On Thu, Dec 06, 2012 at 12:03:43PM +0100, Sjur BRENDELAND wrote:
> Hi Michael,
>
> > > -struct vring_used_elem *vring_add_used_user(struct vring_host *vh,
> > > - unsigned int head, int len)
> > > +
> > > +static inline struct vring_used_elem *_vring_add_used(struct vring_host
> > *vh,
> > > + u32 head, u32 len,
> > > + bool (*cpy)(void *dst,
> > > + void *src,
> > > + size_t s),
> > > + void (*wbarrier)(void))
> > > {
> > > struct vring_used_elem *used;
> > > + u16 last_used;
> > >
> > > /* The virtqueue contains a ring of used buffers. Get a pointer to the
> > > * next entry in that used ring. */
> > > - used = &vh->vr.used->ring[vh->last_used_idx % vh->vr.num];
> > > - if (__put_user(head, &used->id)) {
> > > - pr_debug("Failed to write used id");
> > > + used = &vh->vr.used->ring[vh->last_used_idx & (vh->vr.num - 1)];
> > > + if (!cpy(&used->id, &head, sizeof(used->id)) ||
> > > + !cpy(&used->len, &len, sizeof(used->len)))
> > > return NULL;
> > > - }
> > > - if (__put_user(len, &used->len)) {
> > > - pr_debug("Failed to write used len");
> > > + wbarrier();
> > > + last_used = vh->last_used_idx + 1;
> > > + if (!cpy(&vh->vr.used->idx, &last_used, sizeof(vh->vr.used->idx)))
> > > return NULL;
> >
> > I think this is broken: we need a 16 bit access, this is
> > doing a memcpy which is byte by byte.
>
> I have played around with gcc and -O2 option, and it seems to me
> that GCC is smart enough to optimize the use of sizeof and memcpy
> into MOV operations. But my assembly knowledge is very rusty,
> so a second opinion on this would be good.
Yes but I don't think we should rely on this: this API is not guaranteed
to do the right thing and no one will bother telling us if it's
rewritten or something.
> > > - }
> > > - /* Make sure buffer is written before we update index. */
> > > - smp_wmb();
> > > - if (__put_user(vh->last_used_idx + 1, &vh->vr.used->idx)) {
> > > - pr_debug("Failed to increment used idx");
> > > - return NULL;
> > > - }
> > > - vh->last_used_idx++;
> > > + vh->last_used_idx = last_used;
> > > return used;
> > > }
> > > -EXPORT_SYMBOL(vring_add_used_user);
>
> Thanks,
> Sjur
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