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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 04, 2026 at 12:00:48PM -0700, Borislav Petkov wrote:
> 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.

Yeah, that got fixed when I applied it ;-)

> > ---
> >  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.

Sure, I can do that.