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

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.

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.


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