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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 2 Dec 2016 11:23:09 -0800
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Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@kernel.org>, Andy Lutomirski <luto@kernel.org>,
        Peter Anvin <hpa@zytor.com>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>,
        Borislav Petkov <bp@alien8.de>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Matthew Whitehead <tedheadster@gmail.com>,
        Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Cooper <andrew.cooper3@citrix.com>
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On Fri, Dec 2, 2016 at 11:03 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Dec 2, 2016 at 10:50 AM, Borislav Petkov <bp@kernel.org> wrote:
>>
>> Right, we can try to do something like invalidate_icache() or so in
>> there with the JMP so that the BSP refetches modified code and see where
>> it gets us.
>
> I'd really rather rjust mark it noinline with a comment. That way the
> return from the function acts as the control flow change.
>
>> The good thing is, the early patching paths run before SMP is
>> up but from looking at load_module(), for example, which does
>> post_relocation()->module_finalize()->apply_alternatives(), this can
>> happen late.
>>
>> Now there I'd like to avoid other cores walking into that address being
>> patched. Or are we "safe" there in the sense that load_module() happens
>> on one CPU only sequentially? (I haven't looked at that code to see
>> what's going on there, actually).
>
> 'sync_core()' doesn't help for other CPU's anyway, you need to do the
> cross-call IPI. So worrying about other CPU's is *not* a valid reason
> to keep a "sync_core()" call.
>
> Seriously, the only reason I can see for "sync_core()" really is:
>
>  - some deep non-serialized MSR  access or similar (ie things like
> firmware loading etc really might want it, and a mchine check might
> want it)

Not even firmware loading wants it.  Firmware loading needs
specifically cpuid(eax=1).  It has nothing to do with serializing
anything -- it's just a CPU bug that was turned into "architecture".
I think it really is just cross-address or cross-core modification,
and I'll add a comment to that effect.

--Andy