linux-kernel - Re: [PATCH 3/3] arm64: tlb: skip tlbi broadcast

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Date:   Tue, 31 Mar 2020 10:45:11 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Rafael Aquini <aquini@...hat.com>,
        Mark Salter <msalter@...hat.com>,
        Jon Masters <jcm@...masters.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        Michal Hocko <mhocko@...nel.org>, QI Fuli <qi.fuli@...itsu.com>
Subject: Re: [PATCH 3/3] arm64: tlb: skip tlbi broadcast

Hi Andrea,

On Mon, Mar 16, 2020 at 02:09:07PM +0000, Mark Rutland wrote:
> AFAICT, this series relies on:
> 
> * An ISB completing prior page table walks when updating TTBR. I don't
>   believe this is necessarily the case, given how things work for an
>   EL1->EL2 transition where there can be ongoing EL1 walks.

I've had confirmation that a DSB is necessary (after the MSR and ISB) to
complete any ongoing translation table walks for the stale context.

Without a DSB, those walks can observe subsequent stores and encounter
the usual set of CONSTRAINED UNPREDICTABLE behaviours (e.g. walking into
MMIO with side-effects, continuing from amalgamted entries, etc). Those
issues are purely to do with the walk, and apply regardless of whether
the resulting translations are architecturally consumed.

> * Walks never being initiated for `inactive` contexts within the current
>   translation regime. e.g. while ASID x is installed, never starting a
>   walk for ASID y. I can imagine that the architecture may permit a form
>   of this starting with intermediate walk entries in the TLBs.

I'm still chasing this point.

Thanks,
Mark.