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

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Date:   Mon, 8 Jan 2018 17:14:03 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     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 06/01/2018 03:12, Tim Chen wrote:
> cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature
> IA32_SPEC_CTRL (0x48)
> IA32_SPEC_CTRL, bit0 – Indirect Branch Restricted Speculation (IBRS)
> 
> 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
> 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
> 
> * 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
> 
> * 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>

KVM also needs STIBP, can you please add it, or perhaps split
cpuid(7,0).edx out to its own feature word?

Thanks,

Paolo

> ---
>  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)
> +
>  #define MSR_IA32_PERFCTR0		0x000000c1
>  #define MSR_IA32_PERFCTR1		0x000000c2
>  #define MSR_FSB_FREQ			0x000000cd
> diff --git a/arch/x86/kernel/cpu/scattered.c b/arch/x86/kernel/cpu/scattered.c
> index 05459ad..bc50c40 100644
> --- a/arch/x86/kernel/cpu/scattered.c
> +++ b/arch/x86/kernel/cpu/scattered.c
> @@ -24,6 +24,7 @@ static const struct cpuid_bit cpuid_bits[] = {
>  	{ X86_FEATURE_INTEL_PT,		CPUID_EBX, 25, 0x00000007, 0 },
>  	{ X86_FEATURE_AVX512_4VNNIW,    CPUID_EDX,  2, 0x00000007, 0 },
>  	{ X86_FEATURE_AVX512_4FMAPS,    CPUID_EDX,  3, 0x00000007, 0 },
> +	{ X86_FEATURE_SPEC_CTRL,	CPUID_EDX, 26, 0x00000007, 0 },
>  	{ X86_FEATURE_CAT_L3,		CPUID_EBX,  1, 0x00000010, 0 },
>  	{ X86_FEATURE_CAT_L2,		CPUID_EBX,  2, 0x00000010, 0 },
>  	{ X86_FEATURE_CDP_L3,		CPUID_ECX,  2, 0x00000010, 1 },
> diff --git a/tools/arch/x86/include/asm/cpufeatures.h b/tools/arch/x86/include/asm/cpufeatures.h
> index 800104c..5e56275 100644
> --- a/tools/arch/x86/include/asm/cpufeatures.h
> +++ b/tools/arch/x86/include/asm/cpufeatures.h
> @@ -208,6 +208,7 @@
>  #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 */
>