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 15:08:53 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Waiman Long <longman@...hat.com>,
        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 Thu, Jun 22, 2023 at 11:38:28AM +0200, Peter Zijlstra wrote:

> 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.

Oh, I might be wrong about the preemption thing; it doesn't make sense
to do wrmsr with preemption on, so it could be simpler.

> 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);
> }
> 
> static void update_spec_ctrl(u64 val)
> {
> 	preempt_disable();
> 	__update_spec_ctrl(val);
> 	preempt_enable();
> }
> 
> And then you can use __update_spec_ctrl(). But that would need a wee
> audit of using native_wrmsrl() in all places, probably ok, IIRC Xen
> wasn't using our IBRS stuff anyway.