linux-kernel - Re: [PATCH] x86/memcpy: Introduce memcpy_mcsafe

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Message-ID: <CAPcyv4iTaBNPMwqUwas+J4rxd867QL7JnQBYB8NKnYaTA-R_Tw@mail.gmail.com>
Date:   Mon, 20 Apr 2020 12:29:12 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, X86 ML <x86@...nel.org>,
        stable <stable@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tony Luck <tony.luck@...el.com>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] x86/memcpy: Introduce memcpy_mcsafe_fast

On Mon, Apr 20, 2020 at 12:13 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Apr 20, 2020 at 11:20 AM Dan Williams <dan.j.williams@...el.com> wrote:
[..]
> I really really detest the whole mcsafe garbage. And I absolutely
> *ABHOR* how nobody inside of Intel has apparently ever questioned the
> brokenness at a really fundamental level.
>
> That "I throw my hands in the air and just give up" thing is a
> disease. It's absolutely not "what else could we do".

So I grew up in the early part of my career validating ARM CPUs where
a data-abort was either precise or imprecise and the precise error
could be handled like a page fault as you know which instruction
faulted and how to restart the thread. So I didn't take x86 CPU
designers' word for it, I honestly thought that "hmm the x86 machine
check thingy looks like it's trying to implement precise vs imprecise
data-aborts, and precise / synchronous is maybe a good thing because
it's akin to a page fault". I didn't consider asynchronous to be
better because that means there is a gap between when the data
corruption is detected and when it might escape the system that some
external agent could trust the result and start acting on before the
asynchronous signal is delivered.