[Question about KVM on arm64] Consider putting VINVALL to deactivation

Shaokun Zhang zhangshaokun at hisilicon.com
Wed Jan 15 22:21:49 PST 2020


Hi Marc,

On 2020/1/15 21:50, Marc Zyngier wrote:
> Hi Shaokun,
> 
> On 2020-01-14 14:20, Shaokun Zhang wrote:
>> Hi Marc,
>>
>> On activation, VMAPP command is followed by a VINVALL, which could be
>> quite expensive for the start-up of virtual machine. If a vpeid is allowed
>> successfully, it is not used in system.
> 
> How expensive? This is exactly similar to what happens on a physical machine
> where we perform an INVALL on MAPC. And yet you don't complain about that.
> 

Agree, I didn't consider this before.

> Please provide numbers.
> 
>> We may consider put VINVALL to deactivation to ensure all cache of certain
>> vpeid is invalid, to simplify activation. We consider start-up may be more
>> common and more time-consuming-sensitive than shutdown process.
> 
> In my world, they cost the same thing, and happen just as often. Also, I want
> guarantees that on VMAPP, there is no stale information even if this is the
> first time we're using this VPEid (who knows what happens over kexec, for
> example).
> 
>> Do you think it's all right?
> 
> I don't, for the reasons stated above. You also provide no numbers showing
> how bad the overhead is, so I'm left guessing.
> 

Got it, you are right :-).

Thanks for your explaination
Shaokun

> Thanks,
> 
>         M.




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