Re: [Xen-devel] [PATCH 1/3] docs: specify endianess of xenstore protocol header

[Ian Jackson <ian.jackson@xxxxxxxxxxxxx>]

Juergen Gross writes ("Re: [PATCH 1/3] docs: specify endianess of xenstore 
protocol header"):
> On 08/05/17 12:07, Ian Jackson wrote:
> > Yes, but this is not correct.  On a big-endian cpu, they would be in
> > big-endian.
> 
> We don't support big-endian cpus, right? Do we want to specify the
> protocol for unsupported cpus?

We have in the past supported big-endian CPUs.  There is no
particular reason to think that a future Xen port will be to only a
little-endian CPU.

> > On a bytesexual cpu, the endianness should be specified but it will be
> > the same endianness as shared ring fields, etc.  So this doc probably
> > ought not to contain a list of endiannesses.  Best just to say that
> > the fields are all in host native byte order.
> 
> Hmm, this is problematic. How does a guest started e.g. big-endian on a
> cpu capable of both byte orders know which endianess the host has? I
> think specifying one endianess in this case is the better approach.

The same way that the guest knows the endianness of the other cpu
structures.

> BTW: I'm quite sure we don't support big-endian guests (or host) on ARM
> either, do we?

I have no idea.  If we do, they will need to byteswap things when
talking PV protocols.

> I could reword the paragraph to:
> 
> "The header fields are in the default endianess of the processor, e.g.
> little endian on x86 and ARM."

What information about endianness is in xen/include/public ?

I don't think the xenstore doc should contain its own indication of
endianness.  That leaves open the possibility that the docs might
specify (and someone might implement!) a mixed-endian system, where
the public headers and PV protocols are in one endianness, but
xenstore in another, because of differences in docs wording.

How about if xenstore.txt says something like `the endianness is the
same as that of the structures in the Xen public headers and the Xen
PV protocols' ?

Ian.

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