Re: [Xen-devel] [PATCH 10/17] xen: arm64: disable alignment traps

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-03-20 at 16:21 +0000, Gordan Bobic wrote:
> On 2014-03-20 15:59, Ian Campbell wrote:
> > On Thu, 2014-03-20 at 15:57 +0000, Andrew Cooper wrote:
> >> On 20/03/14 15:45, Ian Campbell wrote:
> >> > The mem* primitives which I am about to import from Linux in a subsequent
> >> > patch rely on the hardware handling misalignment.
> >> >
> >> > The benefits of an optimised memcpy etc oughtweigh the downsides.
> > 
> > Ahem, "outweigh".
> 
> Just FYI, the slow-down from heavy unaligned accesses (with
> hardware alignment fixup, you can't disable it using
> /proc/cpu/alignment) on Cortex A15 is about 40x.

That's pretty staggering -- are you positive this wasn't the kernel
doing the fixups?

> Most of the commonly used code has been fixed recently, but
> there are still some packages that exhibit misaligned access
> traps during their test suites and/or normal operation.
>
> Whether the hardware alignment fixup is less overheady on
> ARM64, I don't know - I haven't been able to get my hands
> on the hardware yet.

arm64 is a lot "friendlier" than arm32 in this regard. I was mostly
taking it on trust that whoever implemented memcpy.S etc found that
memcpy.S with hardware alignment was better than the dumb loop, even if
it wasn't as good as a clever memcpy.S which avoided the alignments.

Ian.


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