Re: Re: [PATCH] KVM: x86: Check irqchip mode before create PIT

[moehanabi]

> On Thu, Jan 25, 2024, Brilliant Hanabi wrote:
> > As the kvm api(https://docs.kernel.org/virt/kvm/api.html) reads,
> > KVM_CREATE_PIT2 call is only valid after enabling in-kernel irqchip
> > support via KVM_CREATE_IRQCHIP.
> > 
> > Without this check, I can create PIT first and enable irqchip-split
> > then, which may cause the PIT invalid because of lacking of in-kernel
> > PIC to inject the interrupt.
> 
> Does this cause actual problems beyond the PIT not working for the guest?  E.g.
> does it put the host kernel at risk?  If the only problem is that the PIT doesn't
> work as expected, I'm tempted to tweak the docs to say that KVM's PIT emulation
> won't work without an in-kernel I/O APIC.  Rejecting the ioctl could theoertically
> break misconfigured setups that happen to work, e.g. because the guest never uses
> the PIT.

I don't think it will put the host kernel at risk. But that's exactly what
kvmtool does: it creates in-kernel PIT first and set KVM_CREATE_IRQCHIP then.
I found this problem because I was working on implementing a userspace PIC
and PIT in kvmtool. As I planned, I'm going to commit a related patch to 
kvmtool if this patch will be applied.

> > Signed-off-by: Brilliant Hanabi <moehanabichan@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/x86.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 27e23714e960..3edc8478310f 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7016,6 +7016,8 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
> >  		r = -EEXIST;
> >  		if (kvm->arch.vpit)
> >  			goto create_pit_unlock;
> > +		if (!pic_in_kernel(kvm))
> > +			goto create_pit_unlock;
> 
> -EEXIST is not an appropriate errno.

Which errno do you think is better?

> >  		r = -ENOMEM;
> >  		kvm->arch.vpit = kvm_create_pit(kvm, u.pit_config.flags);
> >  		if (kvm->arch.vpit)
> > -- 
> > 2.39.3
> >