From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Stefano Garzarella <sgarzare@redhat.com>,
        Jason Wang <jasowang@redhat.com>
Cc:     virtualization@lists.linux-foundation.org,
        Andrey Zhadchenko <andrey.zhadchenko@virtuozzo.com>,
        eperezma@redhat.com, netdev@vger.kernel.org, stefanha@redhat.com,
        linux-kernel@vger.kernel.org,
        "Michael S. Tsirkin" <mst@redhat.com>, kvm@vger.kernel.org
Subject: Re: [PATCH v2 3/8] vringh: replace kmap_atomic() with kmap_local_page()
Date:   Wed, 15 Mar 2023 22:12:04 +0100
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On marted=C3=AC 14 marzo 2023 04:56:08 CET Jason Wang wrote:
> On Thu, Mar 2, 2023 at 7:34=E2=80=AFPM Stefano Garzarella <sgarzare@redha=
t.com>=20
wrote:
> > kmap_atomic() is deprecated in favor of kmap_local_page().
>=20
> It's better to mention the commit or code that introduces this.
>=20
> > With kmap_local_page() the mappings are per thread, CPU local, can take
> > page-faults, and can be called from any context (including interrupts).
> > Furthermore, the tasks can be preempted and, when they are scheduled to
> > run again, the kernel virtual addresses are restored and still valid.
> >=20
> > kmap_atomic() is implemented like a kmap_local_page() which also disabl=
es
> > page-faults and preemption (the latter only for !PREEMPT_RT kernels,
> > otherwise it only disables migration).
> >=20
> > The code within the mappings/un-mappings in getu16_iotlb() and
> > putu16_iotlb() don't depend on the above-mentioned side effects of
> > kmap_atomic(),
>=20
> Note we used to use spinlock to protect simulators (at least until
> patch 7, so we probably need to re-order the patches at least) so I
> think this is only valid when:
>=20
> The vringh IOTLB helpers are not used in atomic context (e.g spinlock,
> interrupts).

I'm probably missing some context but it looks that you are saying that=20
kmap_local_page() is not suited for any use in atomic context (you are=20
mentioning spinlocks).

The commit message (that I know pretty well since it's the exact copy, word=
 by=20
word, of my boiler plate commits) explains that kmap_local_page() is perfec=
tly=20
usable in atomic context (including interrupts).

I don't know this code, however I am not able to see why these vringh IOTLB=
=20
helpers cannot work if used under spinlocks. Can you please elaborate a lit=
tle=20
more?

> If yes, should we document this? (Or should we introduce a boolean to
> say whether an IOTLB variant can be used in an atomic context)?

Again, you'll have no problems from the use of kmap_local_page() and so you=
=20
don't need any boolean to tell whether or not the code is running in atomic=
=20
context.=20

Please take a look at the Highmem documentation which has been recently=20
reworked and extended by me: https://docs.kernel.org/mm/highmem.html

Anyway, I have been ATK 12 or 13 hours in a row. So I'm probably missing th=
e=20
whole picture.

Thanks,=20

=46abio

> Thanks
>=20
> > so that mere replacements of the old API with the new one
> > is all that is required (i.e., there is no need to explicitly add calls
> > to pagefault_disable() and/or preempt_disable()).
> >=20
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> >=20
> > Notes:
> >     v2:
> >     - added this patch since checkpatch.pl complained about deprecation
> >    =20
> >       of kmap_atomic() touched by next patch
> > =20
> >  drivers/vhost/vringh.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >=20
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index a1e27da54481..0ba3ef809e48 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -1220,10 +1220,10 @@ static inline int getu16_iotlb(const struct vri=
ngh
> > *vrh,>=20
> >         if (ret < 0)
> >        =20
> >                 return ret;
> >=20
> > -       kaddr =3D kmap_atomic(iov.bv_page);
> > +       kaddr =3D kmap_local_page(iov.bv_page);
> >=20
> >         from =3D kaddr + iov.bv_offset;
> >         *val =3D vringh16_to_cpu(vrh, READ_ONCE(*(__virtio16 *)from));
> >=20
> > -       kunmap_atomic(kaddr);
> > +       kunmap_local(kaddr);
> >=20
> >         return 0;
> > =20
> >  }
> >=20
> > @@ -1241,10 +1241,10 @@ static inline int putu16_iotlb(const struct vri=
ngh
> > *vrh,>=20
> >         if (ret < 0)
> >        =20
> >                 return ret;
> >=20
> > -       kaddr =3D kmap_atomic(iov.bv_page);
> > +       kaddr =3D kmap_local_page(iov.bv_page);
> >=20
> >         to =3D kaddr + iov.bv_offset;
> >         WRITE_ONCE(*(__virtio16 *)to, cpu_to_vringh16(vrh, val));
> >=20
> > -       kunmap_atomic(kaddr);
> > +       kunmap_local(kaddr);
> >=20
> >         return 0;
> > =20
> >  }
> >=20
> > --
> > 2.39.2