Messages in this thread | | | Date | Sat, 21 Jul 2012 14:45:16 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH 2/2] x86/mce: Add quirk for instruction recovery on Sandy Bridge processors |
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On Fri, Jul 20, 2012 at 09:33:23AM -0700, Tony Luck wrote: > > Maybe define a default empty quirk_no_way_out() on the remaining > > families/vendors so that the compiler can optimize it away and we save > > ourselves the if-test? > > Perhaps I misunderstood your suggestion. I don't see how the compiler will > manage to optimize it all away. I just tried defining > > static void quirk_no_way_out_nop(int bank, struct mce *m, struct pt_regs *regs) > { > } > > and providing that as an initial value for the quirk_no_way_out > function pointer. > > Then I deleted the "if (quirk_no_way_out)". > > Looking at the assembly code produced, I now just have an unconditional call: > > callq *0x9fe992(%rip) # ffffffff81a18668 <quirk_no_way_out> > > > I'd think that a call through a function pointer to an empty function is > more expensive that testing whether that function pointer was NULL.
Agreed.
Right, so I was thinking about something along the lines of what tglx did for the CMCI poll timer: http://lkml.org/lkml/2012/7/18/79 and I've been playing with this for a bit now and am seeing the same thing as you: obviously the compiler cannot optimize away an unconditional function call to an empty function *through* a function pointer (tried gcc 4.6 and 4.7). Maybe because it is a function pointer we're calling...
If you make the function pointer const like this:
static void default_quirk_no_way_out(int bank, struct mce *m, struct pt_regs *regs) { }
static void (* const quirk_no_way_out)(int bank, struct mce *m, struct pt_regs *regs) = default_quirk_no_way_out;
The "call... " is not issued but you cannot assign to it later in __mcheck_cpu_apply_quirks().
Other things which could probably be used are alternatives or jump labels but one if-test is simply not worth the complexity.
Oh well, enough games.
Thanks.
-- Regards/Gruss, Boris.
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