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Subject: Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and
 'MAX_PHYSMEM_BITS' to vmcoreinfo
References: <1548850991-11879-1-git-send-email-bhsharma@redhat.com>
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From: Robin Murphy <robin.murphy@arm.com>
Message-ID: <2d14484d-743e-080a-daf6-a07b0a659c3d@arm.com>
Date: Mon, 4 Feb 2019 15:31:59 +0000
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To: Bhupesh Sharma <bhsharma@redhat.com>, James Morse <james.morse@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>, ard.biesheuvel@linaro.org, catalin.marinas@arm.com, kexec@lists.infradead.org, Will Deacon <will.deacon@arm.com>, AKASHI Takahiro <takahiro.akashi@linaro.org>, bhupesh.linux@gmail.com, linux-arm-kernel@lists.infradead.org

On 04/02/2019 14:35, Bhupesh Sharma wrote:
[...]
>> Also hardcoding the PTE calculation to use the high address bit mask 
>> always will break the backward compatibility with older kernels (which 
>> don't support 52-bit address space extensions).

No it won't. There's no difference between an old kernel, a new kernel 
on a CPU without ARMv8.2-LPA, or a new kernel on a CPU with ARMv8.2-LPA 
in a system which happens to have less than 49 bits of physical memory 
map - in all those cases the relevant bits are either RES0 or just 
actually 0 in the PTE, so replacing 4 bits of zeros with 4 bits of other 
zeros in the final physical address has no effect whatsoever other than 
taking a couple of extra instructions to perform.

If you're running a 64K page kernel on a system with an SMMU, note how 
that's already been "broken" for nearly a year now ;)

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/iommu/io-pgtable-arm.c#n211


Robin.

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