linux-kernel - [PATCH v2 5/5] x86/idle: Disable IBRS entering mwait idle and enable it on wakeup

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Message-Id: <20230620140625.1001886-6-longman@redhat.com>
Date:   Tue, 20 Jun 2023 10:06:25 -0400
From:   Waiman Long <longman@...hat.com>
To:     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>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Len Brown <lenb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-pm@...r.kernel.org, Robin Jarry <rjarry@...hat.com>,
        Joe Mario <jmario@...hat.com>, Waiman Long <longman@...hat.com>
Subject: [PATCH v2 5/5] x86/idle: Disable IBRS entering mwait idle and enable it on wakeup

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.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 arch/x86/include/asm/mwait.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/include/asm/mwait.h b/arch/x86/include/asm/mwait.h
index 778df05f8539..1e36cdc21661 100644
--- a/arch/x86/include/asm/mwait.h
+++ b/arch/x86/include/asm/mwait.h
@@ -108,15 +108,32 @@ static __always_inline void __sti_mwait(unsigned long eax, unsigned long ecx)
 static __always_inline void mwait_idle_with_hints(unsigned long eax, unsigned long ecx)
 {
 	if (static_cpu_has_bug(X86_BUG_MONITOR) || !current_set_polling_and_test()) {
+		bool ibrs_disabled = false;
+		u64 spec_ctrl;
+
 		if (static_cpu_has_bug(X86_BUG_CLFLUSH_MONITOR)) {
 			mb();
 			clflush((void *)&current_thread_info()->flags);
 			mb();
 		}
 
+		if (irqs_disabled() &&
+		    cpu_feature_enabled(X86_FEATURE_KERNEL_IBRS)) {
+			/* NMI always enable IBRS on exception entry */
+			ibrs_disabled = true;
+			spec_ctrl = spec_ctrl_current();
+			__this_cpu_write(x86_spec_ctrl_current, 0);
+			native_wrmsrl(MSR_IA32_SPEC_CTRL, 0);
+		}
+
 		__monitor((void *)&current_thread_info()->flags, 0, 0);
 		if (!need_resched())
 			__mwait(eax, ecx);
+
+		if (ibrs_disabled) {
+			native_wrmsrl(MSR_IA32_SPEC_CTRL, spec_ctrl);
+			__this_cpu_write(x86_spec_ctrl_current, spec_ctrl);
+		}
 	}
 	current_clr_polling();
 }
-- 
2.31.1