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SubjectRe: [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@linux.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.

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

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