On Tue, Sep 22, 2026, Lorenzo Stoakes (ARM) wrote:
> 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 note you dodge the actually difficult question of what this wrapper function
> would look like ;)
Oh, I was thinking we wouldn't bother with a wrapper. Because I agree that
providing a complement to vcpu_load() would get all kinds of messy. Though we
could harden vcpu_load() with a WARN_ON_ONCE(), which in practice would prevent
the vast majority of violations from making it to a final kernel release.
> > > Instead you can make the predicate 'don't prefault on a
> > > not-yet-initialised
> > > vCPU' which is pretty sensible I think, have a specific place to put it
> > > and
> > > all's well with the world.
> >
> > But look at it from an x86 perspective. Pretty much everyone will look at
> > this
> > and expect:
> >
> > bool kvm_arch_vcpu_allow_pre_fault_memory(struct kvm_vcpu *vcpu)
> > {
> > return vcpu->kvm->arch.pre_fault_allowed;
> > }
>
> I'm not sure I really get your point here at all? :)
>
> Why would it matter what people who are too lazy to go check the
> implementation
> assume about an arch hook?
Because I don't hate the people that contribute to KVM, and want to make it as
easy as possible for them to not make mistakes?
