Date:   Thu, 16 Mar 2023 10:25:12 +0100
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Andrey Zhadchenko <andrey.zhadchenko@...tuozzo.com>,
        eperezma@...hat.com, netdev@...r.kernel.org, stefanha@...hat.com,
        linux-kernel@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v2 3/8] vringh: replace kmap_atomic() with kmap_local_page()

On giovedì 16 marzo 2023 09:09:29 CET Stefano Garzarella wrote:
> On Wed, Mar 15, 2023 at 10:12 PM Fabio M. De Francesco
> 
> <fmdefrancesco@...il.com> wrote:
> > On martedì 14 marzo 2023 04:56:08 CET Jason Wang wrote:
> > > On Thu, Mar 2, 2023 at 7:34 PM Stefano Garzarella <sgarzare@...hat.com>
> > 
> > wrote:
> > > > kmap_atomic() is deprecated in favor of kmap_local_page().
> > > 
> > > It's better to mention the commit or code that introduces this.
> > > 
> > > > 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.
> > > > 
> > > > kmap_atomic() is implemented like a kmap_local_page() which also
> > > > disables
> > > > page-faults and preemption (the latter only for !PREEMPT_RT kernels,
> > > > otherwise it only disables migration).
> > > > 
> > > > 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(),
> > > 
> > > 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:
> > > 
> > > 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
> > kmap_local_page() is not suited for any use in atomic context (you are
> > mentioning spinlocks).
> > 
> > The commit message (that I know pretty well since it's the exact copy, 
word
> > by word, of my boiler plate commits)
> 
> I hope it's not a problem for you, should I mention it somehow?

Sorry, I had missed your last message when I wrote a another message few 
minutes ago in this thread.

Obviously, I'm happy that my commit message it's being reused. As I said in 
the other message I would appreciate some kind of crediting me as the author.

I proposed a means you can use, but feel free to ignore my suggestion and do 
differently if you prefer to.

Again thanks,

Fabio

> I searched for the last commits that made a similar change and found
> yours that explained it perfectly ;-)
> 
> Do I need to rephrase?
> 
> > explains that kmap_local_page() is perfectly
> > usable in atomic context (including interrupts).
> > 
> > I don't know this code, however I am not able to see why these vringh 
IOTLB
> > helpers cannot work if used under spinlocks. Can you please elaborate a
> > little 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
> > don't need any boolean to tell whether or not the code is running in 
atomic
> > context.
> > 
> > Please take a look at the Highmem documentation which has been recently
> > 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 
the
> > whole picture.
> 
> Thanks for your useful info!
> Stefano

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