linux-kernel - Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to x86_spec_ctrl

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Date:   Wed, 21 Jun 2023 22:46:33 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, linux-pm@...r.kernel.org,
        Robin Jarry <rjarry@...hat.com>, Joe Mario <jmario@...hat.com>
Subject: Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to
 x86_spec_ctrl_current

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@...hat.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()?

-- 
Josh