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

[Yee Lee <yee.lee@mediatek.com>]

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:
> > On Mon, 2021-08-02 at 16:30 +0100, Catalin Marinas wrote:
> > > On Fri, Jul 30, 2021 at 10:49:53PM +0800, yee.lee@mediatek.com
> > > wrote:
> > > > From: Yee Lee <yee.lee@mediatek.com>
> > > > 
> > > > For some low-end devices with limited resources,
> > > > MTE needs to be optionally disabled to save system
> > > > costs such as tag memory and firmware controls.
> > > 
> > > I understand the cost of using MTE but I don't fully get what you
> > > mean
> > > by firmware controls. If the ID_AA64PFR1_EL1.MTE reports that MTE
> > > is
> > > present, the firmware should have initialised MTE correctly (e.g.
> > > tag
> > > allocation storage, SCR_EL3.ATA) and not rely on a kernel command
> > > line
> > > argument that may or may not be present.
> > > 
> > > > This allows ID_AA64PFR1_EL1.MTE to be overridden on 
> > > > its shadow value by giving "arm64.nomte" on cmdline,
> > > > and to suppress MTE feature.
> > > > 
> > > > Suggested-by: Marc Zyngier <maz@kernel.org>
> > > > Suggested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> > > > Signed-off-by: Yee Lee <yee.lee@mediatek.com>
> > > 
> > > While this patch appears to disable MTE, I don't think it can
> > > fully
> > > prevent the access to the allocation tag storage, so the firmware
> > > must
> > > still initialise it correctly.
> > > 
> > > The issue is that __cpu_setup already configures the MAIR_EL1
> > > register
> > > to use Normal Tagged memory for the kernel mapping and
> > > SCTLR_EL1.ATA is
> > > set. The TCF field is zero, so no tag checking, but I couldn't
> > > figure
> > > out from the ARM ARM whether this also prevents LDR/STR from
> > > attempting
> > > to fetch the allocation tags. I think it's only the ATA bit and
> > > the MAIR
> > > configuration.
> > > 
> > > With this patch, KASAN_HW_TAGS (if configured) won't be used and
> > > MTE
> > > will not be presented to user applications, if that's what you
> > > want, but
> > > does not fully disable MTE.
> > 
> > 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.