[tip: x86/apic] x86/smpboot: Reference count on smpboot_setup_warm_reset_vector()

[tip-bot2 for David Woodhouse]

The following commit has been merged into the x86/apic branch of tip:

Commit-ID:     805ae9dc3b1c4040a842eb1714e7744af27fd30d
Gitweb:        https://git.kernel.org/tip/805ae9dc3b1c4040a842eb1714e7744af27fd30d
Author:        David Woodhouse <dwmw@xxxxxxxxxxxx>
AuthorDate:    Thu, 16 Mar 2023 22:21:01 
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 21 Mar 2023 13:35:53 +01:00

x86/smpboot: Reference count on smpboot_setup_warm_reset_vector()

When bringing up a secondary CPU from do_boot_cpu(), the warm reset flag
is set in CMOS and the starting IP for the trampoline written inside the
BDA at 0x467. Once the CPU is running, the CMOS flag is unset and the
value in the BDA cleared.

To allow for parallel bringup of CPUs, add a reference count to track the
number of CPUs currently bring brought up, and clear the state only when
the count reaches zero.

Since the RTC spinlock is required to write to the CMOS, it can be used
for mutual exclusion on the refcount too.

Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
Signed-off-by: Usama Arif <usama.arif@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: Kim Phillips <kim.phillips@xxxxxxx>
Tested-by: Oleksandr Natalenko <oleksandr@xxxxxxxxxxxxxx>
Tested-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230316222109.1940300-5-usama.arif@xxxxxxxxxxxxx

---
 arch/x86/kernel/smpboot.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index b7d478d..851477f 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -121,17 +121,20 @@ int arch_update_cpu_topology(void)
 	return retval;
 }
 
+
+static unsigned int smpboot_warm_reset_vector_count;
+
 static inline void smpboot_setup_warm_reset_vector(unsigned long start_eip)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(&rtc_lock, flags);
-	CMOS_WRITE(0xa, 0xf);
+	if (!smpboot_warm_reset_vector_count++) {
+		CMOS_WRITE(0xa, 0xf);
+		*((volatile unsigned short *)phys_to_virt(TRAMPOLINE_PHYS_HIGH)) = start_eip >> 4;
+		*((volatile unsigned short *)phys_to_virt(TRAMPOLINE_PHYS_LOW)) = start_eip & 0xf;
+	}
 	spin_unlock_irqrestore(&rtc_lock, flags);
-	*((volatile unsigned short *)phys_to_virt(TRAMPOLINE_PHYS_HIGH)) =
-							start_eip >> 4;
-	*((volatile unsigned short *)phys_to_virt(TRAMPOLINE_PHYS_LOW)) =
-							start_eip & 0xf;
 }
 
 static inline void smpboot_restore_warm_reset_vector(void)
@@ -143,10 +146,12 @@ static inline void smpboot_restore_warm_reset_vector(void)
 	 * to default values.
 	 */
 	spin_lock_irqsave(&rtc_lock, flags);
-	CMOS_WRITE(0, 0xf);
+	if (!--smpboot_warm_reset_vector_count) {
+		CMOS_WRITE(0, 0xf);
+		*((volatile u32 *)phys_to_virt(TRAMPOLINE_PHYS_LOW)) = 0;
+	}
 	spin_unlock_irqrestore(&rtc_lock, flags);
 
-	*((volatile u32 *)phys_to_virt(TRAMPOLINE_PHYS_LOW)) = 0;
 }
 
 /*