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

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

[Richard W.M. Jones]

On Wed, Sep 29, 2021 at 10:01:55AM +0200, Michal Privoznik wrote:
> Apparently, parsing vmx.genid is not as easy as I thought. Anyway, it
> was brought up in a private thread that libvirt doesn't report correct
> UUIDs. For instance for the following input:
> 
>   vm.genid = "-8536691797830587195"
>   vm.genidX = "-1723453263670062919"

(The two lines above come from a VMware .vmx file)

The only thing that really matters is what the guest sees.  I ran
VMGENID.EXE (https://bugzilla.redhat.com/show_bug.cgi?id=1598350#c3
https://docs.microsoft.com/en-gb/windows/win32/hyperv_v2/virtual-machine-generation-identifier)
inside the guest and it showed:

  8987940a09512cc5:e81510634ff550b9

Note these numbers are the hex equivalents of the VMware .vmx numbers:

>>> print("%x" % (2**64-8536691797830587195))
8987940a09512cc5
>>> print("%x" % (2**64-1723453263670062919))
e81510634ff550b9

> Libvirt would report:
> 
>   <genid>8987940a-0951-2cc5-e815-10634ff550b9</genid>
> 
> while virt-v2v would report:
> 
>   <genid>09512cc5-940a-8987-b950-f54f631015e8</genid>

After thinking about this a bit more, IMHO the real problem is
with qemu.  If you pass this to qemu:

  -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0

then inside the guest VMGENID shows 2cc509518987940a:b950f54f631015e8 (wrong)

If you pass this to qemu:

  ...guid=09512cc5-940a-8987-b950-f54f631015e8...

then inside the guest it sees 8987940a09512cc5:e81510634ff550b9
(the correct values, matching VMware).

This is the reason why virt-v2v mangles the GUID, in order to trick
libvirt into passing a mangled GUID which gets mangled again by qemu
which is the same as unmangling it.

So another way to fix this could be for us to fix qemu.  We could just
pass the two numbers to qemu instead of using GUIDs, eg:

  -device vmgenid,low=0x8987940a09512cc5,high=0xe81510634ff550b9,id=vmgenid0

and then we'd fix the implementation in qemu so that the low/high
values match what is seen by VMGENID.EXE in the guest.

Or we could have libvirt use the current GUID in <genid> but to do the
mangling when it gets passed through to qemu (instead of virt-v2v
doing the mangling).

Adding qemu-devel because I'm interesting to see if the qemu
developers would prefer to fix this properly in qemu.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Daniel P. Berrangé]

On Wed, Sep 29, 2021 at 10:20:44AM +0100, Richard W.M. Jones wrote:
> On Wed, Sep 29, 2021 at 10:01:55AM +0200, Michal Privoznik wrote:
> > Apparently, parsing vmx.genid is not as easy as I thought. Anyway, it
> > was brought up in a private thread that libvirt doesn't report correct
> > UUIDs. For instance for the following input:
> > 
> >   vm.genid = "-8536691797830587195"
> >   vm.genidX = "-1723453263670062919"
> 
> (The two lines above come from a VMware .vmx file)
> 
> The only thing that really matters is what the guest sees.  I ran
> VMGENID.EXE (https://bugzilla.redhat.com/show_bug.cgi?id=1598350#c3
> https://docs.microsoft.com/en-gb/windows/win32/hyperv_v2/virtual-machine-generation-identifier)
> inside the guest and it showed:
> 
>   8987940a09512cc5:e81510634ff550b9
> 
> Note these numbers are the hex equivalents of the VMware .vmx numbers:
> 
> >>> print("%x" % (2**64-8536691797830587195))
> 8987940a09512cc5
> >>> print("%x" % (2**64-1723453263670062919))
> e81510634ff550b9
> 
> > Libvirt would report:
> > 
> >   <genid>8987940a-0951-2cc5-e815-10634ff550b9</genid>
> > 
> > while virt-v2v would report:
> > 
> >   <genid>09512cc5-940a-8987-b950-f54f631015e8</genid>
> 
> After thinking about this a bit more, IMHO the real problem is
> with qemu.  If you pass this to qemu:
> 
>   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> 
> then inside the guest VMGENID shows 2cc509518987940a:b950f54f631015e8 (wrong)
> 
> If you pass this to qemu:
> 
>   ...guid=09512cc5-940a-8987-b950-f54f631015e8...
> 
> then inside the guest it sees 8987940a09512cc5:e81510634ff550b9
> (the correct values, matching VMware).
> 
> This is the reason why virt-v2v mangles the GUID, in order to trick
> libvirt into passing a mangled GUID which gets mangled again by qemu
> which is the same as unmangling it.
> 
> So another way to fix this could be for us to fix qemu.  We could just
> pass the two numbers to qemu instead of using GUIDs, eg:
> 
>   -device vmgenid,low=0x8987940a09512cc5,high=0xe81510634ff550b9,id=vmgenid0
> 
> and then we'd fix the implementation in qemu so that the low/high
> values match what is seen by VMGENID.EXE in the guest.
> 
> Or we could have libvirt use the current GUID in <genid> but to do the
> mangling when it gets passed through to qemu (instead of virt-v2v
> doing the mangling).

On the libvirt side, the #1 most important thing is that a given
XML string

  <genid>8987940a-0951-2cc5-e815-10634ff550b9</genid>

results in the same value being exposed to the guest OS, for both
the QEMU and VMWare drivers.  Whehter the guest sees the bytes in
the same order is not essential, but it would be less of a surprise
if it did match.

Ultimately as long as the mapping from libvirt XML to guest visible
string is consistent across drivers, that's sufficient.

> Adding qemu-devel because I'm interesting to see if the qemu
> developers would prefer to fix this properly in qemu.

No matter what order QEMU accepts the data in, it can be said to be
functionally correct. If the order is "unusual" though, it ought to
at least be documented how the QEMU order corresponds to guest visible
order.

Incidentally I wonder how much to trust VMGENID.EXE and whether that
actally reports what it gets out of guest memory "as is", or whether
it has done any byte re-ordering.

I'm curious what Linux kernel sees for the genid on Vmware vs KVM
hosts, as probably I'd trust that data more ?

Regards,
Daniel
-- 
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

On Wed, Sep 29, 2021 at 10:33:43AM +0100, Daniel P. Berrangé wrote:
> On Wed, Sep 29, 2021 at 10:20:44AM +0100, Richard W.M. Jones wrote:
> > On Wed, Sep 29, 2021 at 10:01:55AM +0200, Michal Privoznik wrote:
> > > Apparently, parsing vmx.genid is not as easy as I thought. Anyway, it
> > > was brought up in a private thread that libvirt doesn't report correct
> > > UUIDs. For instance for the following input:
> > > 
> > >   vm.genid = "-8536691797830587195"
> > >   vm.genidX = "-1723453263670062919"
> > 
> > (The two lines above come from a VMware .vmx file)
> > 
> > The only thing that really matters is what the guest sees.  I ran
> > VMGENID.EXE (https://bugzilla.redhat.com/show_bug.cgi?id=1598350#c3
> > https://docs.microsoft.com/en-gb/windows/win32/hyperv_v2/virtual-machine-generation-identifier)
> > inside the guest and it showed:
> > 
> >   8987940a09512cc5:e81510634ff550b9
> > 
> > Note these numbers are the hex equivalents of the VMware .vmx numbers:
> > 
> > >>> print("%x" % (2**64-8536691797830587195))
> > 8987940a09512cc5
> > >>> print("%x" % (2**64-1723453263670062919))
> > e81510634ff550b9
> > 
> > > Libvirt would report:
> > > 
> > >   <genid>8987940a-0951-2cc5-e815-10634ff550b9</genid>
> > > 
> > > while virt-v2v would report:
> > > 
> > >   <genid>09512cc5-940a-8987-b950-f54f631015e8</genid>
> > 
> > After thinking about this a bit more, IMHO the real problem is
> > with qemu.  If you pass this to qemu:
> > 
> >   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> > 
> > then inside the guest VMGENID shows 2cc509518987940a:b950f54f631015e8 (wrong)
> > 
> > If you pass this to qemu:
> > 
> >   ...guid=09512cc5-940a-8987-b950-f54f631015e8...
> > 
> > then inside the guest it sees 8987940a09512cc5:e81510634ff550b9
> > (the correct values, matching VMware).
> > 
> > This is the reason why virt-v2v mangles the GUID, in order to trick
> > libvirt into passing a mangled GUID which gets mangled again by qemu
> > which is the same as unmangling it.
> > 
> > So another way to fix this could be for us to fix qemu.  We could just
> > pass the two numbers to qemu instead of using GUIDs, eg:
> > 
> >   -device vmgenid,low=0x8987940a09512cc5,high=0xe81510634ff550b9,id=vmgenid0
> > 
> > and then we'd fix the implementation in qemu so that the low/high
> > values match what is seen by VMGENID.EXE in the guest.
> > 
> > Or we could have libvirt use the current GUID in <genid> but to do the
> > mangling when it gets passed through to qemu (instead of virt-v2v
> > doing the mangling).
> 
> On the libvirt side, the #1 most important thing is that a given
> XML string
> 
>   <genid>8987940a-0951-2cc5-e815-10634ff550b9</genid>
> 
> results in the same value being exposed to the guest OS, for both
> the QEMU and VMWare drivers.  Whehter the guest sees the bytes in
> the same order is not essential, but it would be less of a surprise
> if it did match.

I don't know why we decided to use a GUID for this.  The feature
itself (https://go.microsoft.com/fwlink/?LinkId=260709) defines it as
an 128 bit / 8 byte number.  The only connection to GUIDs is the size.

> Ultimately as long as the mapping from libvirt XML to guest visible
> string is consistent across drivers, that's sufficient.
> 
> > Adding qemu-devel because I'm interesting to see if the qemu
> > developers would prefer to fix this properly in qemu.
> 
> No matter what order QEMU accepts the data in, it can be said to be
> functionally correct. If the order is "unusual" though, it ought to
> at least be documented how the QEMU order corresponds to guest visible
> order.
> 
> Incidentally I wonder how much to trust VMGENID.EXE and whether that
> actally reports what it gets out of guest memory "as is", or whether
> it has done any byte re-ordering.
> 
> I'm curious what Linux kernel sees for the genid on Vmware vs KVM
> hosts, as probably I'd trust that data more ?

As far as I can tell no driver has successfully made it upstream,
although there have been a few attempts:

  https://lkml.org/lkml/2018/3/1/498

Here's a more recent one from 10 months ago:

  https://lore.kernel.org/linux-doc/3E05451B-A9CD-4719-99D0-72750A304044@amazon.com/

If I have time I'll patch a Linux kernel with the second patch and see
how it relates to the VMware and KVM parameters, but it probably won't
be today.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
Fedora Windows cross-compiler. Compile Windows programs, test, and
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

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.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-builder quickly builds VMs from scratch
http://libguestfs.org/virt-builder.1.html

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

[Daniel P. Berrangé]

On Wed, Sep 29, 2021 at 10:46:39AM +0100, Richard W.M. Jones wrote:
> On Wed, Sep 29, 2021 at 10:33:43AM +0100, Daniel P. Berrangé wrote:
> > Ultimately as long as the mapping from libvirt XML to guest visible
> > string is consistent across drivers, that's sufficient.
> > 
> > > Adding qemu-devel because I'm interesting to see if the qemu
> > > developers would prefer to fix this properly in qemu.
> > 
> > No matter what order QEMU accepts the data in, it can be said to be
> > functionally correct. If the order is "unusual" though, it ought to
> > at least be documented how the QEMU order corresponds to guest visible
> > order.
> > 
> > Incidentally I wonder how much to trust VMGENID.EXE and whether that
> > actally reports what it gets out of guest memory "as is", or whether
> > it has done any byte re-ordering.
> > 
> > I'm curious what Linux kernel sees for the genid on Vmware vs KVM
> > hosts, as probably I'd trust that data more ?
> 
> As far as I can tell no driver has successfully made it upstream,
> although there have been a few attempts:
> 
>   https://lkml.org/lkml/2018/3/1/498
> 
> Here's a more recent one from 10 months ago:
> 
>   https://lore.kernel.org/linux-doc/3E05451B-A9CD-4719-99D0-72750A304044@amazon.com/
> 
> If I have time I'll patch a Linux kernel with the second patch and see
> how it relates to the VMware and KVM parameters, but it probably won't
> be today.

I'm not sure if we actually need the full driver or not for testing
purposes. The the GenID is just in memory somewhere, and the somewhere
is reported via ACPI table entry. For QEMU its easy as the data is
exposed via fw_cfg which can be read from sysfs directly without
even needing to look at ACPI entries to find it. Not sure how we
find it with VMWare/HyperV though.

Regards,
Daniel
-- 
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Daniel P. Berrangé]

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 ?

Regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

On Wed, Sep 29, 2021 at 11:07:30AM +0100, Daniel P. Berrangé wrote:
> I'm not sure if we actually need the full driver or not for testing
> purposes. The the GenID is just in memory somewhere, and the somewhere
> is reported via ACPI table entry. For QEMU its easy as the data is
> exposed via fw_cfg which can be read from sysfs directly without
> even needing to look at ACPI entries to find it. Not sure how we
> find it with VMWare/HyperV though.

This still has the problem that qemu is mangling the vmgenid.
Nevertheless, on qemu-6.1.0-5.fc36.x86_64 I added this to a Linux
guest:

  <genid>11223344-5566-7788-99aa-bbccddeeff00</genid>

which turned into:

  -device vmgenid,guid=11223344-5566-7788-99aa-bbccddeeff00,id=vmgenid0

Inside the guest:

# ls /sys/firmware/qemu_fw_cfg/by_name/etc/vmgenid_guid/ -l
total 0
-r--------. 1 root root 4096 Sep 29 11:16 key
-r--------. 1 root root 4096 Sep 29 11:16 name
-r--------. 1 root root    0 Sep 29 11:16 raw
-r--------. 1 root root 4096 Sep 29 11:16 size
# hexdump -C /sys/firmware/qemu_fw_cfg/by_name/etc/vmgenid_guid/raw 
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00000020  00 00 00 00 00 00 00 00  44 33 22 11 66 55 88 77  |........D3".fU.w|
00000030  99 aa bb cc dd ee ff 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
00001000


But I think what I really need to do is look at the raw physical
address:

# hexdump -C /sys/firmware/qemu_fw_cfg/by_name/etc/vmgenid_addr/raw 
00000000  28 f0 ff 7f 00 00 00 00                           |(.......|
00000008

# dd if=/dev/mem bs=1 skip=$(( 0x7ffff028 )) count=16 | hexdump -C
16+0 records in
16+0 records out
16 bytes copied, 6.0392e-05 s, 265 kB/s
00000000  44 33 22 11 66 55 88 77  99 aa bb cc dd ee ff 00  |D3".fU.w........|
00000010


I think for VMware I'm really going to need the kernel driver, unless
there's some way that iasl can be used to extract the information?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

On Wed, Sep 29, 2021 at 10:46:38AM +0100, Richard W.M. Jones wrote:
> I don't know why we decided to use a GUID for this.  The feature
> itself (https://go.microsoft.com/fwlink/?LinkId=260709) defines it as
> an 128 bit / 8 byte number.  The only connection to GUIDs is the size.

*cough* .. 16 bytes :-)

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

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

[Richard W.M. Jones]

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
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

More data: I found a colleague who has a Hyper-V instance with a
Windows guest and he helped me to understand how Hyper-V represents
generation IDs.  Hyper-V has had support for generation IDs since long
before Microsoft proposed the feature for standardization.  Originally
(I think pre-2013) Hyper-V used an XML description which included:

<generation_id type="string">4a07df4361fdf4c883f97bc30e524b9d</generation_id>

Note this is a pure hex string, not a GUID.

In current Hyper-V, the same representation is used but it's embedded
in a binary file.

My colleague ran two Windows VMs on Hyper-V and examined the
generation_id on the hypervisor and compared it to the output of
VMGENID.EXE inside the guest.

The first guest had this in the binary hypervisor metadata:

000056b0  00 00 0e 67 65 6e 65 72  61 74 69 6f 6e 5f 69 64  |...generation_id|
000056c0  00 40 00 00 00 38 00 30  00 35 00 61 00 32 00 38  |.@...8.0.5.a.2.8|
000056d0  00 37 00 61 00 32 00 35  00 30 00 39 00 38 00 39  |.7.a.2.5.0.9.8.9|
000056e0  00 65 00 34 00 66 00 65  00 36 00 66 00 36 00 39  |.e.4.f.e.6.f.6.9|
000056f0  00 39 00 32 00 62 00 65  00 33 00 33 00 34 00 61  |.9.2.b.e.3.3.4.a|
00005700  00 34 00 33 00 00 00 00  00 00 00 00 00 00 00 00  |.4.3............|

and reported the output of VMGENID in this guest as:

VmCounterValue: fe6f6992be334a43:805a287a250989e4

The second guest had:

00005360  c5 06 00 00 00 0e 67 65  6e 65 72 61 74 69 6f 6e  |......generation|
00005370  5f 69 64 00 40 00 00 00  65 00 62 00 66 00 62 00  |_id.@...e.b.f.b.|
00005380  62 00 37 00 39 00 37 00  33 00 36 00 35 00 37 00  |b.7.9.7.3.6.5.7.|
00005390  32 00 38 00 65 00 37 00  30 00 62 00 33 00 66 00  |2.8.e.7.0.b.3.f.|
000053a0  64 00 33 00 63 00 37 00  31 00 33 00 65 00 63 00  |d.3.c.7.1.3.e.c.|
000053b0  65 00 38 00 34 00 32 00  00 00 00 00 00 00 00 00  |e.8.4.2.........|

and VMGENID was:

VmCounterValue: b3fd3c713ece842:ebfbb797365728e7

Note:

 - In both cases, the generation ID is clearly not a GUID.

 - The two 64 bit words are swapped over somewhere, but individual
   bytes are not byte-swapped, and there is again no evidence of
   UUID-aware byte swapping.

----------------------------------------------------------------------

So how do we get from where we are to a more sane vmgenid in qemu?

I propose we deprecate the guid parameter in:

  -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0

Instead it will be replaced by bytes= which will simply write
the bytes, in the order they appear, into guest memory with no
attempt to interpret or byte-swap.  Something like:

  -device vmgenid,bytes=112233445566778899aabbccddeeff00,id=vmgenid0

(guid although deprecated will need to be kept around for a while,
along with its weird byte-swapping behaviour).

We will then have a plain and simple method to emulate the behaviour
of other hypervisors.  We will look at exactly what bytes they write
to guest memory and copy that behaviour when v2v converting from those
hypervisors.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Laszlo Ersek]

On 09/30/21 09:33, Richard W.M. Jones wrote:
> 
> More data: I found a colleague who has a Hyper-V instance with a
> Windows guest and he helped me to understand how Hyper-V represents
> generation IDs.  Hyper-V has had support for generation IDs since long
> before Microsoft proposed the feature for standardization.  Originally
> (I think pre-2013) Hyper-V used an XML description which included:
> 
> <generation_id type="string">4a07df4361fdf4c883f97bc30e524b9d</generation_id>
> 
> Note this is a pure hex string, not a GUID.
> 
> In current Hyper-V, the same representation is used but it's embedded
> in a binary file.
> 
> My colleague ran two Windows VMs on Hyper-V and examined the
> generation_id on the hypervisor and compared it to the output of
> VMGENID.EXE inside the guest.
> 
> The first guest had this in the binary hypervisor metadata:
> 
> 000056b0  00 00 0e 67 65 6e 65 72  61 74 69 6f 6e 5f 69 64  |...generation_id|
> 000056c0  00 40 00 00 00 38 00 30  00 35 00 61 00 32 00 38  |.@...8.0.5.a.2.8|
> 000056d0  00 37 00 61 00 32 00 35  00 30 00 39 00 38 00 39  |.7.a.2.5.0.9.8.9|
> 000056e0  00 65 00 34 00 66 00 65  00 36 00 66 00 36 00 39  |.e.4.f.e.6.f.6.9|
> 000056f0  00 39 00 32 00 62 00 65  00 33 00 33 00 34 00 61  |.9.2.b.e.3.3.4.a|
> 00005700  00 34 00 33 00 00 00 00  00 00 00 00 00 00 00 00  |.4.3............|
> 
> and reported the output of VMGENID in this guest as:
> 
> VmCounterValue: fe6f6992be334a43:805a287a250989e4
> 
> The second guest had:
> 
> 00005360  c5 06 00 00 00 0e 67 65  6e 65 72 61 74 69 6f 6e  |......generation|
> 00005370  5f 69 64 00 40 00 00 00  65 00 62 00 66 00 62 00  |_id.@...e.b.f.b.|
> 00005380  62 00 37 00 39 00 37 00  33 00 36 00 35 00 37 00  |b.7.9.7.3.6.5.7.|
> 00005390  32 00 38 00 65 00 37 00  30 00 62 00 33 00 66 00  |2.8.e.7.0.b.3.f.|
> 000053a0  64 00 33 00 63 00 37 00  31 00 33 00 65 00 63 00  |d.3.c.7.1.3.e.c.|
> 000053b0  65 00 38 00 34 00 32 00  00 00 00 00 00 00 00 00  |e.8.4.2.........|
> 
> and VMGENID was:
> 
> VmCounterValue: b3fd3c713ece842:ebfbb797365728e7
> 
> Note:
> 
>  - In both cases, the generation ID is clearly not a GUID.
> 
>  - The two 64 bit words are swapped over somewhere, but individual
>    bytes are not byte-swapped, and there is again no evidence of
>    UUID-aware byte swapping.
> 
> ----------------------------------------------------------------------
> 
> So how do we get from where we are to a more sane vmgenid in qemu?
> 
> I propose we deprecate the guid parameter in:
> 
>   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> 
> Instead it will be replaced by bytes= which will simply write
> the bytes, in the order they appear, into guest memory with no
> attempt to interpret or byte-swap.  Something like:
> 
>   -device vmgenid,bytes=112233445566778899aabbccddeeff00,id=vmgenid0
> 
> (guid although deprecated will need to be kept around for a while,
> along with its weird byte-swapping behaviour).
> 
> We will then have a plain and simple method to emulate the behaviour
> of other hypervisors.  We will look at exactly what bytes they write
> to guest memory and copy that behaviour when v2v converting from those
> hypervisors.

I don't have anything against this, I just don't know who's going to
implement the QEMU change.

Thanks
Laszlo

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

[Daniel P. Berrangé]

On Thu, Sep 30, 2021 at 08:33:48AM +0100, Richard W.M. Jones wrote:
> 
> More data: I found a colleague who has a Hyper-V instance with a
> Windows guest and he helped me to understand how Hyper-V represents
> generation IDs.  Hyper-V has had support for generation IDs since long
> before Microsoft proposed the feature for standardization.  Originally
> (I think pre-2013) Hyper-V used an XML description which included:
> 
> <generation_id type="string">4a07df4361fdf4c883f97bc30e524b9d</generation_id>
> 
> Note this is a pure hex string, not a GUID.
> 
> In current Hyper-V, the same representation is used but it's embedded
> in a binary file.
> 
> My colleague ran two Windows VMs on Hyper-V and examined the
> generation_id on the hypervisor and compared it to the output of
> VMGENID.EXE inside the guest.
> 
> The first guest had this in the binary hypervisor metadata:
> 
> 000056b0  00 00 0e 67 65 6e 65 72  61 74 69 6f 6e 5f 69 64  |...generation_id|
> 000056c0  00 40 00 00 00 38 00 30  00 35 00 61 00 32 00 38  |.@...8.0.5.a.2.8|
> 000056d0  00 37 00 61 00 32 00 35  00 30 00 39 00 38 00 39  |.7.a.2.5.0.9.8.9|
> 000056e0  00 65 00 34 00 66 00 65  00 36 00 66 00 36 00 39  |.e.4.f.e.6.f.6.9|
> 000056f0  00 39 00 32 00 62 00 65  00 33 00 33 00 34 00 61  |.9.2.b.e.3.3.4.a|
> 00005700  00 34 00 33 00 00 00 00  00 00 00 00 00 00 00 00  |.4.3............|
> 
> and reported the output of VMGENID in this guest as:
> 
> VmCounterValue: fe6f6992be334a43:805a287a250989e4
> 
> The second guest had:
> 
> 00005360  c5 06 00 00 00 0e 67 65  6e 65 72 61 74 69 6f 6e  |......generation|
> 00005370  5f 69 64 00 40 00 00 00  65 00 62 00 66 00 62 00  |_id.@...e.b.f.b.|
> 00005380  62 00 37 00 39 00 37 00  33 00 36 00 35 00 37 00  |b.7.9.7.3.6.5.7.|
> 00005390  32 00 38 00 65 00 37 00  30 00 62 00 33 00 66 00  |2.8.e.7.0.b.3.f.|
> 000053a0  64 00 33 00 63 00 37 00  31 00 33 00 65 00 63 00  |d.3.c.7.1.3.e.c.|
> 000053b0  65 00 38 00 34 00 32 00  00 00 00 00 00 00 00 00  |e.8.4.2.........|
> 
> and VMGENID was:
> 
> VmCounterValue: b3fd3c713ece842:ebfbb797365728e7
> 
> Note:
> 
>  - In both cases, the generation ID is clearly not a GUID.
> 
>  - The two 64 bit words are swapped over somewhere, but individual
>    bytes are not byte-swapped, and there is again no evidence of
>    UUID-aware byte swapping.
> 
> ----------------------------------------------------------------------
> 
> So how do we get from where we are to a more sane vmgenid in qemu?
> 
> I propose we deprecate the guid parameter in:
> 
>   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> 
> Instead it will be replaced by bytes= which will simply write
> the bytes, in the order they appear, into guest memory with no
> attempt to interpret or byte-swap.  Something like:
> 
>   -device vmgenid,bytes=112233445566778899aabbccddeeff00,id=vmgenid0
> 
> (guid although deprecated will need to be kept around for a while,
> along with its weird byte-swapping behaviour).
> 
> We will then have a plain and simple method to emulate the behaviour
> of other hypervisors.  We will look at exactly what bytes they write
> to guest memory and copy that behaviour when v2v converting from those
> hypervisors.

From the libvirt POV, I'm not expecting anything in QEMU to change
in this respect. If guid is replaced by a new attribute taking data
in a different way, then libvirt will have to remap itself, so that
existing usage in libvirt keeps working the same way as it did with
guid.  Essentially from libvirt's POV, it is simply a documentation
issue to specify how the libvirt XML representation translates to
the guest visible representation, and ensure that all libvirt drivers
implement it the same way. The QEMU genid support arrived first so
that set the standard for how libvirt will represent it, that all
further libvirt hypervisor drivers need to match.


Regards,
Daniel
-- 
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Re: [PATCH 0/1] vmx: Fix <genid/> mapping

[Richard W.M. Jones]

On Thu, Sep 30, 2021 at 09:47:01AM +0100, Daniel P. Berrangé wrote:
> On Thu, Sep 30, 2021 at 08:33:48AM +0100, Richard W.M. Jones wrote:
> > I propose we deprecate the guid parameter in:
> > 
> >   -device vmgenid,guid=8987940a-0951-2cc5-e815-10634ff550b9,id=vmgenid0
> > 
> > Instead it will be replaced by bytes= which will simply write
> > the bytes, in the order they appear, into guest memory with no
> > attempt to interpret or byte-swap.  Something like:
> > 
> >   -device vmgenid,bytes=112233445566778899aabbccddeeff00,id=vmgenid0
> > 
> > (guid although deprecated will need to be kept around for a while,
> > along with its weird byte-swapping behaviour).
> > 
> > We will then have a plain and simple method to emulate the behaviour
> > of other hypervisors.  We will look at exactly what bytes they write
> > to guest memory and copy that behaviour when v2v converting from those
> > hypervisors.
> 
> From the libvirt POV, I'm not expecting anything in QEMU to change
> in this respect. If guid is replaced by a new attribute taking data
> in a different way, then libvirt will have to remap itself, so that
> existing usage in libvirt keeps working the same way as it did with
> guid.  Essentially from libvirt's POV, it is simply a documentation
> issue to specify how the libvirt XML representation translates to
> the guest visible representation, and ensure that all libvirt drivers
> implement it the same way. The QEMU genid support arrived first so
> that set the standard for how libvirt will represent it, that all
> further libvirt hypervisor drivers need to match.

I was going to suggest something like:

  <genid type="guid">aa-bb-cc..</genid>
or
  <genid type="binary">aabbcc..</genid>

with the type defaulting to guid for backwards compatibility.

Does libvirt XML have any other fields were you're passing
essentially small snippets of binary data?

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org

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

[Richard W.M. Jones]

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

On Thu, Sep 30, 2021 at 10:16:20AM +0100, Richard W.M. Jones wrote:
> I was going to suggest something like:
> 
>   <genid type="guid">aa-bb-cc..</genid>
> or
>   <genid type="binary">aabbcc..</genid>

After thinking about this some more, the real implementation on
Windows guest and host is two 64 bit little-endian integers[1].  How
about implementing this exactly the same way as Hyper-V (and VMware):

  <genid type="pair">
    <low>0x8877665544332211</low>
    <high>0x00ffeeddccbbaa99</high>
  </genid>

This would have to be mapped to the following qemu command line:

  qemu -device vmgenid,guid=44332211-6655-8877-99aa-bbccddeeff00

which would unmangle to the following in guest physical memory:

  11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff 00

The equivalent back-compat option would be:

  <genid type="qemu">44332211-6655-8877-99aa-bbccddeeff00</genid>

Rich.

[1] No one has ever thought what to do about big-endian guests.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/

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

[Laszlo Ersek]

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

Thanks,
Laszlo

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

[Richard W.M. Jones]

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.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-top is 'top' for virtual machines.  Tiny program with many
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