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Date:   Wed, 21 Jun 2023 09:32:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     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>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        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 v2 5/5] x86/idle: Disable IBRS entering mwait idle and
 enable it on wakeup
Message-ID: <20230621073212.GC2046280@hirez.programming.kicks-ass.net>
References: <20230620140625.1001886-1-longman@redhat.com>
 <20230620140625.1001886-6-longman@redhat.com>
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On Tue, Jun 20, 2023 at 10:06:25AM -0400, Waiman Long wrote:
> When a user sets "intel_idle.max_cstate=0", it will disable
> intel_idle and fall back to acpi_idle instead. The acpi_idle code
> will then call mwait_idle_with_hints() to enter idle state. So when
> X86_FEATURE_KERNEL_IBRS is enabled, it is necessary to disable IBRS
> within mwait_idle_with_hints() when IRQ was disabled to avoid performance
> degradation on silbing thread running user workload.

Urgh, no, just no. This is nasty.