Re: [PATCH 1/2] virtio-ccw: fix virtio_set_ind_atomic

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 16 Jun 2020 07:58:53 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 
> 
> On 16.06.20 06:50, Halil Pasic wrote:
> > The atomic_cmpxchg() loop is broken because we occasionally end up with
> > old and _old having different values (a legit compiler can generate code
> > that accessed *ind_addr again to pick up a value for _old instead of
> > using the value of old that was already fetched according to the
> > rules of the abstract machine). This means the underlying CS instruction
> > may use a different old (_old) than the one we intended to use if
> > atomic_cmpxchg() performed the xchg part.
> > 
> > Let us use volatile to force the rules of the abstract machine for
> > accesses to *ind_addr. Let us also rewrite the loop so, we that the
> > new old is used to compute the new desired value if the xchg part
> > is not performed.
> > 
> > Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
> > Reported-by: Andre Wild <Andre.Wild1@ibm.com>
> > Fixes: 7e7494627f ("s390x/virtio-ccw: Adapter interrupt support.")
> > ---
> >  hw/s390x/virtio-ccw.c | 18 ++++++++++--------
> >  1 file changed, 10 insertions(+), 8 deletions(-)
> > 
> > diff --git a/hw/s390x/virtio-ccw.c b/hw/s390x/virtio-ccw.c
> > index c1f4bb1d33..3c988a000b 100644
> > --- a/hw/s390x/virtio-ccw.c
> > +++ b/hw/s390x/virtio-ccw.c
> > @@ -786,9 +786,10 @@ static inline VirtioCcwDevice *to_virtio_ccw_dev_fast(DeviceState *d)
> >  static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> >                                       uint8_t to_be_set)
> >  {
> > -    uint8_t ind_old, ind_new;
> > +    uint8_t expected, actual;
> >      hwaddr len = 1;
> > -    uint8_t *ind_addr;
> > +    /* avoid  multiple fetches */
> > +    uint8_t volatile *ind_addr;
> >  
> >      ind_addr = cpu_physical_memory_map(ind_loc, &len, true);
> >      if (!ind_addr) {
> > @@ -796,14 +797,15 @@ static uint8_t virtio_set_ind_atomic(SubchDev *sch, uint64_t ind_loc,
> >                       __func__, sch->cssid, sch->ssid, sch->schid);
> >          return -1;
> >      }
> > +    actual = *ind_addr;
> >      do {
> > -        ind_old = *ind_addr;
> 
> to make things easier to understand. Adding a barrier in here also fixes the issue.
> Reasoning follows below:
> 
> > -        ind_new = ind_old | to_be_set;
> 
> with an analysis from Andreas (cc)
> 
>  #define atomic_cmpxchg__nocheck(ptr, old, new)    ({                    \   
>  
>      typeof_strip_qual(*ptr) _old = (old);                               \   
>  
>      (void)__atomic_compare_exchange_n(ptr, &_old, new, false,           \   
>  
>                                __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);      \   
>  
>      _old;                                                               \   
>  
>  })
>  

There is also the 

#define atomic_cmpxchg(ptr, old, new) __sync_val_compare_and_swap(ptr, old, new)

variant, I guess, when the C11 stuff is not available. I don't know if
that variant is guaranteed to not have problems with multiple loads.



> ind_old is copied into _old in the macro. Instead of doing the copy from the
> register the compiler reloads the value from memory. The result is that _old
> and ind_old end up having different values. _old in r1 with the bits set
> already and ind_old in r10 with the bits cleared. _old gets updated by CS
> and matches ind_old afterwards - both with the bits being 0. So the !=
> compare is false and the loop is left without having set any bits.
> 
> 
> Paolo (to),
> I am asking myself if it would be safer to add a barrier or something like
> this in the macros in include/qemu/atomic.h. 
>

I think accessing the initial value via a volatile pointer initially and
using the value loaded by cmpxchg for subsequent iterations is cleaner.

Regards,
Halil

> 
> > -    } while (atomic_cmpxchg(ind_addr, ind_old, ind_new) != ind_old);
> > -    trace_virtio_ccw_set_ind(ind_loc, ind_old, ind_new);
> > -    cpu_physical_memory_unmap(ind_addr, len, 1, len);
> > +        expected = actual;
> > +        actual = atomic_cmpxchg(ind_addr, expected, expected | to_be_set);
> > +    } while (actual != expected);
> > +    trace_virtio_ccw_set_ind(ind_loc, actual, actual | to_be_set);
> > +    cpu_physical_memory_unmap((void *)ind_addr, len, 1, len);
> >  
> > -    return ind_old;
> > +    return actual;
> >  }
> >  
> >  static void virtio_ccw_notify(DeviceState *d, uint16_t vector)
> > 
> 
> 
>