linux-kernel - Re: [PATCHV2 1/3] x86, ras: Add new infrastructure for machine check fixup tables

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Date:	Mon, 14 Dec 2015 17:00:59 -0800
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-nvdimm@...1.01.org, x86@...nel.org
Subject: Re: [PATCHV2 1/3] x86, ras: Add new infrastructure for machine check
 fixup tables

On Sat, Dec 12, 2015 at 11:11:42AM +0100, Borislav Petkov wrote:
> > +config MCE_KERNEL_RECOVERY
> > +	depends on X86_MCE && X86_64
> > +	def_bool y
> 
> Shouldn't that depend on NVDIMM or whatnot? Looks too generic now.

Not sure what the "whatnot" would be though.  Making it depend on
X86_MCE should keep it out of the tiny configurations. By the time
you have MCE support, this seems like a pretty small incremental
change.

> > +#ifdef CONFIG_MCE_KERNEL_RECOVERY
> > +int fixup_mcexception(struct pt_regs *regs, u64 addr)
> > +{
> 
> If you move the #ifdef here, you can save yourself the ifdeffery in the
> header above.

I realized I didn't need the inline stub function in the header.

> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index 1781e54ea6d3..21bb20d1172a 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -473,6 +473,12 @@
> >  		VMLINUX_SYMBOL(__start___ex_table) = .;			\
> >  		*(__ex_table)						\
> >  		VMLINUX_SYMBOL(__stop___ex_table) = .;			\
> > +	}								\
> > +	. = ALIGN(align);						\
> > +	__mcex_table : AT(ADDR(__mcex_table) - LOAD_OFFSET) {		\
> > +		VMLINUX_SYMBOL(__start___mcex_table) = .;		\
> > +		*(__mcex_table)						\
> > +		VMLINUX_SYMBOL(__stop___mcex_table) = .;		\
> 
> Of all the places, this one is missing #ifdef CONFIG_MCE_KERNEL_RECOVERY.

Is there some cpp magic to use an #ifdef inside a multi-line macro like this?
Impact of not having the #ifdef is two extra symbols (the start/stop ones)
in the symbol table of the final binary. If that's unacceptable I can fall
back to an earlier unpublished version that had separate EXCEPTION_TABLE and
MCEXCEPTION_TABLE macros with both invoked in the x86 vmlinux.lds.S file.

> You can make this one a bit more readable by doing:
> 
> /* Given an address, look for it in the machine check exception tables. */
> const struct exception_table_entry *
> search_mcexception_tables(unsigned long addr)
> {
> #ifdef CONFIG_MCE_KERNEL_RECOVERY
>         return search_extable(__start___mcex_table,
>                                __stop___mcex_table - 1, addr);
> #endif
> }

I got rid of the local variable and the return ... but left the
#ifdef/#endif around the whole function.

-Tony
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