[PATCH] x86/smpboot: Remove 486-isms from the modern AP boot path

[Andrew Cooper]

Linux has an implementation of the Universal Start-up Algorithm (MP spec,
Appendix B.4, Application Processor Startup), which includes unconditionally
writing to the Bios Data Area and CMOS registers.

The warm reset vector is only necessary in the non-integrated Local APIC case.
UV and Jailhouse already have an opt-out for this behaviour, but blindly using
the BDA and CMOS on a UEFI or other reduced hardware system isn't clever.

Drop the warm_reset flag from struct x86_legacy_features, and tie the warm
vector modifications to the integrated-ness of the Local APIC.  This has the
advantage of compiling the warm reset logic out entirely for 64bit builds.

CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
CC: James Morris <jmorris@xxxxxxxxx>
CC: David Howells <dhowells@xxxxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Matthew Garrett <mjg59@xxxxxxxxxx>
CC: Josh Boyer <jwboyer@xxxxxxxxxx>
CC: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx>
CC: Steve Wahl <steve.wahl@xxxxxxx>
CC: Mike Travis <mike.travis@xxxxxxx>
CC: Dimitri Sivanich <dimitri.sivanich@xxxxxxx>
CC: Arnd Bergmann <arnd@xxxxxxxx>
CC: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
CC: Giovanni Gherdovich <ggherdovich@xxxxxxx>
CC: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
CC: Len Brown <len.brown@xxxxxxxxx>
CC: Kees Cook <keescook@xxxxxxxxxxxx>
CC: Martin Molnar <martin.molnar.programming@xxxxxxxxx>
CC: Pingfan Liu <kernelfans@xxxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: jailhouse-dev@xxxxxxxxxxxxxxxx
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
Thomas: I finally found the reference we were discussing in Portland.  Sorry
this patch took so long.

I don't have any non-integrated APIC hardware to test with.  Can anyone help
me out?
---
 arch/x86/include/asm/x86_init.h    |  1 -
 arch/x86/kernel/apic/x2apic_uv_x.c |  1 -
 arch/x86/kernel/jailhouse.c        |  1 -
 arch/x86/kernel/platform-quirks.c  |  1 -
 arch/x86/kernel/smpboot.c          | 21 ++++++++++++---------
 5 files changed, 12 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
index 96d9cd208610..006a5d7fd7eb 100644
--- a/arch/x86/include/asm/x86_init.h
+++ b/arch/x86/include/asm/x86_init.h
@@ -229,7 +229,6 @@ enum x86_legacy_i8042_state {
 struct x86_legacy_features {
 	enum x86_legacy_i8042_state i8042;
 	int rtc;
-	int warm_reset;
 	int no_vga;
 	int reserve_bios_regions;
 	struct x86_legacy_devices devices;
diff --git a/arch/x86/kernel/apic/x2apic_uv_x.c b/arch/x86/kernel/apic/x2apic_uv_x.c
index ad53b2abc859..5afcfd193592 100644
--- a/arch/x86/kernel/apic/x2apic_uv_x.c
+++ b/arch/x86/kernel/apic/x2apic_uv_x.c
@@ -343,7 +343,6 @@ static int __init uv_acpi_madt_oem_check(char *_oem_id, char *_oem_table_id)
 	} else if (!strcmp(oem_table_id, "UVH")) {
 		/* Only UV1 systems: */
 		uv_system_type = UV_NON_UNIQUE_APIC;
-		x86_platform.legacy.warm_reset = 0;
 		__this_cpu_write(x2apic_extra_bits, pnodeid << uvh_apicid.s.pnode_shift);
 		uv_set_apicid_hibit();
 		uv_apic = 1;
diff --git a/arch/x86/kernel/jailhouse.c b/arch/x86/kernel/jailhouse.c
index 6eb8b50ea07e..d628fe92d6af 100644
--- a/arch/x86/kernel/jailhouse.c
+++ b/arch/x86/kernel/jailhouse.c
@@ -210,7 +210,6 @@ static void __init jailhouse_init_platform(void)
 	x86_platform.calibrate_tsc	= jailhouse_get_tsc;
 	x86_platform.get_wallclock	= jailhouse_get_wallclock;
 	x86_platform.legacy.rtc		= 0;
-	x86_platform.legacy.warm_reset	= 0;
 	x86_platform.legacy.i8042	= X86_LEGACY_I8042_PLATFORM_ABSENT;
 
 	legacy_pic			= &null_legacy_pic;
diff --git a/arch/x86/kernel/platform-quirks.c b/arch/x86/kernel/platform-quirks.c
index b348a672f71d..d922c5e0c678 100644
--- a/arch/x86/kernel/platform-quirks.c
+++ b/arch/x86/kernel/platform-quirks.c
@@ -9,7 +9,6 @@ void __init x86_early_init_platform_quirks(void)
 {
 	x86_platform.legacy.i8042 = X86_LEGACY_I8042_EXPECTED_PRESENT;
 	x86_platform.legacy.rtc = 1;
-	x86_platform.legacy.warm_reset = 1;
 	x86_platform.legacy.reserve_bios_regions = 0;
 	x86_platform.legacy.devices.pnpbios = 1;
 
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index d85e91a8aa8c..e2ebb0be2ee3 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1049,18 +1049,21 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
 	 * the targeted processor.
 	 */
 
-	if (x86_platform.legacy.warm_reset) {
+	/*
+	 * APs are typically started in one of two ways:
+	 *
+	 * - On 486-era hardware with a non-integrated Local APIC, a single
+	 *   INIT IPI is sent.  When the AP comes out of reset, the BIOS
+	 *   follows the warm reset vector to start_ip.
+	 * - On everything with an integrated Local APIC, the start_ip is
+	 *   provided in the Startup IPI message, sent as part of an
+	 *   INIT-SIPI-SIPI sequence.
+	 */
+	if (!APIC_INTEGRATED(boot_cpu_apic_version)) {
 
 		pr_debug("Setting warm reset code and vector.\n");
 
 		smpboot_setup_warm_reset_vector(start_ip);
-		/*
-		 * Be paranoid about clearing APIC errors.
-		*/
-		if (APIC_INTEGRATED(boot_cpu_apic_version)) {
-			apic_write(APIC_ESR, 0);
-			apic_read(APIC_ESR);
-		}
 	}
 
 	/*
@@ -1118,7 +1121,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
 		}
 	}
 
-	if (x86_platform.legacy.warm_reset) {
+	if (!APIC_INTEGRATED(boot_cpu_apic_version)) {
 		/*
 		 * Cleanup possible dangling ends...
 		 */
-- 
2.11.0