linux-kernel - [RESEND PATCH] x86: ACPI: cstate: Optimize C3 entry on AMD CPUs

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Date:   Thu, 23 Sep 2021 23:12:05 -0700
From:   Deepak Sharma <deepak.sharma@....com>
To:     <rafael@...nel.org>, <len.brown@...el.com>, <pavel@....cz>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-pm@...r.kernel.org>, Deepak Sharma <deepak.sharma@....com>
Subject: [RESEND PATCH] x86: ACPI: cstate: Optimize C3 entry on AMD CPUs

All Zen or newer CPU which support C3 shares cache. Its not necessary to
flush the caches in software before entering C3. This will cause drop in
performance for the cores which share some caches. ARB_DIS is not used
with current AMD C state implementation. So set related flags correctly.

Signed-off-by: Deepak Sharma <deepak.sharma@....com>
---
 arch/x86/kernel/acpi/cstate.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/kernel/acpi/cstate.c b/arch/x86/kernel/acpi/cstate.c
index 7de599eba7f0..7945eae5b315 100644
--- a/arch/x86/kernel/acpi/cstate.c
+++ b/arch/x86/kernel/acpi/cstate.c
@@ -79,6 +79,21 @@ void acpi_processor_power_init_bm_check(struct acpi_processor_flags *flags,
 		 */
 		flags->bm_control = 0;
 	}
+	if (c->x86_vendor == X86_VENDOR_AMD && c->x86 >= 0x17) {
+		/*
+		 * For all AMD Zen or newer CPUs that support C3, caches
+		 * should not be flushed by software while entering C3
+		 * type state. Set bm->check to 1 so that kernel doesn't
+		 * need to execute cache flush operation.
+		 */
+		flags->bm_check = 1;
+		/*
+		 * In current AMD C state implementation ARB_DIS is no longer
+		 * used. So set bm_control to zero to indicate ARB_DIS is not
+		 * required while entering C3 type state.
+		 */
+		flags->bm_control = 0;
+	}
 }
 EXPORT_SYMBOL(acpi_processor_power_init_bm_check);
 
-- 
2.25.1