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Date:   Mon, 22 Jul 2019 17:38:12 +0100
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Guo Ren <guoren@kernel.org>
Cc:     Julien Grall <julien.grall@arm.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        aou@eecs.berkeley.edu, gary@garyguo.net,
        Atish Patra <Atish.Patra@wdc.com>, hch@infradead.org,
        paul.walmsley@sifive.com, rppt@linux.ibm.com,
        linux-riscv@lists.infradead.org, Anup Patel <anup.Patel@wdc.com>,
        Palmer Dabbelt <palmer@sifive.com>, suzuki.poulose@arm.com,
        Marc Zyngier <marc.zyngier@arm.com>, julien.thierry@arm.com,
        Will Deacon <will.deacon@arm.com>, christoffer.dall@arm.com,
        james.morse@arm.com, linux-csky@vger.kernel.org
Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
 separate file
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On Tue, Jul 16, 2019 at 11:31:27AM +0800, Guo Ren wrote:
> I saw arm64 to prevent speculation by temporarily setting TTBR0.el1 to
> a zero page table. Is that used to prevent speculative execution user
> space code or just prevent ld/st in copy_use_* ?

Only to prevent explicit ld/st from user. On ARMv8.1+, we don't normally
use the TTBR0 trick but rather disable user space access using the PAN
(privileged access never) feature. However, I don't think PAN disables
speculative accesses, only explicit loads/stores. Also, with ARMv8.2
Linux uses the LDTR/STTR instructions in copy_*_user() which don't need
to disable PAN explicitly.

-- 
Catalin