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 05:38, Peter Zijlstra wrote:
> 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@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()?
> 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);
> }

We can put this into asm/nospec-branch.h since x86_spec_ctrl_current is 
defined there as well.

Cheers,
Longman