linux-kernel - Re: [PATCH 09/12] iommu/vt-d: Check device list of domain in domain free path

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Date:   Wed, 1 Jun 2022 19:02:28 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Jason Gunthorpe <jgg@...dia.com>
Cc:     baolu.lu@...ux.intel.com, 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>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/12] iommu/vt-d: Check device list of domain in domain
 free path

On 2022/6/1 17:28, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Friday, May 27, 2022 2:30 PM
>>
>> When the IOMMU domain is about to be freed, it should not be set on any
>> device. Instead of silently dealing with some bug cases, it's better to
>> trigger a warning to report and fix any potential bugs at the first time.
>>
> 
> 
>>   static void domain_exit(struct dmar_domain *domain)
>>   {
>> -
>> -	/* Remove associated devices and clear attached or cached domains
>> */
>> -	domain_remove_dev_info(domain);
>> +	if (WARN_ON(!list_empty(&domain->devices)))
>> +		return;
>>
> 
> warning is good but it doesn't mean the driver shouldn't deal with
> that situation to make it safer e.g. blocking DMA from all attached
> device...

I have ever thought the same thing. :-)

Blocking DMA from attached device should be done when setting blocking
domain to the device. It should not be part of freeing a domain.

Here, the caller asks the driver to free the domain, but the driver
finds that something is wrong. Therefore, it warns and returns directly.
The domain will still be there in use until the next set_domain().

Best regards,
baolu