Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to x86_spec_ctrl_current

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 21, 2023 at 10:46:33PM -0700, Josh Poimboeuf wrote:
> On Wed, Jun 21, 2023 at 08:36:02PM -0400, Waiman Long wrote:
> > When intel_idle_ibrs() is called, it modifies the SPEC_CTRL MSR to
> > 0 in order disable IBRS. However, the new MSR value isn't reflected
> > in x86_spec_ctrl_current which is at odd with the other code that
> > keep track of its state in that percpu variable. Fix that by updating
> > x86_spec_ctrl_current percpu value to always match the content of the
> > SPEC_CTRL MSR.
> 
> Is this fixing an actual bug or is there some other reason for doing
> this?
> 
> > 
> > Signed-off-by: Waiman Long <longman@redhat.com>
> > ---
> >  drivers/idle/intel_idle.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/idle/intel_idle.c b/drivers/idle/intel_idle.c
> > index aa2d19db2b1d..07fa23707b3c 100644
> > --- a/drivers/idle/intel_idle.c
> > +++ b/drivers/idle/intel_idle.c
> > @@ -181,13 +181,17 @@ static __cpuidle int intel_idle_ibrs(struct cpuidle_device *dev,
> >  	u64 spec_ctrl = spec_ctrl_current();
> >  	int ret;
> >  
> > -	if (smt_active)
> > +	if (smt_active) {
> > +		__this_cpu_write(x86_spec_ctrl_current, 0);
> >  		native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
> > +	}
> >  
> >  	ret = __intel_idle(dev, drv, index);
> >  
> > -	if (smt_active)
> > +	if (smt_active) {
> >  		native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
> > +		__this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
> > +	}
> 
> More candidates for update_spec_ctrl()?

Both this and the play_dead case can't use update_spec_ctrl() because
RCU isn't there anymore and all that is noinstr. Additionally, both
sites rely on preemption being off already, where update_spec_ctrl()
can't do that.

That said, I suppose one could write it like so:

static __always_inline __update_spec_ctrl(u64 val)
{
	__this_cpu_write(x86_spec_ctrl_current, val);
	native_wrmsrl(MSR_IA32_SPEC_CTRL, val);
}

static void update_spec_ctrl(u64 val)
{
	preempt_disable();
	__update_spec_ctrl(val);
	preempt_enable();
}

And then you can use __update_spec_ctrl(). But that would need a wee
audit of using native_wrmsrl() in all places, probably ok, IIRC Xen
wasn't using our IBRS stuff anyway.