linux-kernel - Re: [PATCH 10/17] prmem: documentation

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Message-Id: <34204E6C-53C2-427D-A3B2-3D2E091D3E4B@amacapital.net>
Date:   Wed, 31 Oct 2018 13:38:37 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Igor Stoppa <igor.stoppa@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Kees Cook <keescook@...omium.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        James Morris <jmorris@...ei.org>,
        Michal Hocko <mhocko@...nel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-integrity <linux-integrity@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Igor Stoppa <igor.stoppa@...wei.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Laura Abbott <labbott@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/17] prmem: documentation



> On Oct 31, 2018, at 3:11 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
>> On Wed, Oct 31, 2018 at 12:15:46AM +0200, Igor Stoppa wrote:
>> On 30/10/2018 23:02, Andy Lutomirski wrote:
> 
>>> But I dislike allowing regular writes in the protected region. We
>>> really only need four write primitives:
>>> 
>>> 1. Just write one value.  Call at any time (except NMI).
>>> 
>>> 2. Just copy some bytes. Same as (1) but any number of bytes.
>>> 
>>> 3,4: Same as 1 and 2 but must be called inside a special rare write
>>> region. This is purely an optimization.
>> 
>> Atomic? RCU?
> 
> RCU can be done, that's not really a problem. Atomics otoh are a
> problem. Having pointers makes them just work.
> 
> Andy; I understand your reason for not wanting them, but I really don't
> want to duplicate everything. Is there something we can do with static
> analysis to make you more comfortable with the pointer thing?

I’m sure we could do something with static analysis, but I think seeing a real use case where all this fanciness makes sense would be good.

And I don’t know if s390 *can* have an efficient implementation that uses pointers. OTOH they have all kinds of magic stuff, so who knows?