linux-kernel - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Date:   Fri, 5 Jan 2018 19:09:01 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

On Fri, Jan 5, 2018 at 6:52 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The fact is, we have to stop speculating when access_ok() does *not*
> fail - because that's when we'll actually do the access. And it's that
> access that needs to be non-speculative.

I also suspect we should probably do this entirely differently.

Maybe the whole lfence can be part of uaccess_begin() instead (ie
currently 'stac()'). That would fit the existing structure better, I
think. And it would avoid any confusion about the whole "when to stop
speculation".

                 Linus