Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly

From: Rob Clark <robdclark@gmail.com>
To: Will Deacon <will@kernel.org>
Cc: Joerg Roedel <joro@8bytes.org>,
	"list@263.net:IOMMU DRIVERS <iommu@lists.linux-foundation.org>,
	Joerg Roedel <joro@8bytes.org>,"
	<iommu@lists.linux-foundation.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	aarch64-laptops@lists.linaro.org,
	Rob Clark <robdclark@chromium.org>,
	Robin Murphy <robin.murphy@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>,
	Bartosz Golaszewski <brgl@bgdev.pl>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Joe Perches <joe@perches.com>,
	"moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
	<linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>
Subject: Re: [PATCH v2] iommu: add support for drivers that manage iommu explicitly
Date: Tue, 23 Jul 2019 10:40:55 -0700	[thread overview]
Message-ID: <CAF6AEGtL6gqtbmtksf7zCSGrFOEj0ynq-2nwvizLLiS0FTwHpg@mail.gmail.com> (raw)
In-Reply-To: <20190723153822.gm4ossn43nvqbyak@willie-the-truck>

On Tue, Jul 23, 2019 at 8:38 AM Will Deacon <will@kernel.org> wrote:
>
> On Mon, Jul 22, 2019 at 09:23:48AM -0700, Rob Clark wrote:
> > On Mon, Jul 22, 2019 at 8:48 AM Joerg Roedel <joro@8bytes.org> wrote:
> > >
> > > On Mon, Jul 22, 2019 at 08:41:34AM -0700, Rob Clark wrote:
> > > > It is set by the driver:
> > > >
> > > > https://patchwork.freedesktop.org/patch/315291/
> > > >
> > > > (This doesn't really belong in devicetree, since it isn't a
> > > > description of the hardware, so the driver is really the only place to
> > > > set this.. which is fine because it is about a detail of how the
> > > > driver works.)
> > >
> > > It is more a detail about how the firmware works. IIUC the problem is
> > > that the firmware initializes the context mappings for the GPU and the
> > > OS doesn't know anything about that and just overwrites them, causing
> > > the firmware GPU driver to fail badly.
> > >
> > > So I think it is the task of the firmware to tell the OS not to touch
> > > the devices mappings until the OS device driver takes over. On x86 there
> > > is something similar with the RMRR/unity-map tables from the firmware.
> > >
> >
> > Bjorn had a patchset[1] to inherit the config from firmware/bootloader
> > when arm-smmu is probed which handles that part of the problem.  My
> > patch is intended to be used on top of his patchset.  This seems to me
> > like the best solution, if we don't have control over the firmware.
>
> Hmm, but the feedback from Robin on the thread you cite was that this should
> be generalised to look more like RMRR, so there seems to be a clear message
> here.
>

Perhaps it is a lack of creativity, or lack of familiarity w/ iommu vs
virtualization, but I'm not quite seeing how RMRR would help.. in
particular when dealing with both DT and ACPI cases.  So I kinda
prefer, when possible, if arm-smmu can figure out what is going on by
looking at the hw state at boot (since that approach would work
equally well for DT and ACPI).

I *think* (but need to confirm if Bjorn hasn't already) that the
memory for the pagetables that firmware/bootloader sets up is already
removed from the memory map efi passes to kernel, so we don't need to
worry about kernel stomping in-use pagetables.

BR,
-R