linux-kernel - Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC

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Message-Id: <33241b25-4d45-4278-a4e6-ec9c12b0e1f3@www.fastmail.com>
Date:   Thu, 17 Jun 2021 07:00:26 -0700
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Mark Rutland" <mark.rutland@....com>
Cc:     "Russell King (Oracle)" <linux@...linux.org.uk>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Dave Hansen" <dave.hansen@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        linux-mm@...ck.org, "Andrew Morton" <akpm@...ux-foundation.org>,
        "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
        "Nicholas Piggin" <npiggin@...il.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 7/8] membarrier: Remove arm (32) support for SYNC_CORE

On Thu, Jun 17, 2021, at 6:51 AM, Mark Rutland wrote:
> On Thu, Jun 17, 2021 at 06:41:41AM -0700, Andy Lutomirski wrote:

> > In any event, I’m even more convinced that no new SYNC_CORE arches
> > should be added. We need a new API that just does the right thing. 
> 
> My intuition is the other way around, and that this is a gnereally
> useful thing for architectures that require context synchronization.

Except that you can't use it in a generic way.  You have to know the specific rules for your arch.

> 
> It's not clear to me what "the right thing" would mean specifically, and
> on architectures with userspace cache maintenance JITs can usually do
> the most optimal maintenance, and only need help for the context
> synchronization.
> 

This I simply don't believe -- I doubt that any sane architecture really works like this.  I wrote an email about it to Intel that apparently generated internal discussion but no results.  Consider:

mmap(some shared library, some previously unmapped address);

this does no heavyweight synchronization, at least on x86.  There is no "serializing" instruction in the fast path, and it *works* despite anything the SDM may or may not say.

We can and, IMO, should develop a sane way for user programs to install instructions into VMAs, for security-conscious software to verify them (by splitting the read and write sides?), and for their consumers to execute them, without knowing any arch details.  And I think this can be done with no IPIs except for possible TLB flushing when needed, at least on most architectures.  It would require a nontrivial amount of design work, and it would not resemble sys_cacheflush() or SYNC_CORE.

--Andy