Re: [Qemu-devel] [PATCH 6/7] s390x/pci: move the memory region write from pcistg

[Cornelia Huck <cohuck@redhat.com>]

On Mon, 13 Nov 2017 10:39:50 +0100
Pierre Morel <pmorel@linux.vnet.ibm.com> wrote:

> On 10/11/2017 10:51, Cornelia Huck wrote:
> > On Fri, 10 Nov 2017 17:40:12 +0800
> > Yi Min Zhao <zyimin@linux.vnet.ibm.com> wrote:
> >   
> >> 在 2017/11/10 上午3:23, Cornelia Huck 写道:  
> >>> On Tue,  7 Nov 2017 18:24:38 +0100
> >>> Pierre Morel <pmorel@linux.vnet.ibm.com> wrote:
> >>>     
> >>>> Let's move the memory region write from pcistg into a dedicated
> >>>> function.
> >>>> This allows us to prepare a later patch searching for subregions
> >>>> inside of the memory region.  
> >>> OK, so here is the memory region write. Do we have any sleeping
> >>> endianness bugs in there for when we wire up tcg? I'm not sure how this
> >>> plays with the bswaps (see patch 1).
> >>>
> >>> But maybe I've just gotten lost somewhere.  
> >> I think there's no error. For PCI bars' MRs, we got the little-endian data
> >> that is exactly fit to the byte ordering of pcilg instruction. For PCI
> >> config
> >> space, the data has been swapped according to the cpu byte ordering.  
> > 
> > Host or target cpu?
> >   
> >> So we use zpci_swap_endian() to swap the data back to the little-endian
> >> ordering.  
> >   
> 
> 
> I do not see where we use the zpci_swap_endian() function in this patch 
> or in the zpci_write_bar() function.

So, is that swap function only ever used to convert BE register
contents to LE?

> 
> 
> 
> > That swap is unconditional. If we were running on a little-endian host,
> > it would be wrong, wouldn't it?  
> 
> I think there is no problem here, we do not use the swap function but we 
> use the memory_region_dispatch_write() to access a subregion of the PCI 
> device which is defined as DEVICE_LITTLE_ENDIAN
> 
> AFAIU The memory access process the endianness correctly.

OK, if this is all going through generic memory mechanisms, we should
be fine.

> 
> 
> >   
> >>>     
> >>>> Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
> >>>> Reviewed-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
> >>>> ---
> >>>>    hw/s390x/s390-pci-inst.c | 27 +++++++++++++++++----------
> >>>>    1 file changed, 17 insertions(+), 10 deletions(-)  
> >   
> 
>