Messages in this thread | | | Date | Tue, 16 Aug 2022 08:51:47 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH] mte: Follow arm64.nomte override in MMU setup. |
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On Tue, Aug 09, 2022 at 06:24:23PM -0700, Peter Collingbourne wrote: > On Tue, Aug 9, 2022 at 10:29 AM Evgenii Stepanov <eugenis@google.com> wrote: > > On Tue, Aug 9, 2022 at 9:49 AM Marc Zyngier <maz@kernel.org> wrote: > > > In which case what is the tag memory doing in the linear map? > > > Shouldn't it be marked as reserved, not mapped, and in general > > > completely ignored by the NS OS? > > > > That would be wasteful. The idea is to only reserve the parts of the > > tag memory that correspond to the TZ carveout and release the rest to > > the NS OS. > > More generally, one can imagine a system where *any* tagged memory > transaction can result in an SError because the MTE implementation was > not configured by an earlier bootloader phase, e.g. because the > bootloader was configured to disable MTE at runtime. On such systems, > the kernel must refrain from causing tagged memory transactions to be > issued via the linear map, and that's exactly what this patch does.
The problem is that it doesn't. The 8.5 architecture allows any Normal Cacheable (even non-tagged) mapping to fetch tags. It may happen that on certain implementations setting MAIR to non-tagged works but that's not guaranteed and with the Linux kernel we tend to stick to the architected behaviour (with a few exceptions like PMU counters and errata).
There is an ongoing discussion with the architects and partners on whether we can tighten the architecture as not to cause visible side-effects like SError but not sure whether that has been closed yet (just back from holiday).
Until that's sorted, tag storage cannot be reused in an arm64-generic way in the kernel.
-- Catalin
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