Re: [PATCH v5 2/9] x86/split_lock: Avoid runtime reads of the TEST_CTRL MSR

[Xiaoyao Li]

On 3/24/2020 1:06 AM, Thomas Gleixner wrote:
> Xiaoyao Li <xiaoyao.li@xxxxxxxxx> writes:
> > +/*
> > + * Soft copy of MSR_TEST_CTRL initialized when we first read the
> > + * MSR. Used at runtime to avoid using rdmsr again just to collect
> > + * the reserved bits in the MSR. We assume reserved bits are the
> > + * same on all CPUs.
> > + */
> > +static u64 test_ctrl_val;
> > +
> >   /*
> >    * Locking is not required at the moment because only bit 29 of this
> >    * MSR is implemented and locking would not prevent that the operation
> > @@ -1027,16 +1035,14 @@ static void __init split_lock_setup(void)
> >    */
> >   static void __sld_msr_set(bool on)
> >   {
> > -	u64 test_ctrl_val;
> > -
> > -	rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
> > +	u64 val = test_ctrl_val;
> >   
> >   	if (on)
> > -		test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> > +		val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> >   	else
> > -		test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> > +		val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
> >   
> > -	wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
> > +	wrmsrl(MSR_TEST_CTRL, val);
> >   }
> >   
> >   /*
> > @@ -1048,11 +1054,13 @@ static void __sld_msr_set(bool on)
> >    */
> >   static void split_lock_init(struct cpuinfo_x86 *c)
> >   {
> > -	u64 test_ctrl_val;
> > +	u64 val;
> >   
> > -	if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
> > +	if (rdmsrl_safe(MSR_TEST_CTRL, &val))
> >   		goto msr_broken;
> >   
> > +	test_ctrl_val = val;
> > +
> >   	switch (sld_state) {
> >   	case sld_off:
> >   		if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val & ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT))
>
> That's just broken. Simply because
>
>         case sld_warn:
>         case sld_fatal:
>
> set the split lock detect bit, but the cache variable has it cleared
> unless it was set at boot time already.

The test_ctrl_val is not to cache the value of MSR_TEST_CTRL, but cache 
the reserved/unused bits other than MSR_TEST_CTRL_SPLIT_LOCK_DETECT bit.

> Thanks,
>
>          tglx