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Subject: Re: [PATCH v5 7/7] x86/speculation: Add basic IBPB (Indirect Branch
 Prediction Barrier) support
To:     David Woodhouse <dwmw@amazon.co.uk>, arjan@linux.intel.com,
        tglx@linutronix.de, karahmed@amazon.de, x86@kernel.org,
        linux-kernel@vger.kernel.org, bp@alien8.de, peterz@infradead.org,
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        dave.hansen@intel.com, gnomes@lxorguk.ukuu.org.uk,
        ashok.raj@intel.com, mingo@kernel.org
References: <1516896855-7642-1-git-send-email-dwmw@amazon.co.uk>
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From:   Tim Chen <tim.c.chen@linux.intel.com>
Message-ID: <85fc729e-6ff6-3a13-481c-2e0e11407225@linux.intel.com>
Date:   Fri, 26 Jan 2018 13:36:05 -0800
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On 01/25/2018 08:14 AM, David Woodhouse wrote:

>  
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 4ad4108..34e384c 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -218,5 +218,18 @@ static inline void vmexit_fill_RSB(void)
>  #endif
>  }
>  
> +static inline void indirect_branch_prediction_barrier(void)
> +{
> +	asm volatile(ALTERNATIVE("",
> +				 "movl %[msr], %%ecx\n\t"
> +				 "movl %[val], %%eax\n\t"
> +				 "movl $0, %%edx\n\t"
> +				 "wrmsr",
> +				 X86_FEATURE_IBPB)
> +		     : : [msr] "i" (MSR_IA32_PRED_CMD),
> +			 [val] "i" (PRED_CMD_IBPB)
> +		     : "eax", "ecx", "edx", "memory");

With Peter's fixing of paravirt's indirect call for wrmsr,
(https://patchwork.kernel.org/patch/10173547/)
we don't have to worry about indirect call in wrmsr anymore.

Can we use regular wrmsr here?

It will make IBPB tracing straightforward using the
MSR tracepoints.

Tim