Re: [PATCH v2 09/16] iommu/vt-d: Restore IOMMU state and reclaimed domain ids

[Baolu Lu <baolu.lu@linux.intel.com>]

On 4/28/2026 1:56 AM, Samiullah Khawaja wrote:
> During boot fetch the preserved state of IOMMU unit and if found then
> restore the state.
> 
> - Reuse the root_table that was preserved in the previous kernel.
> - Reclaim the domain ids of the preserved domains for each preserved
>    devices so these are not acquired by another domain.
> 
> Signed-off-by: Samiullah Khawaja<skhawaja@google.com>
> ---
>   drivers/iommu/intel/iommu.c      | 55 ++++++++++++++++++++++--------
>   drivers/iommu/intel/iommu.h      |  7 ++++
>   drivers/iommu/intel/liveupdate.c | 57 ++++++++++++++++++++++++++++++++
>   3 files changed, 105 insertions(+), 14 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 68fecd4e57fa..4118a0861f38 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -670,10 +670,17 @@ void dmar_fault_dump_ptes(struct intel_iommu *iommu, u16 source_id,
>   #endif
>   
>   /* iommu handling */
> -static int iommu_alloc_root_entry(struct intel_iommu *iommu)
> +static int iommu_alloc_root_entry(struct intel_iommu *iommu,
> +				  struct iommu_hw_ser *iommu_ser)
>   {
>   	struct root_entry *root;
>   
> +	if (iommu_ser) {
> +		intel_iommu_liveupdate_restore_root_table(iommu, iommu_ser);
> +		__iommu_flush_cache(iommu, iommu->root_entry, ROOT_SIZE);

The root table is already active and being used by the hardware at this
point, correct? Since the CPU should not be modifying an active table,
the __iommu_flush_cache() call seems unnecessary and potentially risky.

Furthermore, if the iommu state has been restored, is it necessary to
call iommu_set_root_entry()? Calling it would essentially replace an
active, in-use root table with an identical one, which appears
redundant.

> +		return 0;
> +	}
> +
>   	root = iommu_alloc_pages_node_sz(iommu->node, GFP_ATOMIC, SZ_4K);
>   	if (!root) {
>   		pr_err("Allocating root entry for %s failed\n",

Thanks,
baolu