linux-kernel - Re: [PATCH v2 1/8] x86/feature: Detect the x86 IBRS feature to control Speculation

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Date:   Sat, 6 Jan 2018 13:56:33 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/8] x86/feature: Detect the x86 IBRS feature to
 control Speculation

On Fri, Jan 05, 2018 at 06:12:16PM -0800, Tim Chen wrote:

<--- This needs an introductory sentence here.

> cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature

You can write that as CPUID(7).RDX[26].

> IA32_SPEC_CTRL (0x48)
> IA32_SPEC_CTRL, bit0 – Indirect Branch Restricted Speculation (IBRS)

Ah, those are MSRs. Please say so.

> If IBRS is set, near returns and near indirect jumps/calls will not allow
> their predicted target address to be controlled by code that executed in
> a less privileged prediction mode before the IBRS mode was last written
> with a value of 1 or on another logical processor so long as all RSB

"RSB" is?

> entries from the previous less privileged prediction mode are overwritten.
> 
> * Thus a near indirect jump/call/return may be affected by code in a
> less privileged prediction mode that executed AFTER IBRS mode was last
> written with a value of 1

End sentences with a full stop.

> * There is no need to clear IBRS before writing it with a value of
> 1. Unconditionally writing it with a value of 1 after the prediction
> mode change is sufficient

This sounds strange. I know of funky MSRs like that but if it is not
the case here, no need to mention it then.

> * Note: IBRS is not required in order to isolate branch predictions for
> SMM or SGX enclaves
> 
> * Code executed by a sibling logical processor cannot control indirect
> jump/call/return predicted target when IBRS is set
> 
> * SMEP will prevent supervisor mode using RSB entries filled by user code;
> this can reduce the need for software to overwrite RSB entries
> 
> CPU performance could be reduced when running with IBRS set.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  arch/x86/include/asm/cpufeatures.h       | 1 +
>  arch/x86/include/asm/msr-index.h         | 4 ++++
>  arch/x86/kernel/cpu/scattered.c          | 1 +
>  tools/arch/x86/include/asm/cpufeatures.h | 1 +
>  4 files changed, 7 insertions(+)
> 
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 07cdd17..5ee0737 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -209,6 +209,7 @@
>  #define X86_FEATURE_AVX512_4FMAPS	( 7*32+17) /* AVX-512 Multiply Accumulation Single precision */
>  
>  #define X86_FEATURE_MBA			( 7*32+18) /* Memory Bandwidth Allocation */
> +#define X86_FEATURE_SPEC_CTRL		( 7*32+19) /* Control Speculation Control */
>  
>  /* Virtualization flags: Linux defined, word 8 */
>  #define X86_FEATURE_TPR_SHADOW		( 8*32+ 0) /* Intel TPR Shadow */
> diff --git a/arch/x86/include/asm/msr-index.h b/arch/x86/include/asm/msr-index.h
> index 34c4922..f881add 100644
> --- a/arch/x86/include/asm/msr-index.h
> +++ b/arch/x86/include/asm/msr-index.h
> @@ -42,6 +42,10 @@
>  #define MSR_PPIN_CTL			0x0000004e
>  #define MSR_PPIN			0x0000004f
>  
> +#define MSR_IA32_SPEC_CTRL		0x00000048
> +#define SPEC_CTRL_FEATURE_DISABLE_IBRS	(0 << 0)
> +#define SPEC_CTRL_FEATURE_ENABLE_IBRS	(1 << 0)

s/_FEATURE//

SPEC_CTRL_{ENABLE,DISABLE}_IBRS is good enough.

> +
>  #define MSR_IA32_PERFCTR0		0x000000c1
>  #define MSR_IA32_PERFCTR1		0x000000c2
>  #define MSR_FSB_FREQ			0x000000cd
...

-- 
Regards/Gruss,
    Boris.

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