From mboxrd@z Thu Jan  1 00:00:00 1970
From: Catalin Marinas <catalin.marinas@arm.com>
Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension
 documentation
Date: Fri, 15 May 2020 13:13:44 +0100
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To: Szabolcs Nagy <szabolcs.nagy@arm.com>
Cc: Dave Martin <Dave.Martin@arm.com>, linux-arch@vger.kernel.org, Richard Earnshaw <Richard.Earnshaw@arm.com>, Andrey Konovalov <andreyknvl@google.com>, Kevin Brodsky <kevin.brodsky@arm.com>, Peter Collingbourne <pcc@google.com>, linux-mm@kvack.org, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org, nd@arm.com

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