linux-kernel - Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync

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Date:   Sat, 3 Dec 2016 13:44:06 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Anvin <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Matthew Whitehead <tedheadster@...il.com>,
        Henrique de Moraes Holschuh <hmh@....eng.br>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()
 implementation

On Fri, Dec 02, 2016 at 09:38:38AM -0800, Andy Lutomirski wrote:
> TBH, I didn't start down this path for performance.  I did it because
> I wanted to kill off a CPUID that was breaking on old CPUs that don't
> have CPUID.  So I propose MOV-to-CR2 followed by an unconditional
> jump.  My goal here is to make the #*!& thing work reliably and not be
> ludicrously slow.  Borislav and I mulled over using an alternative to
> use CPUID if and only if we have CPUID, but that doesn't work because
> we call sync_core() before we're done applying alternatives.

Btw if the noinline thing which Linus suggested, works out, we can still
do the alternatives thing with CPUID in sync_core() because we won't
need it in alternatives.c itself anymore.

Just putting it on the table, I know you complained about the mess
yesterday on IRC and in case the CR2 move looks painful on xen, we can
still do what we initially considered. I.e., that thing:

+	/* Do a CPUID if available, otherwise do a forward jump. */
+	alternative_io("jmp 1f\n\t1:", "cpuid",
+			X86_FEATURE_CPUID,
+			"=a" (tmp),
+			"0" (1)
+			: "ebx", "ecx", "edx", "memory");

-- 
Regards/Gruss,
    Boris.

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