| Message ID | 20230622003603.1188364-2-longman@redhat.com (mailing list archive) |
|---|---|
| State | Changes Requested, archived |
| Headers | show |
| Series | x86/speculation: Disable IBRS when idle | expand |
On 6/21/23 17:36, Waiman Long wrote: > Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle") > disables IBRS when the CPU enters long idle. However, when a CPU > becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS > is enabled. That will impact the performance of a sibling CPU. Mitigate > this performance impact by clearing all the mitigation bits in SPEC_CTRL > MSR when offline. When the CPU is online again, it will be re-initialized > and so restoring the SPEC_CTRL value isn't needed. > > Add a comment to say that native_play_dead() is a __noreturn function, > but it can't be marked as such to avoid confusion about the missing > MSR restoration code. > > Signed-off-by: Waiman Long <longman@redhat.com> > --- > arch/x86/kernel/smpboot.c | 10 ++++++++++ > 1 file changed, 10 insertions(+) > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c > index 352f0ce1ece4..7bc33885518c 100644 > --- a/arch/x86/kernel/smpboot.c > +++ b/arch/x86/kernel/smpboot.c > @@ -84,6 +84,7 @@ > #include <asm/hw_irq.h> > #include <asm/stackprotector.h> > #include <asm/sev.h> > +#include <asm/nospec-branch.h> > > /* representing HT siblings of each logical CPU */ > DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map); > @@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void) > } > } > > +/* > + * naitve_play_dead() is essentially a __noreturn function, but it can't typo: native_play_dead() > + * be marked as such as the compiler may complain about it. > + */ > void native_play_dead(void) > { > + if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) { > + this_cpu_write(x86_spec_ctrl_current, 0); > + native_wrmsrl(MSR_IA32_SPEC_CTRL, 0); > + } > + > play_dead_common(); > tboot_shutdown(TB_SHUTDOWN_WFS); >
On 6/21/23 22:05, Randy Dunlap wrote: > > On 6/21/23 17:36, Waiman Long wrote: >> Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle") >> disables IBRS when the CPU enters long idle. However, when a CPU >> becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS >> is enabled. That will impact the performance of a sibling CPU. Mitigate >> this performance impact by clearing all the mitigation bits in SPEC_CTRL >> MSR when offline. When the CPU is online again, it will be re-initialized >> and so restoring the SPEC_CTRL value isn't needed. >> >> Add a comment to say that native_play_dead() is a __noreturn function, >> but it can't be marked as such to avoid confusion about the missing >> MSR restoration code. >> >> Signed-off-by: Waiman Long <longman@redhat.com> >> --- >> arch/x86/kernel/smpboot.c | 10 ++++++++++ >> 1 file changed, 10 insertions(+) >> >> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c >> index 352f0ce1ece4..7bc33885518c 100644 >> --- a/arch/x86/kernel/smpboot.c >> +++ b/arch/x86/kernel/smpboot.c >> @@ -84,6 +84,7 @@ >> #include <asm/hw_irq.h> >> #include <asm/stackprotector.h> >> #include <asm/sev.h> >> +#include <asm/nospec-branch.h> >> >> /* representing HT siblings of each logical CPU */ >> DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map); >> @@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void) >> } >> } >> >> +/* >> + * naitve_play_dead() is essentially a __noreturn function, but it can't > typo: native_play_dead() Thanks for spotting the typo. Will fix it in the next version. Cheers, Longman > >> + * be marked as such as the compiler may complain about it. >> + */ >> void native_play_dead(void) >> { >> + if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) { >> + this_cpu_write(x86_spec_ctrl_current, 0); >> + native_wrmsrl(MSR_IA32_SPEC_CTRL, 0); >> + } >> + >> play_dead_common(); >> tboot_shutdown(TB_SHUTDOWN_WFS); >>
On Wed, Jun 21, 2023 at 08:36:01PM -0400, Waiman Long wrote: > +/* > + * naitve_play_dead() is essentially a __noreturn function, but it can't > + * be marked as such as the compiler may complain about it. > + */ FWIW, we could in theory do so by marking the smp_ops.play_dead function pointer as __noreturn, but it would be tricky to teach objtool how to understand that. > void native_play_dead(void) > { > + if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) { > + this_cpu_write(x86_spec_ctrl_current, 0); > + native_wrmsrl(MSR_IA32_SPEC_CTRL, 0); > + } Can update_spec_ctrl() be used instead?
On 6/22/23 01:40, Josh Poimboeuf wrote: > On Wed, Jun 21, 2023 at 08:36:01PM -0400, Waiman Long wrote: >> +/* >> + * naitve_play_dead() is essentially a __noreturn function, but it can't >> + * be marked as such as the compiler may complain about it. >> + */ > FWIW, we could in theory do so by marking the smp_ops.play_dead function > pointer as __noreturn, but it would be tricky to teach objtool how to > understand that. I added the comment here because I had taken out the MSR restoration code. We can always replace that later on if there is a better way to do that. > >> void native_play_dead(void) >> { >> + if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) { >> + this_cpu_write(x86_spec_ctrl_current, 0); >> + native_wrmsrl(MSR_IA32_SPEC_CTRL, 0); >> + } > Can update_spec_ctrl() be used instead? Yes, the code is similar to what has been done in update_spec_ctrl(). Using it, however, will require exporting the function either by putting it into a public header or making it a global function. Cheers, Longman >
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 352f0ce1ece4..7bc33885518c 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -84,6 +84,7 @@ #include <asm/hw_irq.h> #include <asm/stackprotector.h> #include <asm/sev.h> +#include <asm/nospec-branch.h> /* representing HT siblings of each logical CPU */ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, cpu_sibling_map); @@ -1836,8 +1837,17 @@ void __noreturn hlt_play_dead(void) } } +/* + * naitve_play_dead() is essentially a __noreturn function, but it can't + * be marked as such as the compiler may complain about it. + */ void native_play_dead(void) { + if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) { + this_cpu_write(x86_spec_ctrl_current, 0); + native_wrmsrl(MSR_IA32_SPEC_CTRL, 0); + } + play_dead_common(); tboot_shutdown(TB_SHUTDOWN_WFS);
Commit bf5835bcdb96 ("intel_idle: Disable IBRS during long idle") disables IBRS when the CPU enters long idle. However, when a CPU becomes offline, the IBRS bit is still set when X86_FEATURE_KERNEL_IBRS is enabled. That will impact the performance of a sibling CPU. Mitigate this performance impact by clearing all the mitigation bits in SPEC_CTRL MSR when offline. When the CPU is online again, it will be re-initialized and so restoring the SPEC_CTRL value isn't needed. Add a comment to say that native_play_dead() is a __noreturn function, but it can't be marked as such to avoid confusion about the missing MSR restoration code. Signed-off-by: Waiman Long <longman@redhat.com> --- arch/x86/kernel/smpboot.c | 10 ++++++++++ 1 file changed, 10 insertions(+)