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

[Oliver Upton <oupton@kernel.org>]

On Tue, Sep 22, 2026 at 10:36:49AM -0700, Sean Christopherson wrote:
> On Tue, Sep 22, 2026, Lorenzo Stoakes (ARM) wrote:
> > On Tue, Sep 22, 2026 at 10:23:43AM -0700, Sean Christopherson wrote:
> > > 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.
> > 
> > ...this is exactly what I started out with.
> > 
> > But then you are in a pickle, because _really_ you need to do that check in
> > vcpu_load(). Which is a void function. Which is called by every single
> > architecture all over the place.
> > 
> > So you'd have actually no way of signalling the error back.
> > 
> > Of course those places are arch code and you could say 'arches should know
> > better and if they call it it's fine not to call the arch 'can you load'
> > function.
> 
> Yes, that's my vote.  It'd be easy enough to clarify that "rule" with a comment
> in linux/kvm_host.h.

I feel like trying to make this generic will wind up under-documenting
the single example we have with the pre fault ioctl. Putting the comment
into a header practically guarantees that nobody will read it either.

I'd favor doing something like below and sticking the comment inline in
the ioctl handler. Unless I'm missing something blatantly obvious, I
don't see why the x86 or s390 pre-conditions can't be tested early too.

But I don't care enough to bikeshed this any further.

Thanks,
Oliver

diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 8b080804bc90..396e64875fe7 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1852,6 +1852,14 @@ static int kvm_arm_vcpu_set_events(struct kvm_vcpu *vcpu,
 	return __kvm_arm_vcpu_set_events(vcpu, events);
 }
 
+int kvm_arch_pre_fault_allowed(struct kvm_vcpu *vcpu)
+{
+	if (!kvm_vcpu_initialized(vcpu))
+		return -ENOEXEC;
+
+	return 0;
+}
+
 long kvm_arch_vcpu_ioctl(struct file *filp,
 			 unsigned int ioctl, unsigned long arg)
 {
diff --git a/arch/s390/kvm/s390/s390.c b/arch/s390/kvm/s390/s390.c
index 5c73f43782a7..47fe032444f4 100644
--- a/arch/s390/kvm/s390/s390.c
+++ b/arch/s390/kvm/s390/s390.c
@@ -5784,6 +5784,14 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, struct kvm_memory_slot *old,
 	s390_kvm_mmu_commit_memory_region(kvm, old, new, change);
 }
 
+int kvm_arch_pre_fault_allowed(struct kvm_vcpu *vcpu)
+{
+	if (kvm_is_ucontrol(vcpu->kvm))
+		return -EINVAL;
+
+	return 0;
+}
+
 /**
  * kvm_arch_vcpu_pre_fault_memory() -- pre-fault and link gmap dat tables
  * @vcpu: the vcpu that shall appear to have generated the fault-in.
@@ -5810,9 +5818,6 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu, struct kvm_pre_fault_
 	gpa_t end;
 	int rc;
 
-	if (kvm_is_ucontrol(vcpu->kvm))
-		return -EINVAL;
-
 	rc = kvm_s390_faultin_gfn(vcpu, NULL, &f);
 	if (rc == PGM_ADDRESSING)
 		return -ENOENT;
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 064ecc33b926..c35fd2868c20 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -5086,6 +5086,14 @@ static int kvm_tdp_page_prefault(struct kvm_vcpu *vcpu, gpa_t gpa,
 	}
 }
 
+int kvm_arch_pre_fault_allowed(struct kvm_vcpu *vcpu)
+{
+	if (!vcpu->kvm->arch.pre_fault_allowed)
+		return -EOPNOTSUPP;
+
+	return 0;
+}
+
 long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 				    struct kvm_pre_fault_memory *range)
 {
@@ -5095,9 +5103,6 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 	u64 end;
 	int r;
 
-	if (!vcpu->kvm->arch.pre_fault_allowed)
-		return -EOPNOTSUPP;
-
 	if (kvm_is_gfn_alias(vcpu->kvm, gpa_to_gfn(range->gpa)))
 		return -EINVAL;
 
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 03bfc92864b6..bff842548c04 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -1693,6 +1693,7 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu);
 bool kvm_arch_dy_runnable(struct kvm_vcpu *vcpu);
 bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
 bool kvm_arch_vcpu_preempted_in_kernel(struct kvm_vcpu *vcpu);
+int kvm_arch_pre_fault_allowed(struct kvm_vcpu *vcpu);
 void kvm_arch_pre_destroy_vm(struct kvm *kvm);
 void kvm_arch_create_vm_debugfs(struct kvm *kvm);
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 65eb26a0520d..07f2ce7a3cb3 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3961,6 +3961,11 @@ bool __weak kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu)
 	return false;
 }
 
+int __weak kvm_arch_pre_fault_allowed(struct kvm_vcpu *vcpu)
+{
+	return 0;
+}
+
 void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 {
 	int nr_vcpus, start, i, idx, yielded;
@@ -4353,7 +4358,7 @@ static int kvm_vcpu_ioctl_get_stats_fd(struct kvm_vcpu *vcpu)
 static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 				     struct kvm_pre_fault_memory *range)
 {
-	int idx;
+	int idx, ret;
 	long r;
 	u64 full_size;
 
@@ -4365,6 +4370,14 @@ static int kvm_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
 	    range->gpa + range->size <= range->gpa)
 		return -EINVAL;
 
+	/*
+	 * Certain architectures (e.g. arm64) need to reject the ioctl 'early'
+	 * before vcpu_load().
+	 */
+	ret = kvm_arch_pre_fault_allowed(vcpu);
+	if (ret)
+		return ret;
+
 	vcpu_load(vcpu);
 	idx = srcu_read_lock(&vcpu->kvm->srcu);

Thanks,
Oliver