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

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Message-ID: <20120426153346.GC26830@phenom.dumpdata.com>
Date:	Thu, 26 Apr 2012 11:33:46 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.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>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen/apic: implement io apic read with
 hypercall

> >>
> >> unsigned int xen_io_apic_read(unsigned apic, unsigned reg)
> >> {
> >>         struct physdev_apic apic_op;
> >>         int ret;
> >>
> >>         apic_op.apic_physbase = mpc_ioapic_addr(apic);
> >>         apic_op.reg = reg;
> >>         ret = HYPERVISOR_physdev_op(PHYSDEVOP_apic_read, &apic_op);
> >>         if (!ret)
> >>                 return apic_op.value;
> >>
> >>         /* emulate register */
> >>         if (reg == 0x1)
> >>                 return 0x00170020;
> >>         else if (reg == 0x0)
> >>                 return apic << 24;
> >>         else
> >>                 return -1;
> >
> >        return 0xfd;
> 
> Where does this magic number 0xfd come from?
> 
> Both native_io_apic_read and xen_io_apic_read does not return 0xfd on error.

That is correct. But that is what it should have been. Suresh
pointed that out sometime and I managed to lose that part in one of the
commits. The earlier patch of this version did that.

Thought thinking about it this some I am not sure if 0xff is a better
choice... In the end it probably does not matter the slighest.

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