On 2/15/2016 2:44 PM, Jan Beulich wrote:

     switch ( mop->op )
     {
     case XEN_DOMCTL_MONITOR_OP_ENABLE:
     case XEN_DOMCTL_MONITOR_OP_DISABLE:
         /* Check if event type is available. */
         if ( unlikely(!(arch_monitor_get_capabilities(d) & (1 << mop->event))) )
             return -EOPNOTSUPP;
         /* Arch-side handles enable/disable ops. */
         return arch_monitor_domctl_event(d, mop);

Ah, I see now that I've mis-read the default: code further below,
which actually calls arch_monitor_domctl_op(), not ..._event().
However, there's an "undefined behavior" issue with the code
above then when mop->event >= 31 - I think you want to left
shift 1U instead of plain 1, and you need to range check
mop->event first.

Jan

Never looked @ that part before, used it the way it was.
I suppose that's because "according to the C specification, the result of a bit shift
operation on a signed argument gives implementation-defined results, so in theory 1U << i is
more portable than 1 << i" (taken from a stackoverflow post).

After changing 1 to 1U though, I don't understand why we should also range-check mop->event.
I'm imagining when (mop->event > 31):
* (1U << mop->event) = 0 or >= (0x1 + 0xFFFFFFFF) (?)
* in both cases arch_monitor_get_capabilities(d) & (1U << mop->event) would be = 0
* in which case we would return -EOPNOTSUPP
, no?

Corneliu.