linux-kernel - Re: [PATCH v3 4/7] iommu/vt-d: Fold dmar_remove_one_dev

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Date:   Wed, 16 Nov 2022 20:08:30 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Cc:     baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 4/7] iommu/vt-d: Fold dmar_remove_one_dev_info() into
 its caller

On 2022/11/16 17:15, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Wednesday, November 16, 2022 4:03 PM
>>
>> On 2022/11/16 13:35, Tian, Kevin wrote:
>>>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>>>> Sent: Wednesday, November 16, 2022 12:36 PM
>>>>
>>>> On 11/16/22 11:53 AM, Tian, Kevin wrote:
>>>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>>>>>> Sent: Monday, November 14, 2022 9:41 AM
>>>>>> @@ -4562,7 +4538,10 @@ static void
>> intel_iommu_release_device(struct
>>>>>> device *dev)
>>>>>>     {
>>>>>>     	struct device_domain_info *info = dev_iommu_priv_get(dev);
>>>>>>
>>>>>> -	dmar_remove_one_dev_info(dev);
>>>>>> +	iommu_disable_pci_caps(info);
>>>>>> +	domain_context_clear(info);
>>>>>> +	device_block_translation(dev);
>>>>> clear context after blocking translation.
>>>> Unfortunately domain_context_clear() needs reference to info->domain
>>>> (for domain id when flushing cache), which is cleared in
>>>> device_block_translation().
>>>>
>>> this sounds an ordering problem. clearing context should be after
>>> blocking translation in concept.
>>
>> At present, when the default domain is attached to the device, we first
>> populate the pasid table entry, and then populate the device context
>> entry. Above code is just the reverse operation.
>>
>> Can you see any practical problems caused by this sequence? If so, it
>> seems that we should carefully consider whether such problems already
>> exist.
>>
> 
> there is no problem with existing code. Just after this patch the order
> looks weird based on the literal name of those functions.
> 
> domain_context_clear() is a big hammer to disable the context entry,
> implying translation must be blocked. Then calling another block
> translation afterwards becomes unnecessary.
> 
> Probably it should be split into two functions with one requiring
> info->domain called before block translation and the rest which
> actually clears the context entry being the last step?

This is what the existing code does. Perhaps I should drop this patch,
or only rename iommu_disable_dev_iotlb() to iommu_disable_pci_caps().

Best regards,
baolu