Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation

Catalin Marinas
2020-05-15T12:14:06Z

On Fri, May 15, 2020 at 01:04:33PM +0100, Szabolcs Nagy wrote:
> The 05/15/2020 12:27, Catalin Marinas wrote:
> > Thanks Szabolcs. While we are at this, no-one so far asked for the
> > GCR_EL1.RRND to be exposed to user (and this implies RGSR_EL1.SEED).
> > Since RRND=1 guarantees a distribution "no worse" than that of RRND=0, I
> > thought there isn't much point in exposing this configuration to the
> > user. The only advantage of RRND=0 I see is that the kernel can change
>
> it seems RRND=1 is the impl specific algorithm.

Yes, that's the implementation specific algorithm which shouldn't be
worse than the standard one.

> > the seed randomly but, with only 4 bits per tag, it really doesn't
> > matter much.
> >
> > Anyway, mentioning it here in case anyone is surprised later about the
> > lack of RRND configurability.
>
> i'm not familiar with how irg works.

It generates a random tag based on some algorithm.

> is the seed per process state that's set up at process startup in some
> way? or shared (and thus effectively irg is non-deterministic in
> userspace)?

The seed is only relevant if the standard algorithm is used (RRND=0).

--
Catalin