Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

[Borislav Petkov]

On Sun, Jan 21, 2018 at 09:49:06AM +0000, David Woodhouse wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> Expose indirect_branch_prediction_barrier() for use in subsequent patches.
> 
> [karahmed: remove the special-casing of skylake for using IBPB (wtf?),
>            switch to using ALTERNATIVES instead of static_cpu_has]
> [dwmw2:    set up ax/cx/dx in the asm too so it gets NOP'd out]
> 
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
> Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/cpufeatures.h   |  1 +
>  arch/x86/include/asm/nospec-branch.h | 16 ++++++++++++++++
>  arch/x86/kernel/cpu/bugs.c           |  7 +++++++
>  3 files changed, 24 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 8c9e5c0..cf28399 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -207,6 +207,7 @@
>  #define X86_FEATURE_RETPOLINE_AMD	( 7*32+13) /* AMD Retpoline mitigation for Spectre variant 2 */
>  #define X86_FEATURE_INTEL_PPIN		( 7*32+14) /* Intel Processor Inventory Number */
>  
> +#define X86_FEATURE_IBPB		( 7*32+16) /* Using Indirect Branch Prediction Barrier */

Right, and as AMD has a separate bit for this in CPUID_80000008_EBX[12],
we probably don't really need the synthetic bit here but simply use the
one at (13*32+12) - word 13.

>  #define X86_FEATURE_AMD_PRED_CMD	( 7*32+17) /* Prediction Command MSR (AMD) */
>  #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
>  #define X86_FEATURE_RSB_CTXSW		( 7*32+19) /* Fill RSB on context switches */
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 4ad4108..c333c95 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -218,5 +218,21 @@ static inline void vmexit_fill_RSB(void)
>  #endif
>  }
>  
> +static inline void indirect_branch_prediction_barrier(void)

I like ibp_barrier() better.

> +{
> +	unsigned long ax, cx, dx;
> +
> +	asm volatile(ALTERNATIVE("",
> +				 "movl %[msr], %%ecx\n\t"
> +				 "movl %[val], %%eax\n\t"
> +				 "movl $0, %%edx\n\t"
> +				 "wrmsr",
> +				 X86_FEATURE_IBPB)
> +		     : "=a" (ax), "=c" (cx), "=d" (dx)
> +		     : [msr] "i" (MSR_IA32_PRED_CMD),
> +		       [val] "i" (PRED_CMD_IBPB)
> +		     : "memory");
> +}

Btw, we can simplify this a bit by dropping the inputs and marking the 3
GPRs as clobbered:

        alternative_input("",
                          "mov $0x49, %%ecx\n\t"
                          "mov $1, %%eax\n\t"
                          "xor %%edx, %%edx\n\t"
                          "wrmsr\n\t",
                          X86_FEATURE_IBPB,
                          ASM_NO_INPUT_CLOBBER("eax", "ecx", "edx", "memory"));


The "memory" clobber is probably not really needed but it wouldn't
hurt...

Also, above says:

> switch to using ALTERNATIVES instead of static_cpu_has]

Why?

	if (static_cpu_has(X86_FEATURE_IBPB))
		wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);

It can't get any more readable than this. Why even f*ck with
alternatives?

> +
>  #endif /* __ASSEMBLY__ */
>  #endif /* __NOSPEC_BRANCH_H__ */
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index 390b3dc..96548ff 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -249,6 +249,13 @@ static void __init spectre_v2_select_mitigation(void)
>  		setup_force_cpu_cap(X86_FEATURE_RSB_CTXSW);
>  		pr_info("Filling RSB on context switch\n");
>  	}
> +
> +	/* Initialize Indirect Branch Prediction Barrier if supported */
> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL) ||
> +	    boot_cpu_has(X86_FEATURE_AMD_PRED_CMD)) {
> +		setup_force_cpu_cap(X86_FEATURE_IBPB);
> +		pr_info("Enabling Indirect Branch Prediction Barrier\n");

We don't really need the pr_info as "ibpb" will appear in /proc/cpuinfo.

-- 
Regards/Gruss,
    Boris.

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