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

[Jason Gunthorpe <jgg@nvidia.com>]

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?

Jason