On March 25, 2020 3:14:31 AM PDT, Andrew Cooper <andrew.coop...@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 <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...
>                */

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.

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