Re: [PATCH 2/2] x86/mce: Add quirk for instruction recovery on SandyBridge processors

[Borislav Petkov]

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.

-- 
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Boris.

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