[aarch64-port-dev ] [Roland Westrelin] Re: Aarch64 port for ZGC, so far

[Andrew Haley]

On 4/13/19 11:33 PM, Stuart Monteith wrote:
> An alternative is to reproduce the multimapping that is on x86, where
> all addresses are real. This has the advantage that when the Memory
> Tagging Extensions (MTE) in AArch64 are implemented, they won't
> encroach on the ZGC coloured bits, and vice versa. Realistically if we
> are ever to use MTE, ZGC may prevent that from happening.
> 
> I believe the only reason we should continue with using the aarch64
> TBI tagged addresses is if it confers a good performance advantage
> over multimapping. My plan is to patch JNI for now in my ZGC patch,
> and then work on a new patch but with x86 style multimapping, which
> ought to be a straight copy. As I understand it, only one bit of
> colour is significant in ZGC at any given time, so the TLB impact
> might not be so bad for the majority of the time, but we'll need to
> check that.

This seems a shame. Multi-mapping is a kludge that we shouldn't need
on AArch64. Do we know that the colour bits will conflict with MTE? I
would have thought that the only place you're likely to see a problem
is with the GetPrimitiveArrayCritical functions, and they can be
corrected for ZGC.

-- 
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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