Re: [PATCH RFC v2 03/10] vringh: reset kiov 'consumed' field in __vringh_iov()

[Jason Wang]

On 2021/2/1 下午6:21, Stefano Garzarella wrote:
> On Mon, Feb 01, 2021 at 01:40:01PM +0800, Jason Wang wrote:
> > On 2021/1/28 下午10:41, Stefano Garzarella wrote:
> > > __vringh_iov() overwrites the contents of riov and wiov, in fact it
> > > resets the 'i' and 'used' fields, but also the consumed field should
> > > be reset to avoid an inconsistent state.
> > >
> > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> >
> >
> > I had a question(I remember we had some discussion like this but I 
> > forget the conclusion):
>
> Sorry, I forgot to update you.
>
> > I see e.g in vringh_getdesc_kern() it has the following comment:
> >
> > /*
> >  * Note that you may need to clean up riov and wiov, even on error!
> >  */
> >
> > So it looks to me the correct way is to call vringh_kiov_cleanup() 
> > before?
>
> Looking at the code the right pattern should be:
>
>     vringh_getdesc_*(..., &out_iov, &in_iov, ...);
>
>     // use out_iov and in_iov
>
>     vringh_kiov_cleanup(&out_iov);
>     vringh_kiov_cleanup(&in_iov);
>
> This because vringh_getdesc_*() calls __vringh_iov() where 
> resize_iovec() is called to allocate the iov wrapped by 'struct 
> vringh_kiov' and vringh_kiov_cleanup() frees that memory.
>
> Looking better, __vringh_iov() is able to extend a 'vringh_kiov' 
> pre-allocated, so in order to avoid to allocate and free the iov for 
> each request we can avoid to call vringh_kiov_cleanup(), but this 
> patch is needed to avoid an inconsistent state.
>
> And also patch "vdpa_sim: cleanup kiovs in vdpasim_free()" is required 
> to free the iov when the device is going away.
>
> Does that make sense to you?


Make sense.


> Maybe I should add a comment in vringh.c to explain this better.


Yes, please.

Thanks


> Thanks,
> Stefano
>
> > Thanks
> >
> >
> > > ---
> > >  drivers/vhost/vringh.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > > index f68122705719..bee63d68201a 100644
> > > --- a/drivers/vhost/vringh.c
> > > +++ b/drivers/vhost/vringh.c
> > > @@ -290,9 +290,9 @@ __vringh_iov(struct vringh *vrh, u16 i,
> > >          return -EINVAL;
> > >      if (riov)
> > > -        riov->i = riov->used = 0;
> > > +        riov->i = riov->used = riov->consumed = 0;
> > >      if (wiov)
> > > -        wiov->i = wiov->used = 0;
> > > +        wiov->i = wiov->used = wiov->consumed = 0;
> > >      for (;;) {
> > >          void *addr;