Re: [PATCH v7] RISC-V: enable XIP

From: Vitaly Wool <vitaly.wool@konsulko.com>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>, Alex Ghiti <alex@ghiti.fr>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org, Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH v7] RISC-V: enable XIP
Date: Fri, 9 Apr 2021 16:42:12 +0200	[thread overview]
Message-ID: <CAM4kBBKyHSYz+NNDpT=fWseWccsQ4HZ3teBc01jYT2g8j7Ze2A@mail.gmail.com> (raw)
In-Reply-To: <YHBdzPsHantT9r8t@linux.ibm.com>

On Fri, Apr 9, 2021 at 3:59 PM Mike Rapoport <rppt@linux.ibm.com> wrote:
>
> On Fri, Apr 09, 2021 at 02:46:17PM +0200, David Hildenbrand wrote:
> > > > > Also, will that memory properly be exposed in the resource tree as
> > > > > System RAM (e.g., /proc/iomem) ? Otherwise some things (/proc/kcore)
> > > > > won't work as expected - the kernel won't be included in a dump.
> > > Do we really need a XIP kernel to included in kdump?
> > > And does not it sound weird to expose flash as System RAM in /proc/iomem? ;-)
> >
> > See my other mail, maybe we actually want something different.
> >
> > >
> > > > I have just checked and it does not appear in /proc/iomem.
> > > >
> > > > Ok your conclusion would be to have struct page, I'm going to implement this
> > > > version then using memblock as you described.
> > >
> > > I'm not sure this is required. With XIP kernel text never gets into RAM, so
> > > it does not seem to require struct page.
> > >
> > > XIP by definition has some limitations relatively to "normal" operation,
> > > so lack of kdump could be one of them.
> >
> > I agree.
> >
> > >
> > > I might be wrong, but IMHO, artificially creating a memory map for part of
> > > flash would cause more problems in the long run.
> >
> > Can you elaborate?
>
> Nothing particular, just a gut feeling. Usually, when you force something
> it comes out the wrong way later.

It's possible still that MTD_XIP is implemented allowing to write to
the flash used for XIP. While flash is being written, memory map
doesn't make sense at all. I can't come up with a real life example
when it can actually lead to problems but it is indeed weird when
System RAM suddenly becomes unreadable. I really don't think exposing
it in /proc/iomem is a good idea.

> > > BTW, how does XIP account the kernel text on other architectures that
> > > implement it?
> >
> > Interesting point, I thought XIP would be something new on RISC-V (well, at
> > least to me :) ). If that concept exists already, we better mimic what
> > existing implementations do.
>
> I had quick glance at ARM, it seems that kernel text does not have memory
> map and does not show up in System RAM.

Exactly, and I believe ARM64 won't do that too when it gets its own
XIP support (which is underway).

Best regards,
   Vitaly