From: Cornelia Huck <cohuck@redhat.com>
To: Peter Collingbourne <pcc@google.com>,
 linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu
Subject: Re: [PATCH v5 5/8] arm64: mte: Lock a page for MTE tag initialisation
In-Reply-To: <20221104011041.290951-6-pcc@google.com>
Organization: Red Hat GmbH
References: <20221104011041.290951-1-pcc@google.com>
 <20221104011041.290951-6-pcc@google.com>
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Date: Mon, 07 Nov 2022 18:16:32 +0100
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Cc: kvm@vger.kernel.org, Peter Collingbourne <pcc@google.com>,
 Catalin Marinas <catalin.marinas@arm.com>, Steven Price <steven.price@arm.com>,
 Marc Zyngier <maz@kernel.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>,
 Will Deacon <will@kernel.org>, Evgenii Stepanov <eugenis@google.com>
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On Thu, Nov 03 2022, Peter Collingbourne <pcc@google.com> wrote:

> From: Catalin Marinas <catalin.marinas@arm.com>
>
> Initialising the tags and setting PG_mte_tagged flag for a page can race
> between multiple set_pte_at() on shared pages or setting the stage 2 pte
> via user_mem_abort(). Introduce a new PG_mte_lock flag as PG_arch_3 and
> set it before attempting page initialisation. Given that PG_mte_tagged
> is never cleared for a page, consider setting this flag to mean page
> unlocked and wait on this bit with acquire semantics if the page is
> locked:
>
> - try_page_mte_tagging() - lock the page for tagging, return true if it
>   can be tagged, false if already tagged. No acquire semantics if it
>   returns true (PG_mte_tagged not set) as there is no serialisation with
>   a previous set_page_mte_tagged().
>
> - set_page_mte_tagged() - set PG_mte_tagged with release semantics.
>
> The two-bit locking is based on Peter Collingbourne's idea.
>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Reviewed-by: Steven Price <steven.price@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Peter Collingbourne <pcc@google.com>
> ---
>  arch/arm64/include/asm/mte.h     | 35 +++++++++++++++++++++++++++++++-
>  arch/arm64/include/asm/pgtable.h |  4 ++--
>  arch/arm64/kernel/cpufeature.c   |  2 +-
>  arch/arm64/kernel/mte.c          | 12 +++--------
>  arch/arm64/kvm/guest.c           | 16 +++++++++------
>  arch/arm64/kvm/mmu.c             |  2 +-
>  arch/arm64/mm/copypage.c         |  2 ++
>  arch/arm64/mm/fault.c            |  2 ++
>  arch/arm64/mm/mteswap.c          | 14 +++++--------
>  9 files changed, 60 insertions(+), 29 deletions(-)

Reviewed-by: Cornelia Huck <cohuck@redhat.com>

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