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Date:   Tue, 1 Oct 2019 13:50:32 +0100
From:   Will Deacon <will@kernel.org>
To:     Jia He <justin.he@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>,
        James Morse <james.morse@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Punit Agrawal <punitagrawal@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>, hejianet@gmail.com,
        Kaly Xin <Kaly.Xin@arm.com>
Subject: Re: [PATCH v10 2/3] arm64: mm: implement arch_faults_on_old_pte() on
 arm64
Message-ID: <20191001125031.7ddm5dlwss6m3dth@willie-the-truck>
References: <20190930015740.84362-1-justin.he@arm.com>
 <20190930015740.84362-3-justin.he@arm.com>
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On Mon, Sep 30, 2019 at 09:57:39AM +0800, Jia He wrote:
> On arm64 without hardware Access Flag, copying fromuser will fail because
> the pte is old and cannot be marked young. So we always end up with zeroed
> page after fork() + CoW for pfn mappings. we don't always have a
> hardware-managed access flag on arm64.
> 
> Hence implement arch_faults_on_old_pte on arm64 to indicate that it might
> cause page fault when accessing old pte.
> 
> Signed-off-by: Jia He <justin.he@arm.com>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> ---
>  arch/arm64/include/asm/pgtable.h | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index 7576df00eb50..e96fb82f62de 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -885,6 +885,20 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
>  #define phys_to_ttbr(addr)	(addr)
>  #endif
>  
> +/*
> + * On arm64 without hardware Access Flag, copying from user will fail because
> + * the pte is old and cannot be marked young. So we always end up with zeroed
> + * page after fork() + CoW for pfn mappings. We don't always have a
> + * hardware-managed access flag on arm64.
> + */
> +static inline bool arch_faults_on_old_pte(void)
> +{
> +	WARN_ON(preemptible());
> +
> +	return !cpu_has_hw_af();
> +}

Does this work correctly in a KVM guest? (i.e. is the MMFR sanitised in that
case, despite not being the case on the host?)

Will