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

Catalin Marinas
2021-12-14T12:02:39Z

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