linux-kernel - Re: [PATCH] drm/ttm: provide default page protection for UML

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Message-ID: <YTjusuDs4UhvCJuc@phenom.ffwll.local>
Date:   Wed, 8 Sep 2021 19:11:14 +0200
From:   Daniel Vetter <daniel@...ll.ch>
To:     David Gow <davidgow@...gle.com>
Cc:     Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>,
        Christian König <christian.koenig@....com>,
        Huang Rui <ray.huang@....com>, dri-devel@...ts.freedesktop.org,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        linux-um <linux-um@...ts.infradead.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>
Subject: Re: [PATCH] drm/ttm: provide default page protection for UML

On Sat, Sep 04, 2021 at 11:50:37AM +0800, David Gow wrote:
> On Thu, Sep 2, 2021 at 10:46 PM Daniel Vetter <daniel@...ll.ch> wrote:
> >
> > On Thu, Sep 02, 2021 at 07:19:01AM +0100, Anton Ivanov wrote:
> > > On 02/09/2021 06:52, Randy Dunlap wrote:
> > > > On 9/1/21 10:48 PM, Anton Ivanov wrote:
> > > > > On 02/09/2021 03:01, Randy Dunlap wrote:
> > > > > > boot_cpu_data [struct cpuinfo_um (on UML)] does not have a struct
> > > > > > member named 'x86', so provide a default page protection mode
> > > > > > for CONFIG_UML.
> > > > > >
> > > > > > Mends this build error:
> > > > > > ../drivers/gpu/drm/ttm/ttm_module.c: In function
> > > > > > ‘ttm_prot_from_caching’:
> > > > > > ../drivers/gpu/drm/ttm/ttm_module.c:59:24: error: ‘struct
> > > > > > cpuinfo_um’ has no member named ‘x86’
> > > > > >    else if (boot_cpu_data.x86 > 3)
> > > > > >                          ^
> > > > > >
> > > > > > Fixes: 3bf3710e3718 ("drm/ttm: Add a generic TTM memcpy move for
> > > > > > page-based iomem")
> > > > > > Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
> > > > > > Cc: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
> > > > > > Cc: Christian König <christian.koenig@....com>
> > > > > > Cc: Huang Rui <ray.huang@....com>
> > > > > > Cc: dri-devel@...ts.freedesktop.org
> > > > > > Cc: Jeff Dike <jdike@...toit.com>
> > > > > > Cc: Richard Weinberger <richard@....at>
> > > > > > Cc: Anton Ivanov <anton.ivanov@...bridgegreys.com>
> > > > > > Cc: linux-um@...ts.infradead.org
> > > > > > Cc: David Airlie <airlied@...ux.ie>
> > > > > > Cc: Daniel Vetter <daniel@...ll.ch>
> > > > > > ---
> > > > > >   drivers/gpu/drm/ttm/ttm_module.c |    4 ++++
> > > > > >   1 file changed, 4 insertions(+)
> > > > > >
> > > > > > --- linux-next-20210901.orig/drivers/gpu/drm/ttm/ttm_module.c
> > > > > > +++ linux-next-20210901/drivers/gpu/drm/ttm/ttm_module.c
> > > > > > @@ -53,6 +53,9 @@ pgprot_t ttm_prot_from_caching(enum ttm_
> > > > > >       if (caching == ttm_cached)
> > > > > >           return tmp;
> > > > > > +#ifdef CONFIG_UML
> > > > > > +    tmp = pgprot_noncached(tmp);
> > > > > > +#else
> > > > > >   #if defined(__i386__) || defined(__x86_64__)
> > > > > >       if (caching == ttm_write_combined)
> > > > > >           tmp = pgprot_writecombine(tmp);
> > > > > > @@ -69,6 +72,7 @@ pgprot_t ttm_prot_from_caching(enum ttm_
> > > > > >   #if defined(__sparc__)
> > > > > >       tmp = pgprot_noncached(tmp);
> > > > > >   #endif
> > > > > > +#endif
> > > > > >       return tmp;
> > > > > >   }
> > > > >
> > > > > Patch looks OK.
> > > > >
> > > > > I have a question though - why all of DRM is not !UML in config. Not
> > > > > like we can use them.
> > > >
> > > > I have no idea about that.
> > > > Hopefully one of the (other) UML maintainers can answer you.
> > >
> > > Touche.
> > >
> > > We will discuss that and possibly push a patch to !UML that part of the
> > > tree. IMHO it is not applicable.
> >
> > I thought kunit is based on top of uml, and we do want to eventually adopt
> > that. Especially for helper libraries like ttm.
> 
> UML is not actually a dependency for KUnit, so it's definitely
> possible to test things which aren't compatible with UML. (In fact,
> there's even now some tooling support to use qemu instead on a number
> of architectures.)
> 
> That being said, the KUnit tooling does use UML by default, so if it's
> not too difficult to keep some level of UML support, it'll make it a
> little easier (and faster) for people to run any KUnit tests.

Yeah my understanding is that uml is the quickest way to spawn a new
kernel, which kunit needs to run. And I really do like that idea, because
having virtualization support in cloud CI systems (which use containers
themselves) is a bit a fun exercise. The less we rely on virtual machines
in containers for that, the better.

Hence why I really like the uml approach for kunit.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch