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

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Message-ID: <1334927566.28331.80.camel@zakaz.uk.xensource.com>
Date:	Fri, 20 Apr 2012 14:12:46 +0100
From:	Ian Campbell <Ian.Campbell@...rix.com>
To:	Andrew Cooper <Andrew.Cooper3@...rix.com>
CC:	Lin Ming <mlin@...pku.edu.cn>,
	"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 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 ;-)

Ian.

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