[Qemu-devel] Re: [PATCH V6 13/32] pci_host: consolidate pci config address access.

[Aurelien Jarno]

On Wed, Nov 04, 2009 at 05:37:13PM +0200, Michael S. Tsirkin wrote:
> On Wed, Nov 04, 2009 at 04:17:46PM +0100, Aurelien Jarno wrote:
> > On Wed, Nov 04, 2009 at 03:14:26PM +0900, Isaku Yamahata wrote:
> > > On Tue, Nov 03, 2009 at 03:45:12PM +0200, Michael S. Tsirkin wrote:
> > > > > --- a/hw/pci_host.c
> > > > > +++ b/hw/pci_host.c
> > > > > @@ -32,6 +32,114 @@ do { printf("pci_host_data: " fmt , ## 
> > > > > __VA_ARGS__); } while (0)
> > > > >  #define PCI_DPRINTF(fmt, ...)
> > > > >  #endif
> > > > >  
> > > > > +static void pci_host_config_writel(void *opaque, target_phys_addr_t 
> > > > > addr,
> > > > > +                                   uint32_t val)
> > > > > +{
> > > > > +    PCIHostState *s = opaque;
> > > > > +
> > > > > +#ifdef TARGET_WORDS_BIGENDIAN
> > > > > +    val = bswap32(val);
> > > > > +#endif
> > > > 
> > > > I know you just copied it, but isn't this just
> > > >         val = le32_to_cpu(val);
> > > > 
> > > > ?
> > > 
> > > Makes sense.
> >  
> > The original code is actually wrong, but le32_to_cpu(val), will break on
> > big endian hosts.
> > 
> > The fact is that QEMU doesn't emulate byteswap on buses. Hopefully on all
> > big endian machines we emulate, the PCI bus is always connected backward,
> > so we can simply do the byteswap depending on TARGET_WORDS_BIGENDIAN.
> > 
> > -- 
> > Aurelien Jarno                              GPG: 1024D/F1BCDB73
> > address@hidden                 http://www.aurel32.net
> 
> Are you speaking about bit endian hosts with little endian guests?
> Ugh ... my head hurts. bswap32 is evil because there's no way to
> figure out what is converted to what. big to little? guest to host?
> 

This is not related to the host and guest endianess, but just of the
way the PCI bus is connected to the CPU on all the big endian guest we 
emulate.

On theses machine, a byteswap is needed when transferring a word between
the PCI bus and the CPU.

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net