Re: [PATCH v2 2/5] x86/idle: Disable IBRS when cpu is offline

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 21, 2023 at 09:59:52AM -0400, Waiman Long wrote:
> 
> On 6/21/23 03:23, Peter Zijlstra wrote:
> > On Tue, Jun 20, 2023 at 10:06:22AM -0400, 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 and restoring the value of the MSR when it becomes
> > > online again.
> > > 
> > > Signed-off-by: Waiman Long <longman@redhat.com>
> > > ---
> > >   arch/x86/kernel/smpboot.c | 13 +++++++++++++
> > >   1 file changed, 13 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
> > > index 352f0ce1ece4..5ff82fef413c 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);
> > > @@ -1838,12 +1839,24 @@ void __noreturn hlt_play_dead(void)
> > >   void native_play_dead(void)
> > >   {
> > > +	u64 spec_ctrl = spec_ctrl_current();
> > > +
> > > +	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);
> > >   	mwait_play_dead();
> > >   	if (cpuidle_play_dead())
> > >   		hlt_play_dead();
> > > +
> > > +	if (cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
> > > +		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
> > > +		this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
> > > +	}
> > >   }
> > play_dead() is marked __noreturn
> 
> There are different versions of play_dead() in the kernel. Some of them are
> indeed marked __noreturn like the non-SMP one in arch/x86/kernel/process.c.
> The native_play_dead() that I am patching isn't one of those.

mostly by accident I think, hlt_play_dead() is, so I'm thinking
everybody (all compiler and objtool) managed to figure out
native_play_dead() is __noreturn too.