linux-kernel - Re: [PATCH 2/2] x86/mce: Add quirk for instruction recovery on Sandy Bridge processors

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Date:	Fri, 20 Jul 2012 09:33:23 -0700
From:	Tony Luck <tony.luck@...el.com>
To:	Borislav Petkov <bp@...64.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Chen Gong <gong.chen@...ux.intel.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Subject: Re: [PATCH 2/2] x86/mce: Add quirk for instruction recovery on Sandy
 Bridge processors

> 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.

-Tony
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