Re: [edk2-devel] [PATCH v2] OvmfPkg/PlatformInitLib: catch QEMU's CPU hotplug reg block regression

["Laszlo Ersek" <lersek@redhat.com>]

On 1/18/23 14:10, Ard Biesheuvel wrote:
> On Wed, 18 Jan 2023 at 12:50, Laszlo Ersek <lersek@redhat.com> wrote:
>>
>> On 1/18/23 08:25, Gerd Hoffmann wrote:
>>> On Tue, Jan 17, 2023 at 05:43:53PM +0100, Ard Biesheuvel wrote:
>>>> On Tue, 17 Jan 2023 at 13:37, Gerd Hoffmann <kraxel@redhat.com> wrote:
>>>>>
>>>>>   Hi,
>>>>>
>>>>>>>> In particular the firmware makes no further decisions based on
>>>>>>>> whether QEMU advertized some of these features.
>>>>>>>
>>>>>>> I was thinking the other way around:  When cpu hotplug is disabled in
>>>>>>> qemu it should be safe to skip the whole cpu hotplug checking dance.
>>>>>>> See test patch below.
>>>>>>>
>>>>>>> That would give us a config switch (turn off cpu hotplug support)
>>>>>>> which would allow edk2 run on qemu versions with broken cpu hotplug.
>>>>>>>
>>>>>>> Does the idea look sane or do I miss something?
>>>>>
>>>>>> This would be wrong.
>>>>>>
>>>>>> [ detailed description snipped here (but stored for later reference,
>>>>>>   thanks for all the details) ]
>>>>>
>>>>> So, the tl;dr version:  cpu hotplug is older than smi feature
>>>>> negotiation, so smi hotplug feature bit being off doesn't imply
>>>>> qemu wouldn't hotplug cpus.
>>>>>
>>>>> So, no easy way out.  Luckily this affects tcg only.
>>>>>
>>>>> For edk2 ci doing (tcg) efi shell test boots switching to Oliver's
>>>>> latest containers with fixed qemu included should handle things
>>>>> (latest series just posted).  So once this is in we should be able to
>>>>> merge this patch without breaking CI.
>>>>
>>>> My head is spinning.
>>>>
>>>> What about running QEMU with only a single CPU, and without any of
>>>> these features? Is there really no way we can make that work without
>>>> turning OVMF into the timebomb that Laszlo describes?
>>>
>>> I can't see any way :(
>>>
>>> ovmf seeing only a single cpu does not imply cpu hotplug can't happen,
>>> it could be "qemu -smp cpus=1,maxcpus=4".  Figuring the maxcpus number
>>> depends on the broken cpu hotplug registers.
>>>
>>>> It's just very annoying that on a non-KVM host and a given QEMU
>>>> binary, you might simply be out of luck entirely, and there is no way
>>>> you can run OVMF with the fix applied. I would like to avoid that if
>>>> possible.
>>>
>>> Indeed.
>>
>> ... you could introduce a new fw_cfg boolean switch (and explain it in
>> the hang message) that meant: "I know what this QEMU bug is, I
>> understand its consequences are obscure, risky, and far-reaching in
>> OVMF, I've been warned, I know what I'm doing". That's a relatively
>> small addition to this patch, and then the risk is assumed by the user.
>> It resolves "being out of luck *entirely*".
>>
> 
> You mean the kind of fw_cfg vairiable that is arbitrarily settable
> from the QEMU command line, right? Yeah, that would at least provide a
> way out.
> 

yes.