Re: [Qemu-devel] [qemu-s390x] [PATCH for-4.1 1/2] s390x/pci: add some fallthrough annotations

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 10 Jul 2019 10:20:41 +0200
Cornelia Huck <cohuck@redhat.com> wrote:

> On Tue, 9 Jul 2019 18:55:34 -0400
> Collin Walling <walling@linux.ibm.com> wrote:
> 
> > On 7/8/19 9:23 AM, Christian Borntraeger wrote:  
> > > 
> > > 
> > > On 08.07.19 14:54, Cornelia Huck wrote:    
> > >> According to the comment, the bits are supposed to accumulate.
> > >>
> > >> Reported-by: Stefan Weil <sw@weilnetz.de>
> > >> Fixes: 5d1abf234462 ("s390x/pci: enforce zPCI state checking")
> > >> Signed-off-by: Cornelia Huck <cohuck@redhat.com>    
> > > 
> > > This patch does not change behaviour, so it is certainly not wrong.
> > > 
> > > So lets have a look at if the bug report was actually a real bug or
> > > just a missing annotation.
> > >     
> > >> ---
> > >>   hw/s390x/s390-pci-inst.c | 2 ++
> > >>   1 file changed, 2 insertions(+)
> > >>
> > >> diff --git a/hw/s390x/s390-pci-inst.c b/hw/s390x/s390-pci-inst.c
> > >> index 61f30b8e55d2..00235148bed7 100644
> > >> --- a/hw/s390x/s390-pci-inst.c
> > >> +++ b/hw/s390x/s390-pci-inst.c
> > >> @@ -1209,8 +1209,10 @@ int stpcifc_service_call(S390CPU *cpu, uint8_t r1, uint64_t fiba, uint8_t ar,
> > >>        * FH Enabled bit is set to one in states of ENABLED, BLOCKED or ERROR. */
> > >>       case ZPCI_FS_ERROR:
> > >>           fib.fc |= 0x20;
> > >> +        /* fallthrough */    
> > > 
> > > This is correct, in case of an error we are also blocked.
> > >     
> > 
> > Agreed. This is definitely correct based on our architecture.
> >   
> > >>       case ZPCI_FS_BLOCKED:
> > >>           fib.fc |= 0x40;
> > >> +        /* fallthrough */    
> > > 
> > > I think this is also correct, but  it would be good if Collin could verify.
> > >     
> > 
> > I failed to find anything to support setting the function control
> > enabled bit when the function state is in error / blocked. I'm
> > assuming this might be some QEMU hack to get things working? I'll have
> > to dive further to understand why this was done this way, as it doesn't
> > align with how the s390x architecture is documented. It's confusing.  
> 
> Might this also be a real issue? Not matching the architecture is not a
> good sign...

Friendly ping. If we still want to have this patch or a fix in 4.1, we
need to find out soon...

> 
> > 
> > Functionally, this doesn't change anything... so I'll at least give it
> > an ACK for now and investigate this further.  
> 
> I prefer to hold off on this patch for now, unless we're really sure
> that the code does not have a problem here. Actually documenting
> something that might be wrong does not sound like the right thing to
> do :/
> 
> >   
> > >>       case ZPCI_FS_ENABLED:
> > >>           fib.fc |= 0x80;
> > >>           if (pbdev->iommu->enabled) {
> > >>    
> > > 
> > >     
> > 
> > Acked-by: Collin Walling <walling@linux.ibm.com>
> > 
> > Side note: is there somewhere that I could access this bug report? :)  
> 
> It's the build log with the extra warnings in
> https://qemu.weilnetz.de/results/build-20190708.txt (referenced in
> <591d71a5-5b10-ab22-4751-01da8613d84c@weilnetz.de> on qemu-devel).