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

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Date:	Fri, 20 Apr 2012 12:43:25 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Lin Ming <mlin@...pku.edu.cn>
Cc:	Ian Campbell <Ian.Campbell@...rix.com>,
	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>
Subject: Re: [Xen-devel] [PATCH] xen/apic: implement io apic read with
 hypercall

On Fri, Apr 20, 2012 at 11:39:24PM +0800, Lin Ming wrote:
> On Fri, 2012-04-20 at 16:06 +0100, Ian Campbell wrote:
> > 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.
> 
> Just checked ioapic_guest_read.
> It will return -EINVAL if an invalid physbase is passed in.
> 
> > > >
> > > >> 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?
> 
> Is BUG_ON() reasonable if invalid physbase passed in?

Just emulate the values in the error case. We don't _need_ them per say - except
to emulate some sensible values.

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