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

From: Catalin Marinas <catalin.marinas@arm.com>
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
Subject: Re: [PATCH v3 23/23] arm64: mte: Add Memory Tagging Extension documentation
Date: Fri, 15 May 2020 12:27:40 +0100	[thread overview]
Message-ID: <20200515112740.GB22393@gaia> (raw)
In-Reply-To: <20200515111359.GC27289@arm.com>

On Fri, May 15, 2020 at 12:14:00PM +0100, Szabolcs Nagy wrote:
> The 05/15/2020 11:38, Catalin Marinas wrote:
> > On Thu, May 14, 2020 at 12:37:22PM +0100, Catalin Marinas wrote:
> > > We have two options with suboptions:
> > > 
> > > 1. prctl() gets an exclude mask with 0xffff illegal even though the
> > >    hardware accepts it:
> > >    a) default exclude mask 0, allowing all tags to be generated by IRG
> > >    b) default exclude mask of 0xfffe so that only tag 0 is generated
> > > 
> > > 2. prctl() gets an include mask with 0 illegal:
> > >    a) default include mask is 0xffff, allowing all tags to be generated
> > >    b) default include mask 0f 0x0001 so that only tag 0 is generated
> > > 
> > > We currently have (2) with mask 0 but could be changed to (2.b). If we
> > > are to follow the hardware description (which makes more sense to me but
> > > I don't write the C library), (1.a) is the most appropriate.
> > 
> > Thinking some more about this, as we are to expose the GCR_EL1.Excl via
> > a ptrace interface as a regset, it makes more sense to move back to an
> > exclude mask here with default 0. That would be option 1.a above.
> 
> i think the libc has to do a prctl call to set
> mte up and at that point it will use whatever
> arguments necessary, so 1.a should work (just
> like the other options).
> 
> likely libc will disable 0 for irg and possibly
> one or two other fixed colors (which will have
> specific use).
> 
> the difference i see between 1 vs 2 is forward
> compatibility if the architecture changes (e.g.
> adding more tag bits) but then likely new prctl
> flag will be needed for handling that so it's
> probably not an issue.

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
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.

-- 
Catalin