[libvirt] [PATCH 2/2] qemu: Remove host-passthrough validation check for host-phys-bits=on

[Jim Fehlig]

On 9/2/22 09:24, Jim Fehlig wrote:
> On 9/2/22 08:50, Dario Faggioli wrote:
>> On Wed, 2022-08-17 at 11:18 -0600, Jim Fehlig wrote:
>>> On 8/17/22 08:47, Jim Fehlig wrote:
>>>> On 8/17/22 02:19, Lin Ma wrote:
>>>>> Besides the -cpu host, The host-phys-bits=on applies to custom or
>>>>> max
>>>>> cpu model, So the host-passthrough validation check is
>>>>> unnecessary for
>>>>> maxphysaddr with mode='passthrough'.
>>>>
>>>> I wonder why Dario added the check? He even added a test for it:
>>>> cpu-phys-bits-passthrough2 in qemuxml2argvtest. He is off now, so
>>>> we'll have to
>>>> await his return for an answer.
>>>
>>> BTW, I'm not doubting your analysis. In fact, I tested with one of
>>> the known CPU
>>> models (-cpu Cascadelake-Server,...,host-phys-bits=on) and it worked
>>> fine with a
>>> 7TB VM. I'd still like to know why Dario added the check. Perhaps he
>>> encountered
>>> some problematic corner cases.
>>>
>> Not really, no. IIRC, QEMU was behaving differently than how it work
>> this days, when I wrote this, but even back then host-phys-bits=on was
>> fine with -cpu != host.
>>
>> The check is there just because I thought that, if you're using a
>> specific CPU model, and not host-passthrough, that's probably because
>> you don't want the host details to "leak" into the VM (for migration or
>> whatever). Hence, you shouldn't use phys-bits passthrough either.
> 
> Hmm, an interesting point...
> 
>> Like, you need the CPU to be CascadeLake-Server for migration purposes,
>> your host has 52 bits and you know you need a large VM so instead of
>> wasting time doing the math, you just go for passthrough (for the phys
>> bits, of course). Then you migrate on an host with 46 bits, which would
>> also have been fine for the size VM, but now, since you've basically
>> used 52, it's a problem.
> 
> but at least for the migration case we have that covered by the ABI stability check
> 
> https://gitlab.com/libvirt/libvirt/-/blob/master/src/conf/cpu_conf.c#L1113

Wait, that just checks the stability of any config changes provided by the user 
when starting the migration. It doesn't check such differences between hosts 
involved in the migration. IIRC that's done elsewhere in the qemu driver, but 
I'll need to refresh my memory...

Regards,
Jim