Re: [PATCH v3 1/1] arm64/cpufeature: Optionally disable MTE via command-line

From: Catalin Marinas <catalin.marinas@arm.com>
To: Yee Lee <yee.lee@mediatek.com>
Cc: linux-kernel@vger.kernel.org, nicholas.Tang@mediatek.com,
	Kuan-Ying.lee@mediatek.com, chinwen.chang@mediatek.com,
	Jonathan Corbet <corbet@lwn.net>, Will Deacon <will@kernel.org>,
	Matthias Brugger <matthias.bgg@gmail.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Maciej W. Rozycki" <macro@orcam.me.uk>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>, Marc Zyngier <maz@kernel.org>,
	David Brazdil <dbrazdil@google.com>,
	Ard Biesheuvel <ardb@kernel.org>, Fuad Tabba <tabba@google.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	"moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)"
	<linux-arm-kernel@lists.infradead.org>,
	 "moderated list:ARM/Mediatek SoC support"
	<linux-mediatek@lists.infradead.org>
Subject: Re: [PATCH v3 1/1] arm64/cpufeature: Optionally disable MTE via command-line
Date: Tue, 14 Dec 2021 12:02:27 +0000	[thread overview]
Message-ID: <YbiH04yqEqW8p8EM@arm.com> (raw)
In-Reply-To: <dc2fd1f8c68e78905f4242c2b530bc720b979cd8.camel@mediatek.com>

On Tue, Dec 14, 2021 at 04:19:05PM +0800, Yee Lee wrote:
> On Fri, 2021-12-03 at 16:33 +0000, Catalin Marinas wrote:
> > On Thu, Nov 25, 2021 at 06:19:29PM +0800, Yee Lee wrote:
> > > As pointed out earlier, the hardware has been verified that still has
> > > transaction sending to DRAM due to mair_el1(Normal_tagged) is
> > > setup.  That means the override in this patch would be incompleted and
> > > cannot achieve to avoid undesired hardware confliction by disabling MTE.
> > > 
> > > Do we have other options to delay the configuration on MAIR_EL1 after
> > > the override? Or maybe another CONFIG to bypass the init in __cpu_setup?
> > 
> > This register is trickier as it may be cached in the TLB (IIRC). I think
> > deferring the setting of SCTLR_EL1.ATA(0) should be sufficient. Can you
> > try the diff I sent in the previous email and confirm that the accesses
> > to the allocation tag storage are blocked?
> 
> Yes, the previous diff is already online. 
> 
> In our experiment, with cmdline, "arm64.nomte", cpu_enable_mte() is
> bypassed and the ATA0 is not set, but the access to tag memory still
> dispatches. Only as MAIR_EL1 remains MAIR_ATTR_NORMAL, instead of
> MAIR_ATTR_NORMAL_TAGGED, the access will stop.
> 
> From the manual, I think ATA only affects TAG instructions like STG,
> IRG, but not the tag access within normal STR/LDR.

The ARM ARM states SCTLR_EL1.ATA0 == 0 means "access to allocation tags
is prevented". The AArch64.MemSingle[] pseudocode ends up with similar
checks:

https://developer.arm.com/documentation/ddi0596/2021-09/Shared-Pseudocode/AArch64-Functions?lang=en#AArch64.MemSingle.read.5

before reading the tags from memory in AArch64.CheckTag():

https://developer.arm.com/documentation/ddi0596/2021-09/Shared-Pseudocode/AArch64-Functions?lang=en#AArch64.CheckTag.4

My suggestion is to raise this with support@arm.com (feel free to cc me)
so that we clarify the hardware behaviour. I don't think it's entirely
correct (it's more like, is there a risk of external aborts caused by
access to allocation tag storage that's not present?)

-- 
Catalin

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