Re: KVM: x86/mmu: __kvm_rmap_lock() preemption assert trips on PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2026-08-27 15:17:57 [-0700], Sean Christopherson wrote:
> > @@ -1745,11 +1754,15 @@ static bool kvm_rmap_age_gfn_range(struct kvm *kvm,
> >  	gfn_t gfn;
> >  	int level;
> >  
> > +#ifdef CONFIG_PREEMPT_RT
> > +	guard(read_lock)(&kvm->mmu_lock);
> > +#endif
> 
> And now I remember why I swept this under the rug.  This really should take
> mmu_lock for write, otherwise concurrent aging threads could theoretically get
> stuck competing for KVM_RMAP_LOCKED.  Which is silly, because they don't actually
> need to take a lock of any kind.  I.e. it's not super trivial?
> 
> Wait, duh.  It is trivial if mmu_lock is held, because then KVM can operate on
> rmaps without any KVM_RMAP_LOCKED shenanigans.  I wouldn't test this because it
> might break horribly, but I think this?

So you drop the bit spinlock and rely on kvm->mmu_lock instead? No sure
why it should break horribly but it looks reasonable.

Sebastian