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

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Date:	Fri, 20 Apr 2012 12:41:51 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Lin Ming <mlin@...pku.edu.cn>,
	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 01:38:28PM +0100, Ian Campbell wrote:
> On Fri, 2012-04-20 at 12:13 +0100, Lin Ming wrote:
> > On Fri, 2012-04-20 at 10:58 +0100, Andrew Cooper wrote:
> > > On 20/04/12 10:25, Lin Ming wrote:
> > > > Implements xen_io_apic_read with hypercall, so it returns proper IO-APIC
> > > > information instead of fabricated one.
> > > >
> > > > Signed-off-by: Lin Ming <mlin@...pku.edu.cn>
> > > > ---
> > > >  arch/x86/xen/apic.c |   16 +++++++++++-----
> > > >  1 files changed, 11 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/arch/x86/xen/apic.c b/arch/x86/xen/apic.c
> > > > index aee16ab..f1f392d 100644
> > > > --- a/arch/x86/xen/apic.c
> > > > +++ b/arch/x86/xen/apic.c
> > > > @@ -1,14 +1,20 @@
> > > >  #include <linux/init.h>
> > > >  #include <asm/x86_init.h>
> > > > +#include <asm/apic.h>
> > > > +#include <xen/interface/physdev.h>
> > > > +#include <asm/xen/hypercall.h>
> > > >  
> > > >  unsigned int xen_io_apic_read(unsigned apic, unsigned reg)
> > > >  {
> > > > -	if (reg == 0x1)
> > > > -		return 0x00170020;
> > > > -	else if (reg == 0x0)
> > > > -		return apic << 24;
> > > > +	struct physdev_apic apic_op;
> > > > +	int ret;
> > > >  
> > > > -	return 0xff;
> > > > +	apic_op.apic_physbase = mpc_ioapic_addr(apic);
> > > > +	apic_op.reg = reg;
> > > > +	ret = HYPERVISOR_physdev_op(PHYSDEVOP_apic_read, &apic_op);
> > > > +	if (ret)
> > > > +		return ret;
> > > > +	return apic_op.value;
> > > 
> > > Hypercall ret errors are negative, yet this function is unsigned.  Given
> > > that the previous function had no possible way to fail, perhaps on error
> > > you should fake up the values as before.
> > 
> > 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?

> 
> Under what circumstances can these hypercalls fail? Would a BUG_ON be
> appropriate/
> 
> > unsigned int ret = apic_read(...);
> > if (ret == -1)
> > 	//handle error.
> > 
> > Thanks,
> > Lin Ming
> > 
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@...ts.xen.org
> > http://lists.xen.org/xen-devel
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org
> http://lists.xen.org/xen-devel
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