linux-kernel - Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

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Date:   Fri, 16 Feb 2018 10:44:06 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     mingo@...nel.org, hpa@...or.com, tglx@...utronix.de,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        dwmw@...zon.co.uk, peterz@...radead.org,
        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/pti] x86/speculation: Use IBRS if available before
 calling into firmware

On 02/11/2018 11:19 AM, tip-bot for David Woodhouse wrote:
		\
>  })
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 300cc15..788c4da 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -162,17 +162,36 @@ static inline void vmexit_fill_RSB(void)
>  #endif
>  }
>  
> +#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"		\
> +				 "wrmsr",			\
> +				 _feature)			\
> +		     : : [msr] "i" (_msr), [val] "i" (_val)	\
> +		     : "eax", "ecx", "edx", "memory")
> +

I encountered hang on a machine but not others when using the above
macro.  It is probably an alignment thing with ALTERNATIVE as the problem went
away after I made the change below:

Tim

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 8f2ff74..0f65bd2 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -148,6 +148,7 @@ extern char __indirect_thunk_end[];
 
 #define alternative_msr_write(_msr, _val, _feature)            \
        asm volatile(ALTERNATIVE("",                            \
+                               ".align 16\n\t"                \
                                "movl %[msr], %%ecx\n\t"       \
                                "movl %[val], %%eax\n\t"       \
                                "movl $0, %%edx\n\t"           \