Re: [PATCH 1/2] drm: add cache support for arm64

From: Christoph Hellwig <hch@lst•de>
To: Mark Rutland <mark.rutland@arm•com>
Cc: Rob Clark <robdclark@chromium•org>,
	Maxime Ripard <maxime.ripard@bootlin•com>,
	Catalin Marinas <catalin.marinas@arm•com>,
	David Airlie <airlied@linux•ie>,
	Maarten Lankhorst <maarten.lankhorst@linux•intel.com>,
	LKML <linux-kernel@vger•kernel.org>,
	dri-devel <dri-devel@lists•freedesktop.org>,
	Sean Paul <sean@poorly•run>, Rob Clark <robdclark@gmail•com>,
	linux-arm-kernel@lists•infradead.org,
	Daniel Vetter <daniel@ffwll•ch>,
	Greg Kroah-Hartman <gregkh@linuxfoundation•org>,
	Thomas Gleixner <tglx@linutronix•de>,
	Will Deacon <will@kernel•org>, Christoph Hellwig <hch@lst•de>,
	Allison Randal <allison@lohutok•net>
Subject: Re: [PATCH 1/2] drm: add cache support for arm64
Date: Thu, 8 Aug 2019 09:58:27 +0200	[thread overview]
Message-ID: <20190808075827.GD30308@lst.de> (raw)
In-Reply-To: <20190807164958.GA44765@lakrids.cambridge.arm.com>

On Wed, Aug 07, 2019 at 05:49:59PM +0100, Mark Rutland wrote:
> I'm fairly confident that the linear/direct map cacheable alias is not
> torn down when pages are allocated. The gneeric page allocation code
> doesn't do so, and I see nothing the shmem code to do so.

It is not torn down anywhere.

> For arm64, we can tear down portions of the linear map, but that has to
> be done explicitly, and this is only possible when using rodata_full. If
> not using rodata_full, it is not possible to dynamically tear down the
> cacheable alias.

Interesting.  For this or next merge window I plan to add support to the
generic DMA code to remap pages as uncachable in place based on the
openrisc code.  Aѕ far as I can tell the requirement for that is
basically just that the kernel direct mapping doesn't use PMD or bigger
mapping so that it supports changing protection bits on a per-PTE basis.
Is that the case with arm64 + rodata_full?

> > My understanding is that a cacheable alias is "ok", with some
> > caveats.. ie. that the cacheable alias is not accessed.  
> 
> Unfortunately, that is not true. You'll often get away with it in
> practice, but that's a matter of probability rather than a guarantee.
> 
> You  cannot prevent a CPU from accessing a VA arbitrarily (e.g. as the
> result of wild speculation). The ARM ARM (ARM DDI 0487E.a) points this
> out explicitly:

Well, if we want to fix this properly we'll have to remap in place
for dma_alloc_coherent and friends.

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