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    SubjectRe: [Patch v3 03/13] x86/speculation: Add static key for Enhanced IBRS
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    On 10/26/2018 09:58 AM, Waiman Long wrote:
    > On 10/17/2018 01:59 PM, Tim Chen wrote:
    >> Add static key to indicate whether we are using Enhanced IBRS to mitigate
    >> Spectre v2. This will be used in later patches to disengage STIBP code
    >> for Spectre v2 mitigation as STIBP is not needed when Enhanced IBRS is
    >> in use.
    >>
    >> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
    >> ---
    >> arch/x86/include/asm/nospec-branch.h | 3 +++
    >> arch/x86/kernel/cpu/bugs.c | 4 ++++
    >> 2 files changed, 7 insertions(+)
    >>
    >> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
    >> index fd2a8c1..d57e84e 100644
    >> --- a/arch/x86/include/asm/nospec-branch.h
    >> +++ b/arch/x86/include/asm/nospec-branch.h
    >> @@ -3,6 +3,7 @@
    >> #ifndef _ASM_X86_NOSPEC_BRANCH_H_
    >> #define _ASM_X86_NOSPEC_BRANCH_H_
    >>
    >> +#include <linux/static_key.h>
    >> #include <asm/alternative.h>
    >> #include <asm/alternative-asm.h>
    >> #include <asm/cpufeatures.h>
    >> @@ -228,6 +229,8 @@ enum ssb_mitigation {
    >> extern char __indirect_thunk_start[];
    >> extern char __indirect_thunk_end[];
    >>
    >> +DECLARE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
    >> +
    >> /*
    >> * On VMEXIT we must ensure that no RSB predictions learned in the guest
    >> * can be followed in the host, by overwriting the RSB completely. Both
    >> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
    >> index b2f6b8b..2fc7b4e 100644
    >> --- a/arch/x86/kernel/cpu/bugs.c
    >> +++ b/arch/x86/kernel/cpu/bugs.c
    >> @@ -142,6 +142,9 @@ static const char *spectre_v2_strings[] = {
    >> [SPECTRE_V2_IBRS_ENHANCED] = "Mitigation: Enhanced IBRS",
    >> };
    >>
    >> +DEFINE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
    >> +EXPORT_SYMBOL(spectre_v2_enhanced_ibrs);
    >> +
    >> #undef pr_fmt
    >> #define pr_fmt(fmt) "Spectre V2 : " fmt
    >>
    >> @@ -386,6 +389,7 @@ static void __init spectre_v2_select_mitigation(void)
    >> /* Force it so VMEXIT will restore correctly */
    >> x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
    >> wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
    >> + static_branch_enable(&spectre_v2_enhanced_ibrs);
    >> goto specv2_set_mode;
    >> }
    >> if (IS_ENABLED(CONFIG_RETPOLINE))
    >
    > Why you need a static key for enhanced IBRS? It is supposed to be set at
    > boot time and never get changed after that. It will be easier to use a
    > feature bit for that instead. We usually use static key when the value
    > can be changed at run time.
    >

    We're close to running out of the feature bits. So I'm trying not to
    use those.

    Tim

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