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Date: Mon, 23 Apr 2012 11:11:23 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Lin Ming <mlin@ss.pku.edu.cn>
Cc: Ian Campbell <Ian.Campbell@citrix.com>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
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Subject: Re: [Xen-devel] [PATCH] xen/apic: implement io apic read with
 hypercall
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> >> > How about return -1 on error?
> >> > The calling function can check -1 for error.
> >>
> >> Isn't -1 potentially (at least theoretically) a valid value to read from
> >> one of these registers?
> >
> > Yeah, but then we are back to crashing at bootup (with dom0) :-)
> >
> > Perhaps the fallback is to emulate (so retain some of the original code)
> > as we have been since .. uh 3.0?
> 
> Do you mean the return value of io_apic_read in 3.0?

No. I meant that we would continue to emulate. The improvement
is that now we do:

      if (reg == 0x1)
              return 0x00170020;
      else if (reg == 0x0)
              return apic << 24;

instead of 0xfdfdfdfd.

> It's 0xffffffff.

Now it is 0xfdfdfdfd.

I am suggesting that instead of BUG_ON(), we fallback to do returning
an emulatated IO_APIC values - like the ones that this original patch
cooked up..