Re: [PATCH v3 01/14] KVM: Allow architectures to disallow pre-fault

[Sean Christopherson <seanjc@google.com>]

On Tue, Sep 22, 2026, Oliver Upton wrote:
> Hi Lorenzo,
> 
> On Tue, Sep 22, 2026 at 03:17:55PM +0100, Lorenzo Stoakes (ARM) wrote:
> > +bool __weak kvm_arch_vcpu_allow_pre_fault_memory(struct kvm_vcpu *vcpu)
> > +{
> > +	return true;
> > +}
> > +
> >  void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> >  {
> >  	int nr_vcpus, start, i, idx, yielded;
> > @@ -4365,6 +4370,9 @@ static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
> >  	    range->gpa + range->size <= range->gpa)
> >  		return -EINVAL;
> >  
> > +	if (!kvm_arch_vcpu_allow_pre_fault_memory(vcpu))
> > +		return -ENOEXEC;
> > +
> 
> nit: it'd be better to let the arch hook return an error of its choosing
> but in reality this is only going to be used by arm64.

Heh, except x86 already has something similar.

	if (!vcpu->kvm->arch.pre_fault_allowed)
		return -EOPNOTSUPP;

As does s390:
	
	if (kvm_is_ucontrol(vcpu->kvm))
		return -EINVAL;

I also don't like that this is subtly about avoiding vcpu_load(); it will be all
too easy to overlook that detail in the future.

Rather than have kvm_arch_vcpu_allow_pre_fault_memory(), what if we add a more
generic kvm_is_vcpu_loadable()?  That way we don't need to worry as much about
the return value, the connection to vcpu_load() is obvious, and we don't need to
add another pre-check if future (or cleaned-up existing?) ioctls want to do
vcpu_load() in common code.

I'd also be tempted to say it can be a macro, not a __weak function.  E.g.

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 27fe0cd5b2d7..99613df254cf 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -1533,6 +1533,7 @@ static inline bool __vcpu_has_feature(const struct kvm_arch *ka, int feature)
 #define vcpu_has_feature(v, f)	__vcpu_has_feature(&(v)->kvm->arch, (f))
 
 #define kvm_vcpu_initialized(v) vcpu_get_flag(v, VCPU_INITIALIZED)
+#define kvm_is_vcpu_loadable kvm_vcpu_initialized
 
 int kvm_trng_call(struct kvm_vcpu *vcpu);
 #ifdef CONFIG_KVM
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3dd04605f2e5..02401b080507 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1053,6 +1053,9 @@ int kvm_trylock_all_vcpus(struct kvm *kvm);
 int kvm_lock_all_vcpus(struct kvm *kvm);
 void kvm_unlock_all_vcpus(struct kvm *kvm);
 
+#ifndef kvm_is_vcpu_loadable
+#define kvm_is_vcpu_loadable(v) true
+#endif
 void vcpu_load(struct kvm_vcpu *vcpu);
 void vcpu_put(struct kvm_vcpu *vcpu);