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

[Samiullah Khawaja <skhawaja@google.com>]

On Thu, May 07, 2026 at 05:05:07PM +0800, Baolu Lu wrote:
>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.

Agreed to both points. I will remove the redundant flush here.

The set_root_entry() is done outside this function in a separate loop
after setup of all iommus, I will check whether the iommu was preserved
there and skip set_root_entry().
>
>>+		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