Re: arm64 pointer tagging, VA-bits, the end of the world...
On Wed, Aug 17, 2016 at 04:40:29PM +0100, Mark Rutland wrote:
> > > ARMv8.2 bumps the maximum address limit to 52 bits [1]. Architecturally,
> > > only the upper 8 bits of address are reserved for tagging (and this has
> > > been the case since the original ARMv8-A release), and all other bits
> > > are reserved.
> > >
> > > Given the above, it seems possible/likely that we may see address spaces
> > > of up to 56 bits in future.
> > >
> > > So shuffling bits along a few places is only likely to buy us some time,
> > > and won't solve the problem entirely.
> >
> > Absolutely - but it's still something we need to do, now.
> >
> > And I'm hoping that by the time > 48-bit VA becomes an issue, it will
> > be an issue for Intel also, and we won't need to do all of the lifting
> > on the ARM side.
>
> Sure. I just wanted to make clear that there are already things in the
> pipe beyond 48-bit.
Indeed. And we need to stay on the ball with that, and try to ensure
any changes we do from this point onwards are at least 56-bit
safe. And start agitating against pointer tagging in general.
> w.r.t. future lifting it depends on how quickly those ARMv8.2 systems
> appear.
Well, I would say it depends on:
- how quickly those ARMv8.2 systems start implementing ginormous++
physical address space ranges for DRAM.
- whether we can separate kernel/user VA handling before that happens.
/
Leif
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