From: Catalin Marinas <catalin.marinas@arm.com>
To: Steven Price <steven.price@arm.com>
Cc: linux-arch@vger.kernel.org, Szabolcs Nagy <szabolcs.nagy@arm.com>,
Andrey Konovalov <andreyknvl@google.com>,
Kevin Brodsky <kevin.brodsky@arm.com>,
Peter Collingbourne <pcc@google.com>,
linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Will Deacon <will@kernel.org>,
Dave P Martin <Dave.Martin@arm.com>,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v9 09/29] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE
Date: Thu, 10 Sep 2020 12:55:19 +0100 [thread overview]
Message-ID: <20200910115519.GB4030@gaia> (raw)
In-Reply-To: <19137fdc-64c6-a5c2-d6f6-ebcf4f553816@arm.com>
On Thu, Sep 10, 2020 at 12:12:27PM +0100, Steven Price wrote:
> On 10/09/2020 11:52, Catalin Marinas wrote:
> > On Thu, Sep 10, 2020 at 11:23:33AM +0100, Steven Price wrote:
> > > On 04/09/2020 11:30, Catalin Marinas wrote:
> > > > --- /dev/null
> > > > +++ b/arch/arm64/lib/mte.S
> > > > @@ -0,0 +1,34 @@
> > > > +/* SPDX-License-Identifier: GPL-2.0-only */
> > > > +/*
> > > > + * Copyright (C) 2020 ARM Ltd.
> > > > + */
> > > > +#include <linux/linkage.h>
> > > > +
> > > > +#include <asm/assembler.h>
> > > > +#include <asm/sysreg.h>
> > > > +
> > > > + .arch armv8.5-a+memtag
> > > > +
> > > > +/*
> > > > + * multitag_transfer_size - set \reg to the block size that is accessed by the
> > > > + * LDGM/STGM instructions.
> > > > + */
> > > > + .macro multitag_transfer_size, reg, tmp
> > > > + mrs_s \reg, SYS_GMID_EL1
> > > > + ubfx \reg, \reg, #SYS_GMID_EL1_BS_SHIFT, #SYS_GMID_EL1_BS_SIZE
> > > > + mov \tmp, #4
> > > > + lsl \reg, \tmp, \reg
> > > > + .endm
> > > > +
> > > > +/*
> > > > + * Clear the tags in a page
> > > > + * x0 - address of the page to be cleared
> > > > + */
> > > > +SYM_FUNC_START(mte_clear_page_tags)
> > > > + multitag_transfer_size x1, x2
> > > > +1: stgm xzr, [x0]
> > > > + add x0, x0, x1
> > > > + tst x0, #(PAGE_SIZE - 1)
> > > > + b.ne 1b
> > > > + ret
> > > > +SYM_FUNC_END(mte_clear_page_tags)
> > >
> > > Could the value of SYS_GMID_EL1 vary between CPUs and do we therefore need a
> > > preempt_disable() around mte_clear_page_tags() (and other functions in later
> > > patches)?
> >
> > If they differ, disabling preemption here is not sufficient. We'd have
> > to trap the GMID_EL1 access at EL2 as well and emulate it (we do this
> > for CTR_EL0 in dcache_line_size).
>
> Hmm, good point. It's actually not possible to properly emulate this - EL2
> can trap GMID_EL1 to provide a different (presumably smaller) size, but
> LDGM/STGM will still read/store the number of tags of the underlying
> hardware. While simple loops like we've got at the moment won't care (we'll
> just end up doing useless work), it won't be architecturally correct. The
> guest can always deduce the underlying value. So I think we can safely
> consider this broken hardware.
I think that's similar to the DC ZVA (and DCZID_EL0.BS) case where
faking it could lead to data corruption if the software assumes it
writes a maximum number of bytes.
(I meant to raise a ticket with the architects to make this a
requirement in the ARM ARM but forgot about it)
> > I don't want to proactively implement this just in case we'll have
> > broken hardware (I feel a bit more optimistic today ;)).
>
> Given the above I think if we do have broken hardware the only sane thing to
> do would be to provide a way of overriding multitag_transfer_size to return
> the smallest size of all CPUs. Which works well enough for the uses we've
> currently got.
If we do have such broken hardware, we should probably drop the STGM
instructions in favour of STG or ST2G. Luckily, STGM/LDGM are not
available in user space.
--
Catalin
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next prev parent reply other threads:[~2020-09-10 11:56 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20200904103029.32083-1-catalin.marinas@arm.com>
[not found] ` <20200904103029.32083-5-catalin.marinas@arm.com>
2020-09-04 10:46 ` [PATCH v9 04/29] arm64: kvm: mte: Hide the MTE CPUID information from the guests Marc Zyngier
[not found] ` <20200904103029.32083-10-catalin.marinas@arm.com>
2020-09-10 10:23 ` [PATCH v9 09/29] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE Steven Price
2020-09-10 10:52 ` Catalin Marinas
2020-09-10 11:12 ` Steven Price
2020-09-10 11:55 ` Catalin Marinas [this message]
2020-09-10 12:43 ` Steven Price
[not found] ` <20200904103029.32083-30-catalin.marinas@arm.com>
2020-09-17 8:11 ` [PATCH v9 29/29] arm64: mte: Add Memory Tagging Extension documentation Will Deacon
2020-09-17 9:02 ` Catalin Marinas
2020-09-17 16:15 ` Dave Martin
2020-09-18 8:30 ` Will Deacon
2020-10-14 23:43 ` Peter Collingbourne
2020-10-15 8:57 ` Will Deacon
2020-10-15 11:14 ` Szabolcs Nagy
2020-09-22 16:04 ` Catalin Marinas
2020-09-22 15:52 ` Szabolcs Nagy
2020-09-22 16:55 ` Catalin Marinas
2020-09-23 9:10 ` Szabolcs Nagy
2020-09-22 12:22 ` Andrey Konovalov
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