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:   Thu, 22 Jun 2023 08:43:31 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     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 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@...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()?
> 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