Re: [PATCH v2 03/12] iommu/vt-d: Remove clearing translation data in disable_dmar_iommu()

[Baolu Lu]

On 2022/6/15 14:22, Tian, Kevin wrote:
> > From: Baolu Lu <baolu.lu@xxxxxxxxxxxxxxx>
> > Sent: Tuesday, June 14, 2022 3:21 PM
> >
> > On 2022/6/14 14:49, Tian, Kevin wrote:
> > > > From: Lu Baolu<baolu.lu@xxxxxxxxxxxxxxx>
> > > > 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