Re: [PATCH v1 1/1] s390/kvm: mask extra bits from program interrupt code

[Heiko Carstens <hca@linux.ibm.com>]

On Thu, Oct 31, 2024 at 01:03:16PM +0100, Claudio Imbrenda wrote:
> The program interrupt code has some extra bits that are sometimes set
> by hardware for various reasons; those bits should be ignored when the
> program interrupt number is needed for interrupt handling.
> 
> Fixes: ce2b276ebe51 ("s390/mm/fault: Handle guest-related program interrupts in KVM")
> Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
> ---
>  arch/s390/kvm/kvm-s390.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 8b3afda99397..f2d1351f6992 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -4737,7 +4737,7 @@ static int vcpu_post_run_handle_fault(struct kvm_vcpu *vcpu)
>  	if (kvm_s390_cur_gmap_fault_is_write())
>  		flags = FAULT_FLAG_WRITE;
>  
> -	switch (current->thread.gmap_int_code) {
> +	switch (current->thread.gmap_int_code & PGM_INT_CODE_MASK) {

Can you give an example? When reviewing your patch I was aware of this, but
actually thought we do want to know when this happens, since the kernel did
something which causes such bits to be set; e.g. single stepping with PER
on the sie instruction. If that happens then such program interruptions
should not be passed for kvm handling, since that would indicate a host
kernel bug (the sie instruction is not allowed to be single stepped).

Or in other words: this should never happen. Of course I might have missed
something; so when could this happen where this is not a bug and the bits
should be ignored?