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From: Shaokun Zhang <zhangshaokun@hisilicon.com>
To: Marc Zyngier <maz@kernel.org>
Cc: wangwudi <wangwudi@hisilicon.com>,
	Lizixian <lizixian@hisilicon.com>,
	jiayanlei@huawei.com, fanhenglong@huawei.com,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [Question about KVM on arm64] Consider putting VINVALL to deactivation
Date: Thu, 16 Jan 2020 14:21:49 +0800
Message-ID: <d41bcfbd-69b1-641b-fc54-9255ac3e82b2@hisilicon.com> (raw)
In-Reply-To: <37126a877e1160ed50ee6d95a03d1574@kernel.org>

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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Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-14 14:20 Shaokun Zhang
2020-01-15 13:50 ` Marc Zyngier
2020-01-16  6:21   ` Shaokun Zhang [this message]

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