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

[hpa@zytor.com]

On March 25, 2020 3:14:31 AM PDT, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>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@linutronix.de>
>CC: Ingo Molnar <mingo@redhat.com>
>CC: Borislav Petkov <bp@alien8.de>
>CC: "H. Peter Anvin" <hpa@zytor.com>
>CC: x86@kernel.org
>CC: Jan Kiszka <jan.kiszka@siemens.com>
>CC: James Morris <jmorris@namei.org>
>CC: David Howells <dhowells@redhat.com>
>CC: Andrew Cooper <andrew.cooper3@citrix.com>
>CC: Matthew Garrett <mjg59@google.com>
>CC: Josh Boyer <jwboyer@redhat.com>
>CC: Zhenzhong Duan <zhenzhong.duan@oracle.com>
>CC: Steve Wahl <steve.wahl@hpe.com>
>CC: Mike Travis <mike.travis@hpe.com>
>CC: Dimitri Sivanich <dimitri.sivanich@hpe.com>
>CC: Arnd Bergmann <arnd@arndb.de>
>CC: "Peter Zijlstra (Intel)" <peterz@infradead.org>
>CC: Giovanni Gherdovich <ggherdovich@suse.cz>
>CC: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
>CC: Len Brown <len.brown@intel.com>
>CC: Kees Cook <keescook@chromium.org>
>CC: Martin Molnar <martin.molnar.programming@gmail.com>
>CC: Pingfan Liu <kernelfans@gmail.com>
>CC: linux-kernel@vger.kernel.org
>CC: jailhouse-dev@googlegroups.com
>Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>---
>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...
> 		 */

We don't support SMP on 486 and haven't for a very long time. Is there any reason to retain that code at all?
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.