Re: [PATCH 0/1] vmx: Fix <genid/> mapping

["Richard W.M. Jones" <rjones@redhat.com>]

On Wed, Sep 29, 2021 at 11:10:35AM +0100, Daniel P. Berrangé wrote:
> On Wed, Sep 29, 2021 at 10:57:19AM +0100, Richard W.M. Jones wrote:
> > Looking at the qemu code the problem IMHO is:
> > 
> > https://gitlab.com/qemu-project/qemu/-/blob/6b54a31bf7b403672a798b6443b1930ae6c74dea/docs/specs/vmgenid.txt#L189
> > https://gitlab.com/qemu-project/qemu/-/blob/6b54a31bf7b403672a798b6443b1930ae6c74dea/hw/acpi/vmgenid.c#L37
> > 
> > This byte swapping makes no sense to me.  How do we know that the
> > guest is little endian?  What will this code do for BE guests?  I
> > think qemu would be better off treating the "GUID" as a list of bytes
> > and writing that exactly into the guest memory.
> 
> This is an artifact of the HyperV only caring about x86 and thus leaving
> endianness unspecified in the spec for GenID. QEMU docs say
> 
> 
> Endian-ness Considerations:
> ---------------------------
> 
> Although not specified in Microsoft's document, it is assumed that the
> device is expected to use little-endian format.
> 
> All GUID passed in via command line or monitor are treated as big-endian.
> GUID values displayed via monitor are shown in big-endian format.
> 
> 
> So by extension if libvirt is passing the value from its XML straight
> to QEMU, then libvirt has effectively defined that the XML is storing
> it big-endian too.
> 
> This could be where the confusion with VMX config is coming into play,
> though the byte re-ordering in v2v seems more complex than just an
> endianess-fiddle ?

qemu's qemu_uuid_bswap function only swaps some of the fields.

I think even more that applying qemu_uuid_bswap to these (not really)
"UUIDs" is a nonsense.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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