Messages in this thread | | | Date | Sat, 3 Dec 2016 13:44:06 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation |
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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.
Good mailing practices for 400: avoid top-posting and trim the reply.
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