linux-kernel - Re: [PATCH 12/14] dma-direct: handle the memory encryption bit in common code

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Message-ID: <5316b479-7e75-d62f-6b17-b6bece55187c@arm.com>
Date:   Mon, 19 Mar 2018 15:37:20 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Christoph Hellwig <hch@....de>
Cc:     x86@...nel.org, Tom Lendacky <thomas.lendacky@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org, Muli Ben-Yehuda <mulix@...ix.org>,
        iommu@...ts.linux-foundation.org,
        David Woodhouse <dwmw2@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 12/14] dma-direct: handle the memory encryption bit in
 common code

On 19/03/18 15:24, Christoph Hellwig wrote:
> On Mon, Mar 19, 2018 at 03:19:04PM +0000, Robin Murphy wrote:
>> As a heads-up, I've just realised there's now a silent (but build-breaking)
>> conflict with the current arm64 queue brewing here, as we've unfortunately
>> had to reintroduce ARCH_HAS_PHYS_TO_DMA as a means of being safe against an
>> ugly architectural corner case - currently commit 1f85b42a691c ("arm64:
>> Revert L1_CACHE_SHIFT back to 6 (64-byte cache line size)") in -next.
> 
> Please revert that arm64 commit.  This condition should be handled
> in common code as it is not arm specific.  And next time please CC
> the iommu list and dma-mapping maintainers before doing such a change.

There didn't seem enough justification to clutter up core SWIOTLB code 
with the ability to force bouncing on a per-device basis, but if you 
think there are real potential users out there then fair enough. For 
arm64, it's extremely unlikely that anyone will ever build a 
sufficiently wacky system to actually hit this code path; we really only 
implemented it for peace of mind per the letter of the architecture.

Robin.