linux-kernel - Re: [PATCH v8 5/7] iommu/arm-smmu-v3: Add support for non-strict mode

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Date:   Fri, 28 Sep 2018 15:01:02 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     joro@...tes.org, thunder.leizhen@...wei.com,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linuxarm@...wei.com, guohanjun@...wei.com, huawei.libin@...wei.com,
        john.garry@...wei.com
Subject: Re: [PATCH v8 5/7] iommu/arm-smmu-v3: Add support for non-strict mode

On 28/09/18 14:55, Will Deacon wrote:
> On Fri, Sep 28, 2018 at 01:47:04PM +0100, Will Deacon wrote:
>> On Fri, Sep 28, 2018 at 01:26:00PM +0100, Robin Murphy wrote:
>>> On 28/09/18 13:19, Will Deacon wrote:
>>>> On Thu, Sep 20, 2018 at 05:10:25PM +0100, Robin Murphy wrote:
>>>>> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
>>>>> index f10c852479fc..db402e8b068b 100644
>>>>> --- a/drivers/iommu/arm-smmu-v3.c
>>>>> +++ b/drivers/iommu/arm-smmu-v3.c
>>>>> @@ -612,6 +612,7 @@ struct arm_smmu_domain {
>>>>>    	struct mutex			init_mutex; /* Protects smmu pointer */
>>>>>    	struct io_pgtable_ops		*pgtbl_ops;
>>>>> +	bool				non_strict;
>>>>>    	enum arm_smmu_domain_stage	stage;
>>>>>    	union {
>>>>> @@ -1407,6 +1408,12 @@ static void arm_smmu_tlb_inv_context(void *cookie)
>>>>>    		cmd.tlbi.vmid	= smmu_domain->s2_cfg.vmid;
>>>>>    	}
>>>>> +	/*
>>>>> +	 * NOTE: when io-pgtable is in non-strict mode, we may get here with
>>>>> +	 * PTEs previously cleared by unmaps on the current CPU not yet visible
>>>>> +	 * to the SMMU. We are relying on the DSB implicit in queue_inc_prod()
>>>>> +	 * to guarantee those are observed before the TLBI. Do be careful, 007.
>>>>> +	 */
>>>>
>>>> Good, so you can ignore my comment on the previous patch :)
>>>
>>> Well, I suppose that comment in io-pgtable *could* have explicitly noted
>>> that same-CPU order is dealt with elsewhere - feel free to fix it up if you
>>> think it would be a helpful reminder for the future.
>>
>> I think I'll move it into the documentation for the new attribute, so that
>> any driver authors wanting to enable lazy invalidation know that they need
>> to provide this guarantee in their full TLB invalidation callback.
> 
> Hmm, so I started doing this but then realised we already required this
> behaviour for tlb_add_flush() afaict. That would mean that mainline
> currently has a bug for arm-smmu.c, because we use the _relaxed I/O
> accessors in there and there's no DSB after clearing the PTE on unmap().
> 
> Am I missing something?

Argh, no - I started having the same suspicion when changing those 
writel()s in patch #7, resolved to either mention it to you or 
investigate it myself as a separate fix, then promptly forgot entirely. 
Thanks for the reminder...

Robin.