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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo