Re: [PATCH v3 1/3] x86/idle: Disable IBRS when cpu is offline

[Waiman Long]

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@xxxxxxxxxx>
> > ---
> >   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);
> >