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

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Date:	Fri, 20 Apr 2012 22:50:57 +0800
From:	Lin Ming <mlin@...pku.edu.cn>
To:	Ian Campbell <Ian.Campbell@...rix.com>
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, 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. How about WARN_ON?

ret = hypercall(...)

if (ret) {
   WARN_ON(1);
   return -1;
}


>
> Ian.
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