Re: [PATCH] arm64, vmcoreinfo : Append 'MAX_USER_VA_BITS' and 'MAX_PHYSMEM_BITS' to vmcoreinfo

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Hi Robin,

On 02/04/2019 09:01 PM, Robin Murphy wrote:
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.

Right, but lets think this from a distribution user p-o-v. Why would a user with newer kernel and CPU which doesn't support ARMv8.2 LPA want to update a user-space utility? Well unless something is broken in the user-space. Requesting an upgrade is easier in this case, as the user-space components like crash-utility/kexec-tools (or for that matter any user-space tool which requires a page-table walk to determine vaddr/paddr values) are indeed broken with this combination.

On the other hand, if a user a having a old kernel, CPU which doesn't support LPA, but if we still want them to upgrade to a newer version of user-space utility (with a upgrade fix note reading something like: "Add 52-bit PA support"), surely that would not be an ideal case requiring an upgrade as his underlying environment doesn't support the same.

That's what I meant when I said that we need to take care of the backward compatibility also when we propose new code changes to user-space packages.

Thanks,
Bhupesh

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.


_______________________________________________
kexec mailing list
kexec@xxxxxxxxxxxxxxxxxxx
http://lists.infradead.org/mailman/listinfo/kexec



[Index of Archives]     [LM Sensors]     [Linux Sound]     [ALSA Users]     [ALSA Devel]     [Linux Audio Users]     [Linux Media]     [Kernel]     [Gimp]     [Yosemite News]     [Linux Media]

  Powered by Linux