Re: [PATCH v3 1/1] KVM: s390: pv: fix external interruption loop not always detected

[Janosch Frank <frankja@linux.ibm.com>]

On 2/13/23 09:55, Nico Boehr wrote:
> To determine whether the guest has caused an external interruption loop
> upon code 20 (external interrupt) intercepts, the ext_new_psw needs to
> be inspected to see whether external interrupts are enabled.
> 
> Under non-PV, ext_new_psw can simply be taken from guest lowcore. Under
> PV, KVM can only access the encrypted guest lowcore and hence the
> ext_new_psw must not be taken from guest lowcore.
> 
> handle_external_interrupt() incorrectly did that and hence was not able
> to reliably tell whether an external interruption loop is happening or
> not. False negatives cause spurious failures of my kvm-unit-test
> for extint loops[1] under PV.
> 
> Since code 20 is only caused under PV if and only if the guest's
> ext_new_psw is enabled for external interrupts, false positive detection
> of a external interruption loop can not happen.
> 
> Fix this issue by instead looking at the guest PSW in the state
> description. Since the PSW swap for external interrupt is done by the
> ultravisor before the intercept is caused, this reliably tells whether
> the guest is enabled for external interrupts in the ext_new_psw.
> 
> Also update the comments to explain better what is happening.
> 
> [1] https://lore.kernel.org/kvm/20220812062151.1980937-4-nrb@linux.ibm.com/
> 
> Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
> ---

Reviewed-by: Janosch Frank <frankja@linux.ibm.com>