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x86/bugs: Optimize SPEC_CTRL MSR writes

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When changing SPEC_CTRL for user control, the WRMSR can be delayed
until return-to-user when KERNEL_IBRS has been enabled.

This avoids an MSR write during context switch.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
This commit is contained in:
Peter Zijlstra 2022-06-14 23:15:54 +02:00 committed by Borislav Petkov
parent 2dbb887e87
commit c779bc1a90
3 changed files with 14 additions and 8 deletions
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2
arch/x86/include/asm/nospec-branch.h
View File

@ -253,7 +253,7 @@ static inline void indirect_branch_prediction_barrier(void)
/* The Intel SPEC CTRL MSR base value cache */
extern u64 x86_spec_ctrl_base;
extern void write_spec_ctrl_current(u64 val);
extern void write_spec_ctrl_current(u64 val, bool force);
/*
* With retpoline, we must use IBRS to restrict branch prediction

18
arch/x86/kernel/cpu/bugs.c
View File

@ -63,13 +63,19 @@ static DEFINE_MUTEX(spec_ctrl_mutex);
* Keep track of the SPEC_CTRL MSR value for the current task, which may differ
* from x86_spec_ctrl_base due to STIBP/SSB in __speculation_ctrl_update().
*/
void write_spec_ctrl_current(u64 val)
void write_spec_ctrl_current(u64 val, bool force)
{
if (this_cpu_read(x86_spec_ctrl_current) == val)
return;
this_cpu_write(x86_spec_ctrl_current, val);
wrmsrl(MSR_IA32_SPEC_CTRL, val);
/*
* When KERNEL_IBRS this MSR is written on return-to-user, unless
* forced the update can be delayed until that time.
*/
if (force || !cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS))
wrmsrl(MSR_IA32_SPEC_CTRL, val);
}
/*
@ -1297,7 +1303,7 @@ static void __init spectre_v2_select_mitigation(void)
if (spectre_v2_in_eibrs_mode(mode)) {
/* Force it so VMEXIT will restore correctly */
x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
write_spec_ctrl_current(x86_spec_ctrl_base);
write_spec_ctrl_current(x86_spec_ctrl_base, true);
}
switch (mode) {
@ -1352,7 +1358,7 @@ static void __init spectre_v2_select_mitigation(void)
static void update_stibp_msr(void * __unused)
{
write_spec_ctrl_current(x86_spec_ctrl_base);
write_spec_ctrl_current(x86_spec_ctrl_base, true);
}
/* Update x86_spec_ctrl_base in case SMT state changed. */
@ -1595,7 +1601,7 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
x86_amd_ssb_disable();
} else {
x86_spec_ctrl_base |= SPEC_CTRL_SSBD;
write_spec_ctrl_current(x86_spec_ctrl_base);
write_spec_ctrl_current(x86_spec_ctrl_base, true);
}
}
@ -1846,7 +1852,7 @@ int arch_prctl_spec_ctrl_get(struct task_struct *task, unsigned long which)
void x86_spec_ctrl_setup_ap(void)
{
if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
write_spec_ctrl_current(x86_spec_ctrl_base);
write_spec_ctrl_current(x86_spec_ctrl_base, true);
if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
x86_amd_ssb_disable();

2
arch/x86/kernel/process.c
View File

@ -600,7 +600,7 @@ static __always_inline void __speculation_ctrl_update(unsigned long tifp,
}
if (updmsr)
write_spec_ctrl_current(msr);
write_spec_ctrl_current(msr, false);
}
static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)