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

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Date:	Fri, 20 Apr 2012 16:06:07 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Lin Ming <mlin@...pku.edu.cn>
CC:	Andrew Cooper <Andrew.Cooper3@...rix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Subject: Re: [Xen-devel] [PATCH] xen/apic: implement io apic read with
 hypercall

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@...rix.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.


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