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 <t...@linutronix.de>
CC: Ingo Molnar <mi...@redhat.com>
CC: Borislav Petkov <b...@alien8.de>
CC: "H. Peter Anvin" <h...@zytor.com>
CC: x...@kernel.org
CC: Jan Kiszka <jan.kis...@siemens.com>
CC: James Morris <jmor...@namei.org>
CC: David Howells <dhowe...@redhat.com>
CC: Andrew Cooper <andrew.coop...@citrix.com>
CC: Matthew Garrett <mj...@google.com>
CC: Josh Boyer <jwbo...@redhat.com>
CC: Zhenzhong Duan <zhenzhong.d...@oracle.com>
CC: Steve Wahl <steve.w...@hpe.com>
CC: Mike Travis <mike.tra...@hpe.com>
CC: Dimitri Sivanich <dimitri.sivan...@hpe.com>
CC: Arnd Bergmann <a...@arndb.de>
CC: "Peter Zijlstra (Intel)" <pet...@infradead.org>
CC: Giovanni Gherdovich <ggherdov...@suse.cz>
CC: "Rafael J. Wysocki" <rafael.j.wyso...@intel.com>
CC: Len Brown <len.br...@intel.com>
CC: Kees Cook <keesc...@chromium.org>
CC: Martin Molnar <martin.molnar.programm...@gmail.com>
CC: Pingfan Liu <kernelf...@gmail.com>
CC: linux-ker...@vger.kernel.org
CC: jailhouse-dev@googlegroups.com
Signed-off-by: Andrew Cooper <andrew.coop...@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...
                 */
-- 
2.11.0

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