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Subject: Re: [PATCH 1/1] dma: coherent: check no-map property for arm64
To:     Christoph Hellwig <hch@lst.de>, Robin Murphy <robin.murphy@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Dong Aisheng <dongas86@gmail.com>,
        Will Deacon <will@kernel.org>,
        Dong Aisheng <aisheng.dong@nxp.com>,
        iommu@lists.linux-foundation.org,
        open list <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@lists.infradead.org>
From:   Marek Szyprowski <m.szyprowski@samsung.com>
Message-ID: <7729cff3-27b4-c233-7f53-95f6140d3828@samsung.com>
Date:   Tue, 15 Jun 2021 09:19:57 +0200
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Hi Christoph,

On 14.06.2021 17:36, Christoph Hellwig wrote:
> On Mon, Jun 14, 2021 at 04:34:05PM +0100, Robin Murphy wrote:
>>> Looking at the rmem_dma_device_init() -> dma_init_coherent_memory(), it
>>> ends up calling memremap(MEMREMAP_WC) which would warn if it intersects
>>> with system RAM regardless of the architecture. If the memory region is
>>> nomap, it doesn't end up as IORESOURCE_SYSTEM_RAM, so memremap() won't
>>> warn. But why is this specific only to arm (or arm64)?
>> Didn't some ARMv7 implementations permit unexpected cache hits for the
>> non-cacheable address if the same PA has been speculatively fetched via the
>> cacheable alias?
> If we care about that we need to change these platforms to change the
> cache attributes of the kernel direct mapping instead of using vmap.
> We already have code to do that for openrisc, someone just needs to
> write the glue code for other platforms.

ARAIR there is a problem with changing cache attributes of the direct 
mappings on ARM 32bit. The whole lowmem is mapped with 2M 'section' 
mappings. Changing cache attributes causes 2 issues. First - one would 
need to split 2M entry into 4K entries. Second - 2M section mappings for 
the whole lowmem area are located in the per-process page tables. 
Changing the cache attributes would require locking all processes and 
iterating over their page table entries, which is a huge task.

Best regards

-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland