we
> would need to duplicate that for loop for read and for write
> descriptors, isn't it?
>
> To duplicate the for loop is the way it is done in the kernel, but I
> actually think the kernel could benefit from abstracting both in the
> same function too. Please let me know if you think otherwise or I've
> missed your point.


Ok, so it's just a suggestion and we can do optimization on top for sure.

Thanks


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