[RFC PATCH 1/3] arm64: kvm: Handle Asymmetric AArch32 systems

[Qais Yousef]

On 10/13/20 12:51, James Morse wrote:
> Hi Marc,
> 
> On 13/10/2020 11:32, Marc Zyngier wrote:
> > On 2020-10-12 16:32, James Morse wrote:
> >> On 09/10/2020 13:48, Qais Yousef wrote:
> >>> On 10/09/20 13:34, Marc Zyngier wrote:
> >>>> You can try setting vcpu->arch.target to -1, which is already caught by
> >>>> kvm_vcpu_initialized() right at the top of this function. This will
> >>
> >>>> prevent any reentry unless the VMM issues a KVM_ARM_VCPU_INIT ioctl.
> >>
> >> This doesn't reset SPSR, so this lets the VMM restart the guest with a
> >> bad value.
> > 
> > That's not my reading of the code. Without a valid target, you cannot enter
> > the guest (kvm_vcpu_initialized() prevents you to do so). To set the target,
> > you need to issue a KVM_ARM_VCPU_INIT ioctl, which eventually calls
> 
> > kvm_reset_vcpu(), which does set PSTATE to something legal.
> 
> aha! So it does, this is what I was missing.
> 
> 
> > Or do you mean the guest's SPSR_EL1? Why would that matter?
> > 
> >> I think we should make it impossible to return to aarch32 from EL2 on
> >> these systems.
> > 
> > And I'm saying that the above fulfills that requirement. Am I missing
> > something obvious?
> 
> Nope, I was.
> 
> I agree the check on entry from user-space isn't needed.

Thanks both.

So using the vcpu->arch.target = -1 is the best way forward. In my experiments
when I did that I considered calling kvm_reset_vcpu() too, does it make sense
to force the reset here too? Or too heavy handed?

Thanks

--
Qais Yousef