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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Tue, 23 Jan 2018 12:38:47 -0800
Message-ID: <CA+55aFxAM0NHYXxXdpKjO=Swer2-Mvjs2J6ekZo_GwPAFRC+jA@mail.gmail.com>
Subject: Re: [PATCH v2 5/5] x86/pti: Do not enable PTI on fixed Intel processors
To:     Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc:     David Woodhouse <dwmw@amazon.co.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Jan 23, 2018 at 10:45 AM, Alan Cox <gnomes@lxorguk.ukuu.org.uk> wrote:
>
> static int in_order_cpu(void)
> {
>         /* Processors with CPU id etc */
>         if (x86_match_cpu(cpu_in_order))
>                 return 1;
>         /* Other rules here */
>         return 0;
> }

Note that "in-order" does *not* necessarily mean "no speculation". You
still do branch prediction, you still have a BTB for all but the
simplest in-order things, and you may still end up running
instructions at least part-way through a pipeline. Whether it gets to
a cache replacement or not is unclear.

So that naming is very misleading.

It is quite possible (even likely) that the old in-order Atoms do not
have this issue, because it's definitely true that in-order tends to
limit speculation a lot, and any mis-predict will _probably_ kill
instructions early enough that you'd never see this.

But meltdown, for example, is based entirely on uarch details in the
memory pipeline, not on in-order vs ooo. Again, an in-order CPU is
probably less aggressive in the memory pipeline too, so there is
probably strong correlation, but there is no causal relationship.

              Linus