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SubjectRe: [patch 05/24] x86/speculation: Disable STIBP when enhanced IBRS is in use
On Wed, Nov 21, 2018 at 09:14:35PM +0100, Thomas Gleixner wrote:
> From: Tim Chen <tim.c.chen@linux.intel.com>
>
> If enhanced IBRS is active, STIBP is redundant for mitigating Spectre v2
> user space exploits from hyperthread sibling.
>
> Disable STIBP when enhanced IBRS is used.
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>
> ---
> arch/x86/kernel/cpu/bugs.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -321,6 +321,10 @@ static bool stibp_needed(void)
> if (spectre_v2_enabled == SPECTRE_V2_NONE)
> return false;
>
> + /* Enhanced IBRS makes using STIBP unnecessary. */
> + if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> + return false;
> +
> if (!boot_cpu_has(X86_FEATURE_STIBP))
> return false;
>
> @@ -846,6 +850,9 @@ static ssize_t l1tf_show_state(char *buf
>
> static char *stibp_state(void)
> {
> + if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> + return "";

If
spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED

then SPEC_CTRL_STIBP should not be set in x86_spec_ctrl_base
(stibp_needed() prevents the setting in arch_smt_update()) so the above
check should not be needed.

I *think*.

--
Regards/Gruss,
Boris.

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

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