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

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Message-ID: <20180207111739.GA8698@pd.tnic>
Date:   Wed, 7 Feb 2018 12:17:39 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     tglx@...utronix.de, torvalds@...ux-foundation.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        tim.c.chen@...ux.intel.com, dave.hansen@...el.com,
        arjan.van.de.ven@...el.com
Subject: Re: [RFC PATCH 3/4] x86/speculation: Use IBRS if available before
 calling into firmware

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.

Good mailing practices for 400: avoid top-posting and trim the reply.