Re: [PATCH v3 14/17] i386: Use CPUCacheInfo.share_level to encode CPUID[4]

[Moger, Babu]

Hi Zhao,


On 8/14/23 03:22, Zhao Liu wrote:
> Hi Babu,
> 
> On Fri, Aug 04, 2023 at 10:48:29AM -0500, Moger, Babu wrote:
>> Date: Fri, 4 Aug 2023 10:48:29 -0500
>> From: "Moger, Babu" <babu.moger@amd.com>
>> Subject: Re: [PATCH v3 14/17] i386: Use CPUCacheInfo.share_level to encode
>>  CPUID[4]
>>
>> Hi Zhao,
>>
>> On 8/4/23 04:48, Zhao Liu wrote:
>>> Hi Babu,
>>>
>>> On Thu, Aug 03, 2023 at 11:41:40AM -0500, Moger, Babu wrote:
>>>> Date: Thu, 3 Aug 2023 11:41:40 -0500
>>>> From: "Moger, Babu" <babu.moger@amd.com>
>>>> Subject: Re: [PATCH v3 14/17] i386: Use CPUCacheInfo.share_level to encode
>>>>  CPUID[4]
>>>>
>>>> Hi Zhao,
>>>>
>>>> On 8/2/23 18:49, Moger, Babu wrote:
>>>>> Hi Zhao,
>>>>>
>>>>> Hitting this error after this patch.
>>>>>
>>>>> ERROR:../target/i386/cpu.c:257:max_processor_ids_for_cache: code should
>>>>> not be reached
>>>>> Bail out! ERROR:../target/i386/cpu.c:257:max_processor_ids_for_cache: code
>>>>> should not be reached
>>>>> Aborted (core dumped)
>>>>>
>>>>> Looks like share_level for all the caches for AMD is not initialized.
>>>
>>> I missed these change when I rebase. Sorry for that.
>>>
>>> BTW, could I ask a question? From a previous discussion[1], I understand
>>> that the cache info is used to show the correct cache information in
>>> new machine. And from [2], the wrong cache info may cause "compatibility
>>> issues".
>>>
>>> Is this "compatibility issues" AMD specific? I'm not sure if Intel should
>>> update the cache info like that. thanks!
>>
>> I was going to comment about that. Good that you asked me.
>>
>> Compatibility is qemu requirement.  Otherwise the migrations will fail.
>>
>> Any changes in the topology is going to cause migration problems.
> 
> Could you please educate me more about the details of the "migration
> problem"?
> 
> I didn't understand why it was causing the problem and wasn't sure if I
> was missing any cases.
> 

I am not an expert on migration but I test VM migration sometimes.
Here are some guidelines.
https://developers.redhat.com/blog/2015/03/24/live-migrating-qemu-kvm-virtual-machines

When you migrate a VM to newer qemu using the same CPU type, migration
should work seamless. That means list of CPU features should be compatible
when you move to newer qemu version with CPU type.

Thanks
Babu