linux-kernel - [PATCH v2 10/38] clocksource: hyper-v: Don't save/restore TSC offset when using HV sched

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Message-ID: <20250227021855.3257188-11-seanjc@google.com>
Date: Wed, 26 Feb 2025 18:18:26 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	Sean Christopherson <seanjc@...gle.com>, Juergen Gross <jgross@...e.com>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu <wei.liu@...nel.org>, 
	Dexuan Cui <decui@...rosoft.com>, Ajay Kaher <ajay.kaher@...adcom.com>, 
	Jan Kiszka <jan.kiszka@...mens.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Daniel Lezcano <daniel.lezcano@...aro.org>, 
	John Stultz <jstultz@...gle.com>
Cc: linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, 
	kvm@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	Tom Lendacky <thomas.lendacky@....com>, Nikunj A Dadhania <nikunj@....com>
Subject: [PATCH v2 10/38] clocksource: hyper-v: Don't save/restore TSC offset
 when using HV sched_clock

Now that Hyper-V overrides the sched_clock save/restore hooks if and only
sched_clock itself is set to the Hyper-V timer, drop the invocation of the
"old" save/restore callbacks.  When the registration of the PV sched_clock
was done separate from overriding the save/restore hooks, it was possible
for Hyper-V to clobber the TSC save/restore callbacks without actually
switching to the Hyper-V timer.

Enabling a PV sched_clock is a one-way street, i.e. the kernel will never
revert to using TSC for sched_clock, and so there is no need to invoke the
TSC save/restore hooks (and if there was, it belongs in common PV code).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 drivers/clocksource/hyperv_timer.c | 10 ----------
 1 file changed, 10 deletions(-)

diff --git a/drivers/clocksource/hyperv_timer.c b/drivers/clocksource/hyperv_timer.c
index 5a52d0acf31f..4a21874e91b9 100644
--- a/drivers/clocksource/hyperv_timer.c
+++ b/drivers/clocksource/hyperv_timer.c
@@ -524,9 +524,6 @@ static __always_inline void hv_setup_sched_clock(void *sched_clock)
 }
 #elif defined CONFIG_PARAVIRT
 static u64 hv_ref_counter_at_suspend;
-static void (*old_save_sched_clock_state)(void);
-static void (*old_restore_sched_clock_state)(void);
-
 /*
  * Hyper-V clock counter resets during hibernation. Save and restore clock
  * offset during suspend/resume, while also considering the time passed
@@ -536,8 +533,6 @@ static void (*old_restore_sched_clock_state)(void);
  */
 static void hv_save_sched_clock_state(void)
 {
-	old_save_sched_clock_state();
-
 	hv_ref_counter_at_suspend = hv_read_reference_counter();
 }
 
@@ -550,8 +545,6 @@ static void hv_restore_sched_clock_state(void)
 	 *                - reference counter (time) now.
 	 */
 	hv_sched_clock_offset -= (hv_ref_counter_at_suspend - hv_read_reference_counter());
-
-	old_restore_sched_clock_state();
 }
 
 static __always_inline void hv_setup_sched_clock(void *sched_clock)
@@ -559,10 +552,7 @@ static __always_inline void hv_setup_sched_clock(void *sched_clock)
 	/* We're on x86/x64 *and* using PV ops */
 	paravirt_set_sched_clock(sched_clock);
 
-	old_save_sched_clock_state = x86_platform.save_sched_clock_state;
 	x86_platform.save_sched_clock_state = hv_save_sched_clock_state;
-
-	old_restore_sched_clock_state = x86_platform.restore_sched_clock_state;
 	x86_platform.restore_sched_clock_state = hv_restore_sched_clock_state;
 }
 #else /* !CONFIG_GENERIC_SCHED_CLOCK && !CONFIG_PARAVIRT */
-- 
2.48.1.711.g2feabab25a-goog