Re: [Qemu-devel] [PATCH v3 1/3] spapr: introduce a fixed IRQ number space

[David Gibson <david@gibson.dropbear.id.au>]

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On Tue, Jul 03, 2018 at 05:19:56PM +0200, Cédric Le Goater wrote:
> On 07/02/2018 01:11 PM, Cédric Le Goater wrote:
> > On 07/02/2018 12:03 PM, Cédric Le Goater wrote:
> >>> --- a/hw/ppc/spapr_vio.c
> >>> +++ b/hw/ppc/spapr_vio.c
> >>> @@ -436,6 +436,9 @@ static void spapr_vio_busdev_reset(DeviceState *qdev)
> >>>      }
> >>>  }
> >>>  
> >>> +/* TODO : poor VIO device indexing ... */
> >>> +static uint32_t vio_index;
> >>
> >> I think we could also use (dev->reg & 0xff) as an index for 
> >> the VIO devices.
> >>
> >> The unit address of the virtual IOA is simply allocated using 
> >> an increment of bus->next_reg, next_reg being initialized at
> >> 0x71000000.
> >>
> >> I did not see any restrictions in the PAPR specs or in QEMU 
> >> that would break the above.
> > 
> > That was until I discovered this macro : 
> > 
> >   #define DEFINE_SPAPR_PROPERTIES(type, field)           \
> >         DEFINE_PROP_UINT32("reg", type, field.reg, -1)
> >  
> > so 'reg' could have any value. We can not use it ...
> 
> Would moving vio_index under the bus and incrementing it each time
> a VIO device is created be acceptable ? 

Not really, no.

> It does look like an allocator but I really don't know what else to 
> propose :/ See below.

Not only is it a stealth allocator, it also means we have two
different unique ids for VIO devices - the 'reg' and this new index.
That sounds like a recipe for confusion.

I think we can do better.  I had a look at how these are allocated and
it seems to be this:

In qemu:
	VIO devices start at reg=0x71000000, and just increment by one
	from there.

In libvirt:
	VIO net devices start at reg=0x1000
	VIO scsi devices start at reg=0x2000
	VIO nvram devices start at reg=0x3000
	VIO vty devices start at reg=0x30000000
	    and increment by 0x1000 each type

So we could go for say:
	irq = (reg & 0xf) ^ ((reg >> 12) & 0xf);

Obviously it's easily to construct cases where that will result in
collisions, but I don't think it'll happen for anyone not going out of
there way to make it happen.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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