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

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

On Mon, Oct 04, 2021 at 04:50:51PM +0200, Laszlo Ersek wrote:
> On 10/04/21 11:59, Richard W.M. Jones wrote:
> > It turns out that changing the qemu implementation is painful,
> > particularly if we wish to maintain backwards compatibility of the
> > command line and live migration.
> >
> > Instead I opted to document comprehensively what all the
> > different hypervisors do:
> >
> >   https://github.com/libguestfs/virt-v2v/blob/master/docs/vm-generation-id-across-hypervisors.txt
> 
> > Unfortunately QEMU made a significant mistake when implementing this
> > feature.  Because the string is 128 bits wrong, they decided it must
>                                   ^^^^^^^^^^^^^^
> 
> Haha, that's a great typo :)
> 
> > be a UUID (as you can see above there is no evidence that Microsoft
> > who wrote the original spec thought it was).  Following from this
> > incorrect assumption, they stated that the "UUID" must be supplied to
> > qemu in big endian format and must be byteswapped when writing it to
> > guest memory.  Their documentation says that they only do this for
> > little endian guests, but this is not true of their implementation
> > which byte swaps it for all guests.
> 
> I don't think this is what section "Endian-ness Considerations" in
> "docs/specs/vmgenid.txt" says. That text says that the *device* uses
> little-endian format. That's independent of the endianness of *CPU* of
> the particular guest architecture.
> 
> So the byte-swapping in the QEMU code occurs unconditionally because
> QEMU's UUID type is inherently big endian, and the device model in
> question is fixed little endian. The guest CPU's endianness is
> irrelevant as far as the device is concerned.
> 
> If a BE guest CPU intends to read the generation ID and to interpret it
> as a set of integers, then the guest CPU is supposed to byte-swap the
> appropriate fields itself.
> 
> > References
> 
> I suggest adding two links in this section, namely to:
> - docs/specs/vmgenid.txt
> - hw/acpi/vmgenid.c

Fair enough - I've made the changes you suggest (same URL as above).

Thanks,

Rich.

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