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 10:21, Janosch Frank wrote:
> 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>
> 

I'll add this when picking:
Fixes: 201ae986ead7 ("KVM: s390: protvirt: Implement interrupt injection")