linux-kernel - Re: [PATCH v2 03/12] iommu/vt-d: Remove clearing translation data in disable_dmar

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Date:   Wed, 15 Jun 2022 21:10:19 +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 v2 03/12] iommu/vt-d: Remove clearing translation data in
 disable_dmar_iommu()

On 2022/6/15 14:22, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, June 14, 2022 3:21 PM
>>
>> On 2022/6/14 14:49, Tian, Kevin wrote:
>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Sent: Tuesday, June 14, 2022 10:51 AM
>>>>
>>>> The disable_dmar_iommu() is called when IOMMU initialization fails or
>>>> the IOMMU is hot-removed from the system. In both cases, there is no
>>>> need to clear the IOMMU translation data structures for devices.
>>>>
>>>> On the initialization path, the device probing only happens after the
>>>> IOMMU is initialized successfully, hence there're no translation data
>>>> structures.
>>> Out of curiosity. With kexec the IOMMU may contain stale mappings
>>> from the old kernel. Then is it meaningful to disable IOMMU after the
>>> new kernel fails to initialize it properly?
>>
>> For kexec kernel, if the IOMMU is detected to be pre-enabled, the IOMMU
>> driver will try to copy tables from the old kernel. If copying table
>> fails, the IOMMU driver will disable IOMMU and do the normal
>> initialization.
>>
> 
> What about an error occurred after copying table in the initialization
> path? The new kernel will be in a state assuming iommu is disabled
> but it is still enabled using an old mapping for certain devices...
>   

If copying table failed, the translation will be disabled and a clean
root table will be used.

if (translation_pre_enabled(iommu)) {
         pr_info("Translation already enabled - trying to copy 
translation structures\n");

         ret = copy_translation_tables(iommu);
         if (ret) {
                 /*
                  * We found the IOMMU with translation
                  * enabled - but failed to copy over the
                  * old root-entry table. Try to proceed
                  * by disabling translation now and
                  * allocating a clean root-entry table.
                  * This might cause DMAR faults, but
                  * probably the dump will still succeed.
                  */
                 pr_err("Failed to copy translation tables from previous 
kernel for %s\n",
                        iommu->name);
                 iommu_disable_translation(iommu);
                 clear_translation_pre_enabled(iommu);
         } else {
                 pr_info("Copied translation tables from previous kernel 
for %s\n",
                         iommu->name);
         }
}

Best regards,
baolu