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

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Date:   Mon, 4 May 2020 11:33:46 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        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 Sun, May 3, 2020 at 5:57 AM David Laight <David.Laight@...lab.com> wrote:
>
> From: Linus Torvalds
> > Sent: 01 May 2020 19:29
> ...
> > And as DavidL pointed out - if you ever have "iomem" as a source or
> > destination, you need yet another case. Not because they can take
> > another kind of fault (although on some platforms you have the machine
> > checks for that too), but because they have *very* different
> > performance profiles (and the ERMS "rep movsb" sucks baby donkeys
> > through a straw).
>
>
> I was actually thinking that the nvdimm accesses need to be treated
> much more like (cached) memory mapped io space than normal system
> memory.
> So treating them the same as "iomem" and then having access functions
> that report access failures (which the current readq() doesn't)
> might make sense.

While I agree that something like copy_mc_iomem_to_{user,kernel} could
have users, nvdimm is not one of them.

> If you are using memory that 'might fail' for kernel code or data
> you really get what you deserve.

nvdimms are no less "might fail" than DRAM, recall that some nvdimms
are just DRAM with a platform promise that their contents are battery
backed.

> OTOH system response to PCIe errors is currently rather problematic.
> Mostly reads time out and return ~0u.
> This can be checked for and, if possibly valid, a second location read.

Yes, the ambiguous ~0u return needs careful handling.