Re: [Xen-devel] [PATCH] xen/apic: implement io apic read with hypercall

[Ian Campbell <Ian.Campbell@citrix.com>]

On Fri, 2012-04-20 at 15:50 +0100, Lin Ming wrote:
> On Fri, Apr 20, 2012 at 9:12 PM, Ian Campbell <Ian.Campbell@citrix.com> wrote:
> > On Fri, 2012-04-20 at 13:53 +0100, Andrew Cooper wrote:
> >> >
> >> > Under what circumstances can these hypercalls fail? Would a BUG_ON be
> >> > appropriate/
> >>
> >> -EFAULT, -EPERM, anything xsm_apic() could return (which looks only to
> >> be -EPERM).
> >
> > So either the guest has called a hypercall which it is not permitted to
> > or it has called it with invalid parameters of one sort or another. Both
> > of these would be a code bug in the guest and therefore asserting that
> > no failure occurred is reasonable?
> >
> > What could the caller do with the error other than log it and collapse?
> >
> >> The call into Xen itself will return 0 as a value if an
> >> invalid physbase is passed in the hypercall.
> >
> >> So a BUG_ON() is not safe/sensible for domU.
> >
> > I think you have successfully argued that it is ;-)
> 
> BUG_ON is too severe.

Why? Under what circumstances can this be correctly called in a way
which would result in the hypercall failing?

>  How about WARN_ON?
> 
> ret = hypercall(...)
> 
> if (ret) {
>    WARN_ON(1);
>    return -1;
> }
> 
> 
> >
> > Ian.