[PATCH v2] KVM: arm64: Restore the VM's feature bitmap when kvm_setup_vcpu() fails

[Lorenzo Stoakes (ARM)]

On Mon, Sep 21, 2026 at 11:39:33AM -0700, Oliver Upton wrote:
> Hey Lorenzo,
>
> On Mon, Sep 21, 2026 at 02:34:12PM +0100, Lorenzo Stoakes (ARM) wrote:
> > On Mon, Sep 21, 2026 at 08:24:56AM +0100, Fuad Tabba wrote:
> > > Hi Oliver,
> > >
> > > On Mon, 21 Sep 2026 00:09:17 -0700, Oliver Upton <oupton at kernel.org> wrote:
> > > [...]
> > > > > Nothing in the tree loads a vCPU whose init failed, so this is latent.
> > > > > The upcoming series that enables KVM_PRE_FAULT_MEMORY for arm64 exposes
> > > > > it: the generic kvm_vcpu_pre_fault_memory() calls vcpu_load() whether
> > > > > or not the vCPU has been initialised.
> > > >
> > > > That's a bug, not a feature. We should require an initialized vcpu for
> > > > the ioctl.
> > >
> > > That one is for Lorenzo then :) I'll reword the message so the stale
> > > bitmap comes first and the pre-fault crash is just how it showed up.
> >
> > In general this is the behaviour of _generic_ KVM code
> > (kvm_vcpu_pre_fault_memory()), which itself does not require an initialised
> > vCPU, and no other arch seems to requires that either in the arch-specific
> > pre-fault code.
> >
> > The actual pre-faulting implementation intentionally only targets the
> > canonical MMU to fault in S2 page tables, which doesn't need initialisation
> > to have taken place AFAICT.
> >
> > However I could add:
> >
> > 	if (!kvm_vcpu_initialized(vcpu))
> > 		return -ENOEXEC;
> >
> > To kvm_arch_vcpu_pre_fault_memory(), which would make this a requirement,
> > though that would bring us out of line with other archs.
>
> Actually, we need this enforcement to happen before the arch-generic
> implementation calls vcpu_load(). So that would be some amount of
> kvm_arch_* boilerplate added to the generic code to enforce this
> requirement like we do on the other vCPU ioctls.

Ack, will do!

>
> I'm not too worried about aligning this with other architectures; the
> idea of a half-baked vCPU getting loaded is worrying.

Yeah and I don't think anybody in their right mind is relying on being able
to pre-fault before init anyway.

>
> > OTOH, I don't see why anybody would pre-fault BEFORE init, feels like
> > something that should be done later on in VMM bring up right?
> >
> > So I doubt doing this would really break any sane user.
> >
> > Let me know if you want that change in the series and I can respin it.
>
> Please do, thank you for addressing this.

No worries, will put a commit or 2 at the start of the series to action
this and respin.

>
> Best,
> Oliver

--
Cheers, Lorenzo