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

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

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

> On 23/02/2017 16:39, Alex Williamson wrote:
> > 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.  
> 
> But ramd MemoryRegionOps do not use file read/write, they use memory
> read/write, so they talk directly to the device.

Ah ha!  Ding! ;)  Sorry, I forgot that.