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Date:   Thu, 22 Jun 2023 11:40:51 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     Waiman Long <longman@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Jacob Pan <jacob.jun.pan@linux.intel.com>,
        Len Brown <lenb@kernel.org>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-pm@vger.kernel.org,
        Robin Jarry <rjarry@redhat.com>, Joe Mario <jmario@redhat.com>
Subject: Re: [PATCH v3 2/3] intel_idle: Sync up the SPEC_CTRL MSR value to
 x86_spec_ctrl_current
Message-ID: <20230622094051.GF4253@hirez.programming.kicks-ass.net>
References: <20230622003603.1188364-1-longman@redhat.com>
 <20230622003603.1188364-3-longman@redhat.com>
 <20230622054633.ulrurzzvzjijvdhn@treble>
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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?

No actual bug, he did this for his debugfs file -- which is no longer
part of the series. With that on, you can observe the
x86_spec_ctrl_current value while idle.

Arguably that's not even inconsistent because we disable/enable the
thing with IRQs disabled, so nothing can observe the difference.