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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
Download Patch File Download Diff File Expand all files Collapse all files

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 */ /* The Intel SPEC CTRL MSR base value cache */
extern u64 x86_spec_ctrl_base; 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 * 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 * 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(). * 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) if (this_cpu_read(x86_spec_ctrl_current) == val)
return; return;
this_cpu_write(x86_spec_ctrl_current, val); 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)) { if (spectre_v2_in_eibrs_mode(mode)) {
/* Force it so VMEXIT will restore correctly */ /* Force it so VMEXIT will restore correctly */
x86_spec_ctrl_base |= SPEC_CTRL_IBRS; 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) { switch (mode) {
@@ -1352,7 +1358,7 @@ static void __init spectre_v2_select_mitigation(void)
static void update_stibp_msr(void * __unused) 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. */ /* 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(); x86_amd_ssb_disable();
} else { } else {
x86_spec_ctrl_base |= SPEC_CTRL_SSBD; 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) void x86_spec_ctrl_setup_ap(void)
{ {
if (boot_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) 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) if (ssb_mode == SPEC_STORE_BYPASS_DISABLE)
x86_amd_ssb_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) if (updmsr)
write_spec_ctrl_current(msr); write_spec_ctrl_current(msr, false);
} }
static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk) static unsigned long speculation_ctrl_update_tif(struct task_struct *tsk)