Re: [RFC PATCH 3/4] x86/speculation: Use IBRS if available before calling into firmware

[Borislav Petkov]

Just some random thoughts:

On Wed, Feb 07, 2018 at 12:03:13AM +0000, David Woodhouse wrote:
> +#define alternative_msr_write(_msr, _val, _feature)		\
> +	asm volatile(ALTERNATIVE("",				\
> +				 "movl %[msr], %%ecx\n\t"	\
> +				 "movl %[val], %%eax\n\t"	\
> +				 "movl $0, %%edx\n\t"		\

We'll never write anything except 0 in %edx?

> +				 "wrmsr",			\
> +				 _feature)			\
> +		     : : [msr] "i" (_msr), [val] "i" (_val)	\
> +		     : "eax", "ecx", "edx", "memory")

So I'm not crazy about making it a separate macro because TBH it doesn't
look too generic to do that but then again what do I know, considering
recent history. :-)

	 [ Maybe I need to not look at the spectral meltdown for a
	   couple of weeks and simply take a break. ]

I guess it is fine if it is in nospec-branch.h and not prefix it with
alternative_ so that it doesn't give people ideas.

Oh and firmware_restrict_branch_speculation_start/end() is just too long
a name.

All IMHO, of course.

-- 
Regards/Gruss,
    Boris.

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