linux-kernel - Re: [Question about gic vmovp cmd] Consider adding VINVALL after VMOVP

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Message-ID: <ec4cba70-ba6f-2de6-29e8-945b0f52ed88@hisilicon.com>
Date:   Sat, 27 May 2023 17:51:50 +0800
From:   wangwudi <wangwudi@...ilicon.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <linux-kernel@...r.kernel.org>
Subject: Re: [Question about gic vmovp cmd] Consider adding VINVALL after
 VMOVP



在 2023/5/27 16:56, wangwudi 写道:
> 
> 
> -----邮件原件-----
> 发件人: Marc Zyngier [mailto:maz@...nel.org] 
> 发送时间: 2023年5月26日 15:03
> 收件人: wangwudi <wangwudi@...ilicon.com>
> 抄送: linux-kernel@...r.kernel.org
> 主题: Re: [Question about gic vmovp cmd] Consider adding VINVALL after VMOVP
> 
> On Fri, 26 May 2023 07:04:34 +0100,
> wangwudi <wangwudi@...ilicon.com> wrote:
>>
>> Hi Marc,
>>
>> During vpe migration, VMOVP needs to be executed.
>> If the vpe is migrated for the first time, especially before it is 
>> scheduled for the first time, there may be some unusual hanppens over 
>> kexec.
> 
> What may happen?

Actually, I'm not sure.

> 
>> We might consider adding a VINVALL cmd after VMOVP to increase 
>> robustness.
> 
> What are you trying to guarantee by adding this? From a performance perspective, this is terrible as you're forcing the ITS to drop its caches and reload everything, making the interrupt latency far worse than what it should be on each and every vcpu migration.

Agree, this reduces performance.

> 
> We already issue a VINVALL when a VPE is mapped. Why would you need anything else?
> 

It is just for robustness, like the VINALL when a VPE is mapped.

>>
>> @@ -1327,6 +1327,7 @@ static void its_send_vmovp(struct its_vpe *vpe)
>>
>>                 desc.its_vmovp_cmd.col = &its->collections[col_id];
>>                 its_send_single_vcommand(its, its_build_vmovp_cmd, 
>> &desc);
>> +               its_send_vinvall(its, vpe);
>>         }
>>
>> Do you think it's all right?
> 
> I think this is pretty bad. If your HW requires this, then we can add it as a workaround for your particular platform, but in general, this is not needed.

Got it, you are right :-).

Thanks for your explaination
Wudi

> 
> Thanks,
> 
> 	M.
> 
> --
> Without deviation from the norm, progress is not possible.