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

[Waiman Long]

On 6/22/23 01:46, 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?
It is not a bug per se. It is mainly to make the per cpu variable more 
up to date.
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   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()?

I don't think we can use update_spec_ctrl() here simply because of the 
noinstr requirement. See commit 9b461a6faae7 ("cpuidle, intel_idle: Fix 
CPUIDLE_FLAG_IBRS").

Cheers,
Longman