linux-kernel - Re: [PATCH v2 0/3] Fix several bugs in KVM stage 2 translation

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Date:   Wed, 2 Dec 2020 20:50:14 +0800
From:   "wangyanan (Y)" <wangyanan55@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     Will Deacon <will@...nel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Gavin Shan <gshan@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        <wanghaibin.wang@...wei.com>, <yezengruan@...wei.com>,
        <zhukeqian1@...wei.com>, <yuzenghui@...wei.com>,
        <jiangkunkun@...wei.com>, <wangjingyi11@...wei.com>,
        <lushenming@...wei.com>
Subject: Re: [PATCH v2 0/3] Fix several bugs in KVM stage 2 translation


On 2020/12/2 20:23, Marc Zyngier wrote:
> Hi Yanan,
>
> [...]
>
>> BTW: there are two more things below that I want to talk about.
>>
>> 1.  Recently, I have been focusing on the ARMv8.4-TTRem feature which
>> is aimed at changing block size in stage 2 mapping.
>>
>> I have a plan to implement this feature for stage 2 translation when
>> splitting a block into tables or merging tables into a block.
>>
>> This feature supports changing block size without performing
>> *break-before-make*, which might have some improvement on performance.
>>
>> What do you think about this?
>
> It would be interesting if you can demonstrate some significant
> performance improvements compared to the same workload with BBM.
>
> I'm not completely convinced this would change much, given that
> it is only when moving from a table to a block mapping that you
> can elide BBM when the support level is 1 or 2. As far as I can
> tell, this only happens in the "stop logging" case.
>
> Is that something that happens often enough to justify the added
> complexity? Having to handle TLB Conflict Abort is annoying, for
> example.

I will take more consideration about the necessity  and maybe some tests

on the performance will be made later.


Thanks,


Yanan

>
>> 2. Given that the issues we discussed before were found in practice
>> when guest state changes from dirty logging to dirty logging canceled.
>>
>> I could add a test file testing on this case to selftests/ or kvm unit
>> tests/, if it's necessary.
>
> That would be awesome, and I'd be very grateful if you did. It is the
> second time we break this exact case, and having a reliable way to
> verify it would definitely help.
>
> Thanks,
>
>         M.