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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

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@xxxxxxxxxxxxx>
> > > ---
> > >  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?

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
> 
> 
> 
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