Re: [PATCH] hvmloader: Update to SMBIOS 2.6

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.08.2025 12:31, Andrew Cooper wrote:
> On 28/08/2025 11:22 am, Jan Beulich wrote:
>> On 28.08.2025 12:01, Andrew Cooper wrote:
>>> On 28/08/2025 8:07 am, Jan Beulich wrote:
>>>> On 27.08.2025 19:47, Andrew Cooper wrote:
>>>>> On 22/08/2025 2:47 pm, Teddy Astie wrote:
>>>>>> Currently, hvmloader uses SMBIOS 2.4, however, when using OVMF, the
>>>>>> SMBIOS is patched to 2.8, which has clarified the UUID format (as GUID).
>>>>>>
>>>>>> In Linux, if the SMBIOS version is >= 2.6, the GUID format is used, else
>>>>>> (undefined as per SMBIOS spec), big endian is used (used by Xen). 
>>>>>> Therefore,
>>>>>> you have a endian mismatch causing the UUIDs to mismatch in the guest.
>>>>>>
>>>>>> $ cat /sys/hypervisor/uuid
>>>>>> e865e63f-3d30-4f0b-83e0-8fdfc1e30eb7
>>>>>> $ cat /sys/devices/virtual/dmi/id/product_uuid
>>>>>> 3fe665e8-303d-0b4f-83e0-8fdfc1e30eb7
>>>>>> $ cat /sys/devices/virtual/dmi/id/product_serial
>>>>>> e865e63f-3d30-4f0b-83e0-8fdfc1e30eb7
>>>>>>
>>>>>> This patch updates the SMBIOS version from 2.4 to 2.6 and fixup the UUID
>>>>>> written in the table; which effectively fix this endianness mismatch with
>>>>>> OVMF; while the UUID displayed by Linux is still the same for SeaBIOS.
>>>>>>
>>>>>> Signed-off-by: Teddy Astie <teddy.astie@xxxxxxxxxx>
>>>>>> ---
>>>>>> This effectively changes the UUID seen with UEFI guests as it was
>>>>>> actually inconsistent with SeaBIOS and SMBIOS expectations.
>>>>>> ---
>>>>> I agree this is a real bug and needs fixing.
>>>> Hmm, I didn't realize this is a bug, and hence put the patch off as 4.22
>>>> material. If there is a bug being fixed: Teddy, please add a Fixes: tag.
>>> I'm not sure if this has a reasonable Fixes tag.
>>>
>>> It's a combination of an ill-specified domain handle format, and using
>>> an ill-specified version of the SMBios spec.
>> But the problem was still introduced into the code base at some point. Afaict
>> in c683914ef913 ("Add code to generate SMBIOS tables to hvmloader"), i.e. 
>> when
>> smbios.c was first added.
> 
> The thing that changed was the SMBios spec, in version 2.6.  It went
> from having an ill-defined statement of what a UUID was (and for which
> Xen's code was a valid interpretation), to stating a MSFT GUID format.

Fair enough. I'd still prefer to have the (slightly questionable) Fixes:
tag, to serve as an indication that there will be a need for backporting.
Or else at the very least a statement to the effect of there not being
any good Fixes: tag to use.

Jan