Re: [kvm-ppc-devel] RFC: MMIO endianness flag

[Hollis Blanchard <hollisb-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

On Sun, 2008-01-13 at 11:42 +0200, Avi Kivity wrote:

> Do we really need to propagate endianness all the way to the user?  
> Perhaps libkvm could call the regular mmio functions and do the 
> transformation itself.
> 
> Or maybe even the kernel can do this by itself?

The kernel *already* does this by itself, and I'm attempting to explain
why that is not sufficient.

My point is precisely that the endianness information must be propagated
to the user, otherwise the user may not have all the information it
needs to emulate it.

Here is the concrete example:
      * guest writes to MMIO
      * KVM passes MMIO information (physical address, number of bytes,
        value) to qemu
      * Qemu knows from the address that this access is for a passthough
        device, a special case the administrator has pre-configured
      * Qemu does mmap(/dev/mem), and writes "length" bytes of "value"
        at offset "address".

Now here's the catch: what endianness does qemu use when doing the
write? If qemu only does BE, then a LE access from the guest will be
byte-reversed when presented to the real hardware.

-- 
Hollis Blanchard
IBM Linux Technology Center


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