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    SubjectRe: [PATCH] x86/speculation: Avoid LFENCE in FILL_RETURN_BUFFER on CPUs that lack it
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    On Fri, 2022-08-19 at 13:01 +0200, Peter Zijlstra wrote:
    > On Fri, Aug 19, 2022 at 10:47:21AM +0200, Peter Zijlstra wrote:
    > > On Fri, Aug 19, 2022 at 02:33:08AM +0200, Ben Hutchings wrote:
    > > > From: Ben Hutchings <benh@debian.org>
    > > >
    > > > The mitigation for PBRSB includes adding LFENCE instructions to the
    > > > RSB filling sequence. However, RSB filling is done on some older CPUs
    > > > that don't support the LFENCE instruction.
    > > >
    > >
    > > Wait; what? There are chips that enable the RSB mitigations and DONT
    > > have LFENCE ?!?
    >
    > So I gave in and clicked on the horrible bugzilla thing. Apparently this
    > is P3/Athlon64 era crud.
    >
    > Anyway, the added LFENCE isn't because of retbleed; it is because you
    > can steer the jnz and terminate the loop early and then not actually
    > complete the RSB stuffing.

    I know, I corrected that further down.

    > New insights etc.. So it's a geniune fix for the existing rsb stuffing.
    >
    > I'm not entirly sure what to do here. On the one hand, it's 32bit, so
    > who gives a crap, otoh we shouldn't break these ancient chips either I
    > suppose.
    >
    > How's something like so then? It goes on top of my other patch cleaning
    > up this RSB mess:
    >
    > https://lkml.kernel.org/r/Yv9m%2FhuNJLuyviIn%40worktop.programming.kicks-ass.net
    [...]

    That should be:
    https://lore.kernel.org/all/Yv9m%2FhuNJLuyviIn@worktop.programming.kicks-ass.net/
    (the redirector unescapes the URL-escaped /).

    So that puts the whole __FILL_RETURN_BUFFER inside an alternative, and
    we can't have nested alternatives. That's unfortunate.

    Ben.

    --
    Ben Hutchings
    Beware of bugs in the above code;
    I have only proved it correct, not tried it. - Donald Knuth
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