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

[Claudio Imbrenda]

On Thu, 31 Oct 2024 13:38:15 +0100
Heiko Carstens <hca@xxxxxxxxxxxxx> wrote:

> 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@xxxxxxxxxxxxx>
> > Signed-off-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
> > ---
> >  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?

in some cases some guest indication bits might be set when a
host exception happens.

I was also unaware of those and found out the hard way.