Re: [PATCH 1/5] x86/paravirt: Use static_call() for the paravirt spinlock ops

[Borislav Petkov <bp@alien8.de>]

On Tue, Aug 04, 2026 at 07:15:41AM +0000, Dmitry Ilvokhin wrote:
> From: Peter Zijlstra <peterz@infradead.org>
> 
> queued_spin_lock_slowpath() and queued_spin_unlock() are dispatched
> through pv_ops_lock via the paravirt-ops ALTERNATIVE machinery, which
> picks the target (native inline store / hypervisor call) once at boot
> and cannot change at runtime.
> 
> Convert both to static_call(). The site becomes a direct call patched in
> place (one byte smaller), and on native the unlock still collapses to
> the inline "movb $0, (%rdi)" store, so the fast path is unchanged.
> 
> Unlike the ALTERNATIVE mechanism, a static_call() target can also be
> updated at runtime via static_call_update(). This is a prerequisite for
> the contended_release tracepoint, which has to swap in a traced unlock
> while the system is running.
> 
> [ ilvokhin: commit message; fix PARAVIRT_SPINLOCKS=n build; teach
>   __static_call_validate() about the inline unlock insn; make the
>   slowpath site module-safe: static_call_mod() +
>   EXPORT_STATIC_CALL_TRAMP(); pass @lock to the callee-save unlock,
>   fixing a boot hang under CALL_DEPTH_TRACKING. Boot tested native + KVM
>   PV guest. ]
> 
> Link: https://lore.kernel.org/all/20260603120811.GW3493090@noisy.programming.kicks-ass.net/
> Co-developed-by: Dmitry Ilvokhin <d@ilvokhin.com>
> Signed-off-by: Dmitry Ilvokhin <d@ilvokhin.com>

This needs Peter's SOB.

> ---
>  arch/x86/hyperv/hv_spinlock.c            |  4 ++--
>  arch/x86/include/asm/cpufeatures.h       |  1 -
>  arch/x86/include/asm/paravirt-spinlock.h | 19 +++++++++++------
>  arch/x86/kernel/kvm.c                    |  5 ++---
>  arch/x86/kernel/paravirt-spinlocks.c     | 12 +++++------
>  arch/x86/kernel/static_call.c            | 27 ++++++++++++++++++++++++
>  arch/x86/xen/spinlock.c                  |  5 ++---
>  tools/arch/x86/include/asm/cpufeatures.h |  1 -
>  8 files changed, 51 insertions(+), 23 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_spinlock.c b/arch/x86/hyperv/hv_spinlock.c
> index 210b494e4de0..6b4bdea18218 100644
> --- a/arch/x86/hyperv/hv_spinlock.c
> +++ b/arch/x86/hyperv/hv_spinlock.c
> @@ -78,8 +78,8 @@ void __init hv_init_spinlocks(void)
>  	pr_info("PV spinlocks enabled\n");
>  
>  	__pv_init_lock_hash();
> -	pv_ops_lock.queued_spin_lock_slowpath = __pv_queued_spin_lock_slowpath;
> -	pv_ops_lock.queued_spin_unlock = PV_CALLEE_SAVE(__pv_queued_spin_unlock);
> +	static_call_update(queued_spin_lock_slowpath, __pv_queued_spin_lock_slowpath);
> +	static_call_update(queued_spin_unlock, __raw_callee_save___pv_queued_spin_unlock);
>  	pv_ops_lock.wait = hv_qlock_wait;
>  	pv_ops_lock.kick = hv_qlock_kick;
>  	pv_ops_lock.vcpu_is_preempted = PV_CALLEE_SAVE(hv_vcpu_is_preempted);
> diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> index 1b4a48bff18f..e41fe5c24841 100644
> --- a/arch/x86/include/asm/cpufeatures.h
> +++ b/arch/x86/include/asm/cpufeatures.h
> @@ -225,7 +225,6 @@
>  #define X86_FEATURE_EPT_AD		( 8*32+17) /* "ept_ad" Intel Extended Page Table access-dirty bit */
>  #define X86_FEATURE_VMCALL		( 8*32+18) /* Hypervisor supports the VMCALL instruction */
>  #define X86_FEATURE_VMW_VMMCALL		( 8*32+19) /* VMware prefers VMMCALL hypercall instruction */
> -#define X86_FEATURE_PVUNLOCK		( 8*32+20) /* PV unlock function */

No, do:

/* free: was #define X86_FEATURE_PVUNLOCK		( 8*32+20) /* PV unlock function */

so that we can reuse it by finding it easier.

>  #define X86_FEATURE_VCPUPREEMPT		( 8*32+21) /* PV vcpu_is_preempted function */
>  #define X86_FEATURE_TDX_GUEST		( 8*32+22) /* "tdx_guest" Intel Trust Domain Extensions Guest */

-- 
Regards/Gruss,
    Boris.

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