Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 23 Feb 2017 16:21:47 +0100
Paolo Bonzini <pbonzini@redhat.com> wrote:

> On 23/02/2017 15:35, Peter Maydell wrote:
> > On 23 February 2017 at 12:53, Paolo Bonzini <pbonzini@redhat.com> wrote:  
> >>
> >>
> >> On 23/02/2017 13:26, Peter Maydell wrote:  
> >>> On 23 February 2017 at 11:43, Paolo Bonzini <pbonzini@redhat.com> wrote:  
> >>>> On 23/02/2017 12:34, Peter Maydell wrote:  
> >>>>> We should probably update the doc comment to note that the
> >>>>> pointer is to host-endianness memory (and that this is not
> >>>>> like normal RAM which is target-endian)...  
> >>>>
> >>>> I wouldn't call it host-endianness memory, and I disagree that normal
> >>>> RAM is target-endian---in both cases it's just a bunch of bytes.
> >>>>
> >>>> However, the access done by the MemoryRegionOps callbacks needs to match
> >>>> the endianness declared by the MemoryRegionOps themselves.  
> >>>
> >>> Well, if the guest stores a bunch of integers to the memory, which
> >>> way round do you see them when you look at the bunch of bytes?  
> >>
> >> You see them in whatever endianness the guest used.  
> > 
> > I'm confused. I said "normal RAM and this ramdevice memory are
> > different", and you seem to be saying they're the same. I don't
> > think they are (in particular I think with a BE guest on an
> > LE host they'll look different).  
> 
> No, they look entirely the same.  The only difference is that they go
> through MemoryRegionOps instead of memcpy.

Is this true for vfio use case?  If we use memcpy we're talking directly
to the device with no endian conversions.  If we use read/write then
there is an endian conversion in the host kernel.