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Subject[PATCH v10 6/8] x86/smpboot: Support parallel startup of secondary CPUs
Date
From: David Woodhouse <dwmw@amazon.co.uk>

Rework the real-mode startup code to allow for APs to be brought up in
parallel. This is in two parts:

1. Introduce a bit-spinlock to prevent them from all using the real
mode stack at the same time.

2. Avoid the use of global variables for passing per-CPU information to
the APs.

To achieve the latter, export the cpuid_to_apicid[] array so that each
AP can find its own per_cpu data (and thus initial_gs, initial_stack and
early_gdt_descr) by searching therein based on its APIC ID.

Introduce a global variable 'smpboot_control' indicating to the AP how
it should find its APIC ID. For a serialized bringup, the APIC ID is
explicitly passed in the low bits of smpboot_control, while for parallel
mode there are flags directing the AP to find its APIC ID in CPUID leaf
0x0b (for X2APIC mode) or CPUID leaf 0x01 where 8 bits are sufficient.

Parallel startup may be disabled by a command line option, and also if:
• AMD SEV-ES is in use, since the AP may not use CPUID that early.
• X2APIC is enabled, but CPUID leaf 0xb is not present and correect.
• X2APIC is not enabled but not even CPUID leaf 0x01 exists.

Aside from the fact that APs will now look up their per-cpu data via the
newly-exported cpuid_to_apicid[] table, there is no behavioural change
intended yet, since new parallel CPUHP states have not — yet — been
added.

[ tglx: Initial proof of concept patch with bitlock and APIC ID lookup ]
[ dwmw2: Rework and testing, commit message, CPUID 0x1 and CPU0 support ]
[ seanc: Fix stray override of initial_gs in common_cpu_up() ]
[ Oleksandr Natalenko: reported suspend/resume issue fixed in
x86_acpi_suspend_lowlevel ]
Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Usama Arif <usama.arif@bytedance.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Kim Phillips <kim.phillips@amd.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
---
.../admin-guide/kernel-parameters.txt | 3 +
arch/x86/include/asm/realmode.h | 3 +
arch/x86/include/asm/smp.h | 10 +-
arch/x86/kernel/acpi/sleep.c | 7 ++
arch/x86/kernel/apic/apic.c | 2 +-
arch/x86/kernel/head_64.S | 99 ++++++++++++++++++-
arch/x86/kernel/smpboot.c | 62 +++++++++++-
arch/x86/realmode/init.c | 3 +
arch/x86/realmode/rm/trampoline_64.S | 14 +++
kernel/smpboot.c | 2 +-
10 files changed, 197 insertions(+), 8 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf..ee099b8aac6d 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3819,6 +3819,9 @@

nomodule Disable module load

+ no_parallel_bringup
+ [X86,SMP] Disable parallel brinugp of secondary cores.
+
nopat [X86] Disable PAT (page attribute table extension of
pagetables) support.

diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
index a336feef0af1..f0357cfe2fb0 100644
--- a/arch/x86/include/asm/realmode.h
+++ b/arch/x86/include/asm/realmode.h
@@ -52,6 +52,7 @@ struct trampoline_header {
u64 efer;
u32 cr4;
u32 flags;
+ u32 lock;
#endif
};

@@ -65,6 +66,8 @@ extern unsigned long initial_stack;
extern unsigned long initial_vc_handler;
#endif

+extern u32 *trampoline_lock;
+
extern unsigned char real_mode_blob[];
extern unsigned char real_mode_relocs[];

diff --git a/arch/x86/include/asm/smp.h b/arch/x86/include/asm/smp.h
index b4dbb20dab1a..33c0d5fd8af6 100644
--- a/arch/x86/include/asm/smp.h
+++ b/arch/x86/include/asm/smp.h
@@ -199,5 +199,13 @@ extern void nmi_selftest(void);
#define nmi_selftest() do { } while (0)
#endif

-#endif /* __ASSEMBLY__ */
+extern unsigned int smpboot_control;
+
+#endif /* !__ASSEMBLY__ */
+
+/* Control bits for startup_64 */
+#define STARTUP_SECONDARY 0x80000000
+#define STARTUP_APICID_CPUID_0B 0x40000000
+#define STARTUP_APICID_CPUID_01 0x20000000
+
#endif /* _ASM_X86_SMP_H */
diff --git a/arch/x86/kernel/acpi/sleep.c b/arch/x86/kernel/acpi/sleep.c
index 3b7f4cdbf2e0..47e75c056cb5 100644
--- a/arch/x86/kernel/acpi/sleep.c
+++ b/arch/x86/kernel/acpi/sleep.c
@@ -16,6 +16,7 @@
#include <asm/cacheflush.h>
#include <asm/realmode.h>
#include <asm/hypervisor.h>
+#include <asm/smp.h>

#include <linux/ftrace.h>
#include "../../realmode/rm/wakeup.h"
@@ -57,6 +58,7 @@ asmlinkage acpi_status __visible x86_acpi_enter_sleep_state(u8 state)
*/
int x86_acpi_suspend_lowlevel(void)
{
+ unsigned int __maybe_unused saved_smpboot_ctrl;
struct wakeup_header *header =
(struct wakeup_header *) __va(real_mode_header->wakeup_header);

@@ -115,6 +117,8 @@ int x86_acpi_suspend_lowlevel(void)
early_gdt_descr.address =
(unsigned long)get_cpu_gdt_rw(smp_processor_id());
initial_gs = per_cpu_offset(smp_processor_id());
+ /* Force the startup into boot mode */
+ saved_smpboot_ctrl = xchg(&smpboot_control, 0);
#endif
initial_code = (unsigned long)wakeup_long64;
saved_magic = 0x123456789abcdef0L;
@@ -127,6 +131,9 @@ int x86_acpi_suspend_lowlevel(void)
pause_graph_tracing();
do_suspend_lowlevel();
unpause_graph_tracing();
+
+ if (IS_ENABLED(CONFIG_64BIT) && IS_ENABLED(CONFIG_SMP))
+ smpboot_control = saved_smpboot_ctrl;
return 0;
}

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 20d9a604da7c..ac1d7e5da1f2 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -2377,7 +2377,7 @@ static int nr_logical_cpuids = 1;
/*
* Used to store mapping between logical CPU IDs and APIC IDs.
*/
-static int cpuid_to_apicid[] = {
+int cpuid_to_apicid[] = {
[0 ... NR_CPUS - 1] = -1,
};

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 222efd4a09bc..0e4e53d231db 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -25,6 +25,7 @@
#include <asm/export.h>
#include <asm/nospec-branch.h>
#include <asm/fixmap.h>
+#include <asm/smp.h>

/*
* We are not able to switch in one step to the final KERNEL ADDRESS SPACE
@@ -241,6 +242,83 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
UNWIND_HINT_EMPTY
ANNOTATE_NOENDBR // above

+#ifdef CONFIG_SMP
+ /*
+ * Is this the boot CPU coming up? If so everything is available
+ * in initial_gs, initial_stack and early_gdt_descr.
+ */
+ movl smpboot_control(%rip), %edx
+ testl $STARTUP_SECONDARY, %edx
+ jz .Lsetup_cpu
+
+ /*
+ * Secondary CPUs find out the offsets via the APIC ID. For parallel
+ * boot the APIC ID is retrieved from CPUID, otherwise it's encoded
+ * in smpboot_control:
+ * Bit 31 STARTUP_SECONDARY flag (checked above)
+ * Bit 30 STARTUP_APICID_CPUID_0B flag (use CPUID 0x0b)
+ * Bit 29 STARTUP_APICID_CPUID_01 flag (use CPUID 0x01)
+ * Bit 0-24 APIC ID if STARTUP_APICID_CPUID_xx flags are not set
+ */
+ testl $STARTUP_APICID_CPUID_0B, %edx
+ jnz .Luse_cpuid_0b
+ testl $STARTUP_APICID_CPUID_01, %edx
+ jnz .Luse_cpuid_01
+ andl $0x0FFFFFFF, %edx
+ jmp .Lsetup_AP
+
+.Luse_cpuid_01:
+ mov $0x01, %eax
+ cpuid
+ mov %ebx, %edx
+ shr $24, %edx
+ jmp .Lsetup_AP
+
+.Luse_cpuid_0b:
+ mov $0x0B, %eax
+ xorl %ecx, %ecx
+ cpuid
+
+.Lsetup_AP:
+ /* EDX contains the APIC ID of the current CPU */
+ xorq %rcx, %rcx
+ leaq cpuid_to_apicid(%rip), %rbx
+
+.Lfind_cpunr:
+ cmpl (%rbx,%rcx,4), %edx
+ jz .Linit_cpu_data
+ inc %ecx
+ cmpl nr_cpu_ids(%rip), %ecx
+ jb .Lfind_cpunr
+
+ /* APIC ID not found in the table. Drop the trampoline lock and bail. */
+ movq trampoline_lock(%rip), %rax
+ lock
+ btrl $0, (%rax)
+
+1: cli
+ hlt
+ jmp 1b
+
+.Linit_cpu_data:
+ /* Get the per cpu offset for the given CPU# which is in ECX */
+ leaq __per_cpu_offset(%rip), %rbx
+ movq (%rbx,%rcx,8), %rbx
+ /* Save it for GS BASE setup */
+ movq %rbx, initial_gs(%rip)
+
+ /* Calculate the GDT address */
+ movq $gdt_page, %rcx
+ addq %rbx, %rcx
+ movq %rcx, early_gdt_descr_base(%rip)
+
+ /* Find the idle task stack */
+ movq idle_threads(%rbx), %rcx
+ movq TASK_threadsp(%rcx), %rcx
+ movq %rcx, initial_stack(%rip)
+#endif /* CONFIG_SMP */
+
+.Lsetup_cpu:
/*
* We must switch to a new descriptor in kernel space for the GDT
* because soon the kernel won't have access anymore to the userspace
@@ -281,6 +359,14 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
*/
movq initial_stack(%rip), %rsp

+ /* Drop the realmode protection. For the boot CPU the pointer is NULL! */
+ movq trampoline_lock(%rip), %rax
+ testq %rax, %rax
+ jz .Lsetup_idt
+ lock
+ btrl $0, (%rax)
+
+.Lsetup_idt:
/* Setup and Load IDT */
pushq %rsi
call early_setup_idt
@@ -372,7 +458,14 @@ SYM_CODE_END(secondary_startup_64)
SYM_CODE_START(start_cpu0)
ANNOTATE_NOENDBR
UNWIND_HINT_EMPTY
- movq initial_stack(%rip), %rsp
+ /* Load the per-cpu base for CPU#0 */
+ leaq __per_cpu_offset(%rip), %rbx
+ movq (%rbx), %rbx
+
+ /* Find the idle task stack */
+ movq idle_threads(%rbx), %rcx
+ movq TASK_threadsp(%rcx), %rsp
+
jmp .Ljump_to_C_code
SYM_CODE_END(start_cpu0)
#endif
@@ -426,6 +519,7 @@ SYM_DATA(initial_vc_handler, .quad handle_vc_boot_ghcb)
* reliably detect the end of the stack.
*/
SYM_DATA(initial_stack, .quad init_thread_union + THREAD_SIZE - FRAME_SIZE)
+SYM_DATA(trampoline_lock, .quad 0);
__FINITDATA

__INIT
@@ -660,6 +754,9 @@ SYM_DATA_END(level1_fixmap_pgt)
SYM_DATA(early_gdt_descr, .word GDT_ENTRIES*8-1)
SYM_DATA_LOCAL(early_gdt_descr_base, .quad INIT_PER_CPU_VAR(gdt_page))

+ .align 16
+SYM_DATA(smpboot_control, .long 0)
+
.align 16
/* This must match the first entry in level2_kernel_pgt */
SYM_DATA(phys_base, .quad 0x0)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index b18c1385e181..6924d91b69ca 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -797,6 +797,16 @@ static int __init cpu_init_udelay(char *str)
}
early_param("cpu_init_udelay", cpu_init_udelay);

+static bool do_parallel_bringup __ro_after_init = true;
+
+static int __init no_parallel_bringup(char *str)
+{
+ do_parallel_bringup = false;
+
+ return 0;
+}
+early_param("no_parallel_bringup", no_parallel_bringup);
+
static void __init smp_quirk_init_udelay(void)
{
/* if cmdline changed it from default, leave it alone */
@@ -1084,8 +1094,6 @@ int common_cpu_up(unsigned int cpu, struct task_struct *idle)
#ifdef CONFIG_X86_32
/* Stack for startup_32 can be just as for start_secondary onwards */
per_cpu(pcpu_hot.top_of_stack, cpu) = task_top_of_stack(idle);
-#else
- initial_gs = per_cpu_offset(cpu);
#endif
return 0;
}
@@ -1110,9 +1118,14 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle,
start_ip = real_mode_header->trampoline_start64;
#endif
idle->thread.sp = (unsigned long)task_pt_regs(idle);
- early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(cpu);
initial_code = (unsigned long)start_secondary;
- initial_stack = idle->thread.sp;
+
+ if (IS_ENABLED(CONFIG_X86_32)) {
+ early_gdt_descr.address = (unsigned long)get_cpu_gdt_rw(cpu);
+ initial_stack = idle->thread.sp;
+ } else if (!do_parallel_bringup) {
+ smpboot_control = STARTUP_SECONDARY | apicid;
+ }

/* Enable the espfix hack for this CPU */
init_espfix_ap(cpu);
@@ -1512,6 +1525,47 @@ void __init native_smp_prepare_cpus(unsigned int max_cpus)

speculative_store_bypass_ht_init();

+ /*
+ * We can do 64-bit AP bringup in parallel if the CPU reports
+ * its APIC ID in CPUID (either leaf 0x0B if we need the full
+ * APIC ID in X2APIC mode, or leaf 0x01 if 8 bits are
+ * sufficient). Otherwise it's too hard. And not for SEV-ES
+ * guests because they can't use CPUID that early.
+ */
+ if (IS_ENABLED(CONFIG_X86_32) || boot_cpu_data.cpuid_level < 1 ||
+ (x2apic_mode && boot_cpu_data.cpuid_level < 0xb) ||
+ cc_platform_has(CC_ATTR_GUEST_STATE_ENCRYPT))
+ do_parallel_bringup = false;
+
+ if (do_parallel_bringup && x2apic_mode) {
+ unsigned int eax, ebx, ecx, edx;
+
+ /*
+ * To support parallel bringup in x2apic mode, the AP will need
+ * to obtain its APIC ID from CPUID 0x0B, since CPUID 0x01 has
+ * only 8 bits. Check that it is present and seems correct.
+ */
+ cpuid_count(0xb, 0, &eax, &ebx, &ecx, &edx);
+
+ /*
+ * AMD says that if executed with an umimplemented level in
+ * ECX, then it will return all zeroes in EAX. Intel says it
+ * will return zeroes in both EAX and EBX. Checking only EAX
+ * should be sufficient.
+ */
+ if (eax) {
+ pr_debug("Using CPUID 0xb for parallel CPU startup\n");
+ smpboot_control = STARTUP_SECONDARY | STARTUP_APICID_CPUID_0B;
+ } else {
+ pr_info("Disabling parallel bringup because CPUID 0xb looks untrustworthy\n");
+ do_parallel_bringup = false;
+ }
+ } else if (do_parallel_bringup) {
+ /* Without X2APIC, what's in CPUID 0x01 should suffice. */
+ pr_debug("Using CPUID 0x1 for parallel CPU startup\n");
+ smpboot_control = STARTUP_SECONDARY | STARTUP_APICID_CPUID_01;
+ }
+
snp_set_wakeup_secondary_cpu();
}

diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
index af565816d2ba..788e5559549f 100644
--- a/arch/x86/realmode/init.c
+++ b/arch/x86/realmode/init.c
@@ -154,6 +154,9 @@ static void __init setup_real_mode(void)

trampoline_header->flags = 0;

+ trampoline_lock = &trampoline_header->lock;
+ *trampoline_lock = 0;
+
trampoline_pgd = (u64 *) __va(real_mode_header->trampoline_pgd);

/* Map the real mode stub as virtual == physical */
diff --git a/arch/x86/realmode/rm/trampoline_64.S b/arch/x86/realmode/rm/trampoline_64.S
index e38d61d6562e..49ebc1636ffd 100644
--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -49,6 +49,19 @@ SYM_CODE_START(trampoline_start)
mov %ax, %es
mov %ax, %ss

+ /*
+ * Make sure only one CPU fiddles with the realmode stack
+ */
+.Llock_rm:
+ btl $0, tr_lock
+ jnc 2f
+ pause
+ jmp .Llock_rm
+2:
+ lock
+ btsl $0, tr_lock
+ jc .Llock_rm
+
# Setup stack
movl $rm_stack_end, %esp

@@ -241,6 +254,7 @@ SYM_DATA_START(trampoline_header)
SYM_DATA(tr_efer, .space 8)
SYM_DATA(tr_cr4, .space 4)
SYM_DATA(tr_flags, .space 4)
+ SYM_DATA(tr_lock, .space 4)
SYM_DATA_END(trampoline_header)

#include "trampoline_common.S"
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index 2c7396da470c..a18a21dff9bc 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -25,7 +25,7 @@
* For the hotplug case we keep the task structs around and reuse
* them.
*/
-static DEFINE_PER_CPU(struct task_struct *, idle_threads);
+DEFINE_PER_CPU(struct task_struct *, idle_threads);

struct task_struct *idle_thread_get(unsigned int cpu)
{
--
2.25.1
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