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From: Robin Murphy <robin.murphy@arm.com>
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
Subject: Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo
Date: Mon, 4 Feb 2019 15:31:59 +0000
Message-ID: <2d14484d-743e-080a-daf6-a07b0a659c3d@arm.com> (raw)
In-Reply-To: <a646e38d-9351-e187-1571-c49d3558b164@redhat.com>

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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Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-30 12:23 Bhupesh Sharma
2019-01-30 15:21 ` James Morse
2019-01-30 21:39   ` Bhupesh Sharma
2019-02-04 14:35     ` Bhupesh Sharma
2019-02-04 15:31       ` Robin Murphy [this message]
2019-02-12  4:55         ` Bhupesh Sharma
2019-02-12 10:49           ` Robin Murphy
2019-02-04 16:56       ` James Morse
2019-01-31  1:48 ` Dave Young
2019-01-31 10:00   ` Bhupesh Sharma
2019-01-31 14:03   ` Dave Anderson
2019-02-04 16:04   ` Kazuhito Hagio
2019-02-12  5:07     ` Bhupesh Sharma
2019-02-12 10:44       ` Dave Young
2019-02-12 19:59         ` Bhupesh Sharma
2019-02-12 23:03           ` Kazuhito Hagio
2019-02-13 11:15             ` Dave Young
2019-02-13 18:22               ` James Morse
2019-02-13 19:52                 ` Kazuhito Hagio
2019-02-15 17:34                   ` James Morse
2019-02-15 18:01                     ` Bhupesh Sharma
2019-02-18 15:27                       ` Steve Capper
2019-02-21 16:08                         ` Bhupesh Sharma
2019-02-19 20:47                       ` Kazuhito Hagio
2019-02-21 16:20                         ` Bhupesh Sharma
2019-02-21 16:42                           ` Dave Anderson
2019-02-21 19:02                             ` Kazuhito Hagio
2019-03-01  4:01                               ` Bhupesh Sharma
2019-02-14 19:30                 ` Bhupesh Sharma

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