Re: [Qemu-devel] [PATCH v2 0/5] AArch64 TLB performance improvements

[Christopher Covington <cov@codeaurora.org>]

On 08/04/2014 06:32 AM, Peter Maydell wrote:
> On 4 August 2014 11:23, Alex Bennée <alex.bennee@linaro.org> wrote:
>> Peter Maydell writes:
>>> So:
>>>  * there's clearly a big perf win to be had here
>>>  * this patchset gives us that for 4K pages on AArch64
>>>  * but it doesn't help for 4K pages on AArch32 (really
>>>     common)
>>
>> Well for the AArch32 profile if you ran under qemu-system-aarch64 you
>> would be OK surely?
> 
> Yes, but that's pretty non-obvious, and also it doesn't
> make much sense to the user to say "these 32 bit
> CPUs should be run under qemu-system-aarch64 but
> these other ones should be under qemu-system-arm".
> 
>>>  * and it's not going to be good for 64K pages on AArch64
>>>    either (which I suspect will not be a rare choice)
>>
>> Does the kernel already use 64k pages for it's code?
> 
> There's a config option, which will cause it to use 64K
> pages for everything including userspace.
> (There's also 16K pages but I forget if Linux has support
> for those.) I think the kernel can also use 64K pages in
> some cases even in a 4K page config, but I don't know the
> details.

Linux support for the 16K granule has not been merged nor have I seen any
patches for it.

With a 4K granule one can early out to 1GiB blocks ("gigabyte kernel logical
mappings") [1] or 2MiB blocks ("huge pages") [2].

With a 64K granule, one can early out to 512MiB blocks ("huge pages") [2].

1. http://permalink.gmane.org/gmane.linux.ports.arm.kernel/322436
2. http://comments.gmane.org/gmane.linux.kernel.mm/100651

Regards,
Christopher

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