linux-kernel - Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a separate file

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Date:   Mon, 22 Jul 2019 17:38:12 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Guo Ren <guoren@...nel.org>
Cc:     Julien Grall <julien.grall@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        aou@...s.berkeley.edu, gary@...yguo.net,
        Atish Patra <Atish.Patra@....com>, hch@...radead.org,
        paul.walmsley@...ive.com, rppt@...ux.ibm.com,
        linux-riscv@...ts.infradead.org, Anup Patel <anup.Patel@....com>,
        Palmer Dabbelt <palmer@...ive.com>, suzuki.poulose@....com,
        Marc Zyngier <marc.zyngier@....com>, julien.thierry@....com,
        Will Deacon <will.deacon@....com>, christoffer.dall@....com,
        james.morse@....com, linux-csky@...r.kernel.org
Subject: Re: [PATCH RFC 11/14] arm64: Move the ASID allocator code in a
 separate file

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