Re: [PATCH 5/6] iommu: document missing def_domain_type return

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 12/9/24 9:57 PM, Baolu Lu wrote:
> On 12/10/24 03:24, Matthew Rosato wrote:
>> In addition to IOMMU_DOMAIN_DMA, def_domain_type can also return
>> IOMMU_DOMAIN_DMA_FQ when applicable, else flush queues will never be
>> used.
>>
>> Signed-off-by: Matthew Rosato<mjrosato@linux.ibm.com>
>> ---
>>   include/linux/iommu.h | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
>> index 05279109c732..d0da1918d2de 100644
>> --- a/include/linux/iommu.h
>> +++ b/include/linux/iommu.h
>> @@ -585,6 +585,7 @@ iommu_copy_struct_from_full_user_array(void *kdst, size_t kdst_entry_size,
>>    * @def_domain_type: device default domain type, return value:
>>    *        - IOMMU_DOMAIN_IDENTITY: must use an identity domain
>>    *        - IOMMU_DOMAIN_DMA: must use a dma domain
>> + *              - IOMMU_DOMAIN_DMA_FQ: dma domain with batch invalidation
> 
> In which case must an iommu driver return IOMMU_DOMAIN_DMA_FQ?
> 
> The flush queue is a policy of "when and how to synchronize the IOTLB"
> in dma-iommu.c. The iommu driver actually has no need to understand such
> policy.

If you look ahead to the next patch where I implement def_domain_type for s390, I found that if I only ever return IOMMU_DOMAIN_DMA from ops->def_domain_type then when go through iommu_dma_init_domain() we will never call iommu_dma_init_fq() regardless of IOMMU_CAP_DEFERRED_FLUSH because of the if (domain->type == IOMMU_DOMAIN_DMA_FQ) check.  So something isn't right here.

It looks to me like the following is happening:

We first have the iommu_def_domain_type set in iommu_subsys_init or via one of the set_default routines, e.g.:
	if (!iommu_default_passthrough() && !iommu_dma_strict)
		iommu_def_domain_type = IOMMU_DOMAIN_DMA_FQ;

But when we arrive at iommu_group_alloc_default_domain()...

if we have no ops->def_domain_type() defined we will call __iommu_group_alloc_default_domain using what is in iommu_def_domain_type, which could be IOMMU_DOMAIN_DMA, IOMMU_DOMAIN_DMA_FQ or IOMMU_DOMAIN_IDENTITY based on strict/passthrough settings.  Testing an s390 scenario today without this series applied, we will call __iommu_group_alloc_default_domain with IOMMU_DOMAIN_DMA_FQ, as long as iommu.strict/passthrough is not specified, so then later in dma-iommu:iommu_dma_init_domain() we can use FQ based on IOMMU_CAP_DEFERRED_FLUSH.

but once we add ops->def_domain_type() then we end up calling iommu_group_alloc_default_domain() with a req_type == the return value from ops->def_domain_type(), which by the current definition can only be IOMMU_DOMAIN_DMA or IOMMU_DOMAIN_IDENTITY.  We will then call __iommu_group_alloc_default_domain with that req_type; so without this patch + the DMA_FQ path in patch 6 we would always end up allocating IOMMU_DOMAIN_DMA instead of IOMMU_DOMAIN_DMA_FQ by default, so when we arrive at dma:iommu_dma_init_domain() we won't check for IOMMU_CAP_DEFERRED_FLUSH because of the type.

So unless I'm missing something I think either we have to
1) be more flexible in what ops->default_domain_type() is allowed to return as this patch does 
or 
2) iommu core needs to look at the return from ops->default_domain_type() and decide whether it's OK to convert IOMMU_DOMAIN_DMA->IOMMU_DOMAIN_DMA_FQ based on strict setting.  This removes the decision from the individual drivers and dma-iommu can later decide whether or not to use it or not based on IOMMU_CAP_DEFERRED_FLUSH?  But would also affect other users of def_domain_type() today that perhaps did not want DMA_FQ?  Unsure.  What I mean is something like (untested):

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 6bdede4177ff..275daa7f819d 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -1744,9 +1744,11 @@ static int iommu_get_def_domain_type(struct iommu_group *group,
                 */
                type = ops->default_domain->type;
        } else {
-               if (ops->def_domain_type)
+               if (ops->def_domain_type) {
                        type = ops->def_domain_type(dev);
-               else
+                       if (type == IOMMU_DOMAIN_DMA && !iommu_dma_strict)
+                               type = IOMMU_DOMAIN_DMA_FQ;
+               } else
                        return cur_type;
        }
        if (!type || cur_type == type)