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

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Date:   Thu, 16 Jun 2022 04:00:40 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Jason Gunthorpe" <jgg@...dia.com>
CC:     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()

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Wednesday, June 15, 2022 9:10 PM
> 
> 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);
>          }
> }
> 

I meant copying table succeeds but another error occurs in the
remaining path of initialization...