linux-kernel - Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy

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Date:   Thu, 30 Apr 2020 13:25:16 -0700
From:   "Luck, Tony" <tony.luck@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        stable <stable@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Paul Mackerras <paulus@...ba.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Erwin Tsaur <erwin.tsaur@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] Replace and improve "mcsafe" with copy_safe()

On Thu, Apr 30, 2020 at 12:50:40PM -0700, Linus Torvalds wrote:

I see your point about the namimg being important.  I think Dan's
case is indeed "copy from pmem to user" where only options for faulting
are #MC on the source addresses, and #PF on the destination.

> The only *fundamental* access would likely be a single read/write
> operation, not a copy operation. Think "get_user()" instead of
> "copy_from_user()".  Even there you get combinatorial explosions with
> access sizes, but you can often generate those automatically or with
> simple patterns, and then you can build up the copy functions from
> that if you really need to.

That's maybe very clean. But it looks like it would be hard to build
a high performance interface on top of that primitive. Remember that
for Dan's copy 99.999999999367673%[1] of copies will not hit a machine
check on the read from pmem.

Dan wants (whatever the function name) to get to a "REP MOVS" with an
exception table entry to handle the cases where there is a fault.

-Tony

[1] Likely several more '9's in there