Re: [Xen-devel] Re: [PATCH 1/3] xen: pvhvm: allow user to request no emulated device unplug

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Thu, 2010-08-19 at 17:44 +0100, Jeremy Fitzhardinge wrote:
> On 08/19/2010 09:34 AM, Gianni Tedesco wrote:
> > On Thu, 2010-08-19 at 17:10 +0100, Jeremy Fitzhardinge wrote:
> >> On 08/19/2010 03:54 AM, Stefano Stabellini wrote:
> >>> On Thu, 19 Aug 2010, Ian Campbell wrote:
> >>>> On Thu, 2010-08-19 at 11:50 +0100, Ian Campbell wrote:
> >>>>> On Thu, 2010-08-19 at 11:37 +0100, Stefano Stabellini wrote:
> >>>>>> On Thu, 19 Aug 2010, Ian Campbell wrote:
> >>>>>>>       if (r && !(r == XEN_PLATFORM_ERR_MAGIC &&
> >>>>>>> +                     (xen_emul_unplug != -1) &&
> >>>>>>>                       (xen_emul_unplug & XEN_UNPLUG_IGNORE)))
> >>>>>> I wouldn't add xen_emul_unplug != -1 because it should be clear that
> >>>>>> xen_emul_unplug & XEN_UNPLUG_IGNORE always implies xen_emul_unplug != 
> >>>>>> -1.
> >>>>> That's not correct since -1 is all 1s. So you can get a false positive
> >>>>> for "xen_emul_unplug & XEN_UNPLUG_IGNORE" if xen_emul_unplug == -1.
> >>>> IOW if we were to rewrite the test to use less boolean logic the patch
> >>>> might look like:
> >>>>
> >>>>  if (r) {
> >>>>          if (r != XEN_PLATFORM_ERR_MAGIC)
> >>>>                  return;
> >>>> +                if (xen_emul_unplug == -1)
> >>>> +                        return;
> >>>>          if (!(xen_emul_unplug & XEN_UNPLUG_IGNORE))
> >>>>                  return;
> >>>>  }
> >>>>
> >>>> Perhaps this refactoring is worthwhile in any case? It certainly makes
> >>>> my head hurt less ;-)
> >>>>
> >>>  
> >>> Yeah, it is probably worth it anyway :)
> >> Treating a variable as an integer and a bitfield seems like a bad idea.
> > Should be fine? I'd just #define XEN_UNPLUG_ALL_THE_BITS ~0U for the
> > sake of cosmetics...
> 
> In a bitfield the bits are typically independent, whereas in an integer
> you interpret all the bits to get a value.  Making "all bits set" mean
> something other than all the bitfield bits are set is just asking for
> something bad to happen to you.
> 
> Why not define a bit to mean whatever "-1" currently means?

I'll add an explicit XEN_UNPLUG_NEVER instead.

Ian.


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