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

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

[Shaokun Zhang]

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. 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.

Do you think it's all right?

Thanks,
Shaokun


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

[Marc Zyngier]

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.

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.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...

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

[Shaokun Zhang]

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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