Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo
From: Robin Murphy <robin.murphy@arm.com>
Date: 2019-02-04 15:32:16
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On 04/02/2019 14:35, Bhupesh Sharma wrote: [...]
quoted
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. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel