Re: [PATCH 1/1] iommu/arm-smmu-v3: add support for BBML

From: Robin Murphy <robin.murphy@arm.com>
To: Keqian Zhu <zhukeqian1@huawei.com>, Will Deacon <will@kernel.org>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	iommu <iommu@lists.linux-foundation.org>,
	Yang Yingliang <yangyingliang@huawei.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/1] iommu/arm-smmu-v3: add support for BBML
Date: Thu, 28 Jan 2021 16:17:35 +0000	[thread overview]
Message-ID: <301590f1-817b-d8eb-1585-31208cba18af@arm.com> (raw)
In-Reply-To: <d3aaa7e7-edb0-f867-0519-9a4e0bee35d2@huawei.com>

On 2021-01-28 15:18, Keqian Zhu wrote:
> 
> 
> On 2021/1/27 17:39, Robin Murphy wrote:
>> On 2021-01-27 07:36, Keqian Zhu wrote:
>>>
>>>
>>> On 2021/1/27 10:01, Leizhen (ThunderTown) wrote:
>>>>
>>>>
>>>> On 2021/1/26 18:12, Will Deacon wrote:
>>>>> On Mon, Jan 25, 2021 at 08:23:40PM +0000, Robin Murphy wrote:
>>>>>> Now we probably will need some degreee of BBML feature awareness for the
>>>>>> sake of SVA if and when we start using it for CPU pagetables, but I still
>>>>>> cannot see any need to consider it in io-pgtable.
>>>>>
>>>>> Agreed; I don't think this is something that io-pgtable should have to care
>>>>> about.
>>> Hi,
>>>
>>> I have a question here :-).
>>> If the old table is not live, then the break procedure seems unnecessary. Do I miss something?
>>
>> The MMU is allowed to prefetch translations at any time, so not following the proper update procedure could still potentially lead to a TLB conflict, even if there's no device traffic to worry about disrupting.
>>
>> Robin.
> 
> Thanks. Does the MMU you mention here includes MMU and SMMU? I know that at SMMU side, ATS can prefetch translation.

Yes, both - VMSAv8 allows speculative translation table walks, so SMMUv3 
inherits from there (per 3.21.1 "Translation tables and TLB invalidation 
completion behavior").

Robin.

> 
> Keqian
>>
>>> Thanks,
>>> Keqian
>>>
>>>>
>>>> Yes, the SVA works in stall mode, and the failed device access requests are not
>>>> discarded.
>>>>
>>>> Let me look for examples. The BBML usage scenario was told by a former colleague.
>>>>
>>>>>
>>>>> Will
>>>>>
>>>>> .
>>>>>
>>>>
>>>>
>>>> _______________________________________________
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>>>> linux-arm-kernel@lists.infradead.org
>>>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>>> .
>>>>
>>> _______________________________________________
>>> iommu mailing list
>>> iommu@lists.linux-foundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/iommu
>>>
>> .
>>