Re: [patch 19/48] clockevents: Provide support for clocksource coupled comparators

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 03, 2026 at 06:44:59PM +0000, Michael Kelley wrote:

> > +static inline bool clockevent_set_next_coupled(struct clock_event_device *dev, ktime_t expires)
> > +{
> > +	u64 cycles;
> > +
> > +	if (unlikely(!(dev->features & CLOCK_EVT_FEAT_CLOCKSOURCE_COUPLED)))
> > +		return false;
> > +
> > +	if (unlikely(!ktime_expiry_to_cycles(dev->cs_id, expires, &cycles)))
> > +		return false;
> > +
> > +	if (IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED_INLINE))
> 
> Since COUPLED_INLINE is always selected for x64, there's no way to add the Hyper-V
> clockevent that is coupled but not inline. Adding the machinery to allow a second
> inline clockevent type may not be worth it, but adding a second coupled but not
> inline clockevent type on x64 should be supported. Thoughts?
> 
> After fixing the u64 typo, and temporarily not always selecting COUPLED_INLINE in
> arch/x86/Kconfig, the coupled Hyper-V TSC page clocksource and timer seem to work
> correctly, though I'm still doing some testing. I'm also working on counting the number
> of time reads to confirm the expected benefit.
> 
> Michael
> 
> > +		arch_inlined_clockevent_set_next_coupled(cycles, dev);

How about something deliciously insane like this? :-)

Then you can update the static_call to point to an asm function of your
choice that pretends to be WRMSR, while the 'native' case replaces the
CALL with CS CS CS WRMSR.

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 42447b1e1dff..5426c6fd8ec8 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1568,3 +1568,12 @@ SYM_FUNC_START(clear_bhb_loop)
 SYM_FUNC_END(clear_bhb_loop)
 EXPORT_SYMBOL_FOR_KVM(clear_bhb_loop)
 STACK_FRAME_NON_STANDARD(clear_bhb_loop)
+
+.pushsection .text, "ax"
+SYM_CODE_START(x86_clockevent_set_next_coupled_thunk)
+	ANNOTATE_NOENDBR
+	UNWIND_HINT_FUNC
+	wrmsr
+	RET
+SYM_CODE_END(x86_clockevent_set_next_coupled_thunk)
+.popsection
diff --git a/arch/x86/include/asm/clock_inlined.h b/arch/x86/include/asm/clock_inlined.h
index b2dee8db2fb9..587f2005ef60 100644
--- a/arch/x86/include/asm/clock_inlined.h
+++ b/arch/x86/include/asm/clock_inlined.h
@@ -2,6 +2,9 @@
 #ifndef _ASM_X86_CLOCK_INLINED_H
 #define _ASM_X86_CLOCK_INLINED_H
 
+#include <linux/static_call_types.h>
+#include <asm/msr-index.h>
+#include <asm/asm.h>
 #include <asm/tsc.h>
 
 struct clocksource;
@@ -13,10 +16,18 @@ static __always_inline u64 arch_inlined_clocksource_read(struct clocksource *cs)
 
 struct clock_event_device;
 
+extern void x86_clockevent_set_next_coupled_thunk(void);
+
+DECLARE_STATIC_CALL(x86_clockevent_set_next_coupled, x86_clockevent_set_next_coupled_thunk);
+
 static __always_inline void
 arch_inlined_clockevent_set_next_coupled(u64 cycles, struct clock_event_device *evt)
 {
-	native_wrmsrq(MSR_IA32_TSC_DEADLINE, cycles);
+	asm volatile("1: call " STATIC_CALL_TRAMP_STR(x86_clockevent_set_next_coupled) " \n"
+		     "2:\n"
+		     _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
+		     : ASM_CALL_CONSTRAINT
+		     : "c" (MSR_IA32_TSC_DEADLINE), "a" ((u32)cycles), "d" ((u32)(cycles >> 32)) : "memory");
 }
 
 #endif
diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index 60cab20b7901..194209f857b0 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -67,6 +67,7 @@
 #include <asm/intel-family.h>
 #include <asm/irq_regs.h>
 #include <asm/cpu.h>
+#include <asm/clock_inlined.h>
 
 #include "local.h"
 
@@ -430,6 +431,8 @@ static int lapic_next_deadline(unsigned long delta, struct clock_event_device *e
 	return 0;
 }
 
+DEFINE_STATIC_CALL(x86_clockevent_set_next_coupled, x86_clockevent_set_next_coupled_thunk);
+
 static int lapic_timer_shutdown(struct clock_event_device *evt)
 {
 	unsigned int v;
diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index 61592e41a6b1..4821d155102f 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -3,6 +3,7 @@
 #include <linux/memory.h>
 #include <linux/bug.h>
 #include <asm/text-patching.h>
+#include <asm/clock_inlined.h>
 
 enum insn_type {
 	CALL = 0, /* site call */
@@ -31,6 +32,11 @@ static const u8 retinsn[] = { RET_INSN_OPCODE, 0xcc, 0xcc, 0xcc, 0xcc };
  */
 static const u8 warninsn[] = { 0x67, 0x48, 0x0f, 0xb9, 0x3a };
 
+/*
+ * cs cs cs wrmsr
+ */
+static const u8 wrmsrinsn[] = { 0x2e, 0x2e, 0x2e, 0x0f, 0x30 };
+
 static u8 __is_Jcc(u8 *insn) /* Jcc.d32 */
 {
 	u8 ret = 0;
@@ -78,6 +84,10 @@ static void __ref __static_call_transform(void *insn, enum insn_type type,
 			emulate = code;
 			code = &warninsn;
 		}
+		if (func == x86_clockevent_set_next_coupled_thunk) {
+			emulate = code;
+			code = &wrmsrinsn;
+		}
 		break;
 
 	case NOP: