Re: [PATCH v2 4/8] vringh: support VA with iotlb

[Stefano Garzarella]

On Fri, Mar 17, 2023 at 10:49:27AM +0100, Eugenio Perez Martin wrote:
> On Thu, Mar 16, 2023 at 5:07 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> > On Fri, Mar 3, 2023 at 3:39 PM Eugenio Perez Martin <eperezma@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Mar 2, 2023 at 12:35 PM Stefano Garzarella <sgarzare@xxxxxxxxxx> wrote:
> > > >
> > > > vDPA supports the possibility to use user VA in the iotlb messages.
> > > > So, let's add support for user VA in vringh to use it in the vDPA
> > > > simulators.
> > > >
> > > > Signed-off-by: Stefano Garzarella <sgarzare@xxxxxxxxxx>
> > > > ---
> > > >
> > > > Notes:
> > > >     v2:
> > > >     - replace kmap_atomic() with kmap_local_page() [see previous patch]
> > > >     - fix cast warnings when build with W=1 C=1
> > > >
> > > >  include/linux/vringh.h            |   5 +-
> > > >  drivers/vdpa/mlx5/net/mlx5_vnet.c |   2 +-
> > > >  drivers/vdpa/vdpa_sim/vdpa_sim.c  |   4 +-
> > > >  drivers/vhost/vringh.c            | 247 ++++++++++++++++++++++++------
> > > >  4 files changed, 205 insertions(+), 53 deletions(-)
> > > >
> >
> > [...]
> >
> > >
> > > It seems to me iotlb_translate_va and iotlb_translate_pa are very
> > > similar, their only difference is that the argument is that iov is
> > > iovec instead of bio_vec. And how to fill it, obviously.
> > >
> > > It would be great to merge both functions, only differing with a
> > > conditional on vrh->use_va, or generics, or similar. Or, if following
> > > the style of the rest of vringh code, to provide a callback to fill
> > > iovec (although I like conditional more).
> > >
> > > However I cannot think of an easy way to perform that without long
> > > macros or type erasure.
> >
> > Thank you for pushing me :-)
> > I finally managed to avoid code duplication (partial patch attached,
> > but not yet fully tested).
> >
> > @Jason: with this refactoring I removed copy_to_va/copy_to_pa, so I
> > also avoided getu16_iotlb_va/pa.
> >
> > I will send the full patch in v3, but I would like to get your opinion
> > first ;-)
> >
> >
> >
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index 0ba3ef809e48..71dd67700e36 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -1096,8 +1096,7 @@ EXPORT_SYMBOL(vringh_need_notify_kern);
> >
> >  static int iotlb_translate(const struct vringh *vrh,
> >                            u64 addr, u64 len, u64 *translated,
> > -                          struct bio_vec iov[],
> > -                          int iov_size, u32 perm)
> > +                          void *iov, int iov_size, bool iovec, u32 perm)
>
> I think this is an improvement, but we're doing type erasure here. I
> don't think it is a big deal since the function is not exported, it's
> pretty contained in this file, so I'd ack this version too. I'm just
> throwing ideas here:
>
> a) typedef the union {iovec, bio_vec} and use that type in the parameter.
>
> As a drawback, that union feels out of place in this file. Is this the
> only place where it is needed? I don't see other similar uses in the
> kernel.

iov_iter has something similar, but they are const pointers, so IIUC
it is not supposed to be used to set the bvec contents, just iterate it.

Anyway I thought something similar and should be doable, but since
it was internal API I went to type erasure.

> b) To convert from iov to bio_iov at return
> The drawback is the extra processing if the compiler is not smart
> enough to inline it. I prefer the previous one but I didn't want to
> omit it, just in case.

Yep, I prefer too the previous one, so let's go in that direction for
v3 ;-)

Thanks,
Stefano