Re: [PATCH v2 1/2] amd/iommu: Preserve domain ids inside the kdump kernel

[Sairaj Kodilkar <sarunkod@amd.com>]

On 11/24/2025 8:14 PM, Jason Gunthorpe wrote:
> On Fri, Nov 21, 2025 at 02:41:15PM +0530, Sairaj Kodilkar wrote:
>> Currently AMD IOMMU driver does not reserve domain ids programmed in the
>> DTE while reusing the device table inside kdump kernel. This can cause
>> reallocation of these domain ids for newer domains that are created by
>> the kdump kernel, which can lead to potential IO_PAGE_FAULTs
>>
>> Hence reserve these ids inside pdom_ids.
>>
>> Fixes: 38e5f33ee359 ("iommu/amd: Reuse device table for kdump")
>> Signed-off-by: Sairaj Kodilkar <sarunkod@amd.com>
>> Reported-by: Jason Gunthorpe <jgg@nvidia.com>
>> Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
>> ---
>>   drivers/iommu/amd/init.c | 23 ++++++++++++++++++++++-
>>   1 file changed, 22 insertions(+), 1 deletion(-)
> This seems OK
>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
>
> But the a point of this work was to remove this code:
>
> diff --git a/drivers/iommu/amd/iommu.c b/drivers/iommu/amd/iommu.c
> index 48bca4dc8eb61f..1cd799913cbcd6 100644
> --- a/drivers/iommu/amd/iommu.c
> +++ b/drivers/iommu/amd/iommu.c
> @@ -2024,7 +2024,6 @@ static void set_dte_entry(struct amd_iommu *iommu,
>                            phys_addr_t top_paddr, unsigned int top_level)
>   {
>          u16 domid;
> -       u32 old_domid;
>          struct dev_table_entry *initial_dte;
>          struct dev_table_entry new = {};
>          struct protection_domain *domain = dev_data->domain;
> @@ -2080,7 +2079,6 @@ static void set_dte_entry(struct amd_iommu *iommu,
>          if (dev_data->ats_enabled)
>                  new.data[1] |= DTE_FLAG_IOTLB;
>   
> -       old_domid = READ_ONCE(dte->data[1]) & DEV_DOMID_MASK;
>          new.data[1] |= domid;
>   
>          /*
> @@ -2096,15 +2094,6 @@ static void set_dte_entry(struct amd_iommu *iommu,
>          set_dte_gcr3_table(iommu, dev_data, &new);
>   
>          update_dte256(iommu, dev_data, &new);
> -
> -       /*
> -        * A kdump kernel might be replacing a domain ID that was copied from
> -        * the previous kernel--if so, it needs to flush the translation cache
> -        * entries for the old domain ID that is being overwritten
> -        */
> -       if (old_domid) {
> -               amd_iommu_flush_tlb_domid(iommu, old_domid);
> -       }
>   }
>   
>   /*
>
> Under the reasoning that:
>   - domids in use by the prior kernel are reserved in the IDA and are
>     never used by this kernel
>   - domids in the IDA must be clean
>   - There is no reason to flush a domid until it is returned to the IDA
>   - detach_device() calls amd_iommu_domain_flush_all() before the
>     domain can be freed and the domid returned the IDA which clears the
>     IOTLB
>
> Please add a patch?

Thanks for the explaination
Will add the patch for it

Thanks
Sairaj