Re: [PATCH 1/6] vfio/iommu_type1: Make an explicit "promote" semantic

[Keqian Zhu]

On 2021/1/16 6:42, Alex Williamson wrote:
> On Thu, 7 Jan 2021 12:43:56 +0800
> Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote:
> 
>> When we want to promote the pinned_page_dirty_scope of vfio_iommu,
>> we call the "update" function to visit all vfio_group, but when we
>> want to downgrade this, we can set the flag as false directly.
> 
> I agree that the transition can only go in one direction, but it's
> still conditional on the scope of all groups involved.  We are
> "updating" the iommu state based on the change of a group.  Renaming
> this to "promote" seems like a matter of personal preference.
> 
>> So we'd better make an explicit "promote" semantic to the "update"
>> function. BTW, if vfio_iommu already has been promoted, then return
>> early.
> 
> Currently it's the caller that avoids using this function when the
> iommu scope is already correct.  In fact the changes induces a
> redundant test in the pin_pages code path, we're changing a group from
> non-pinned-page-scope to pinned-page-scope, therefore the iommu scope
> cannot initially be scope limited.  In the attach_group call path,
> we're moving that test from the caller, so at best we've introduced an
> additional function call.
> 
> The function as it exists today is also more versatile whereas the
> "promote" version here forces it to a single task with no appreciable
> difference in complexity or code.  This seems like a frivolous change.
> Thanks,
OK, I will adapt your idea that maintenance a counter of non-pinned groups.
Then we keep the "update" semantic, and the target is the counter ;-).

Thanks,
Keqian

> 
> Alex
> 
>> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
>> ---
>>  drivers/vfio/vfio_iommu_type1.c | 30 ++++++++++++++----------------
>>  1 file changed, 14 insertions(+), 16 deletions(-)
>>
>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>> index 0b4dedaa9128..334a8240e1da 100644
>> --- a/drivers/vfio/vfio_iommu_type1.c
>> +++ b/drivers/vfio/vfio_iommu_type1.c
>> @@ -148,7 +148,7 @@ static int put_pfn(unsigned long pfn, int prot);
>>  static struct vfio_group *vfio_iommu_find_iommu_group(struct vfio_iommu *iommu,
>>  					       struct iommu_group *iommu_group);
>>  
>> -static void update_pinned_page_dirty_scope(struct vfio_iommu *iommu);
>> +static void promote_pinned_page_dirty_scope(struct vfio_iommu *iommu);
>>  /*
>>   * This code handles mapping and unmapping of user data buffers
>>   * into DMA'ble space using the IOMMU
>> @@ -714,7 +714,7 @@ static int vfio_iommu_type1_pin_pages(void *iommu_data,
>>  	group = vfio_iommu_find_iommu_group(iommu, iommu_group);
>>  	if (!group->pinned_page_dirty_scope) {
>>  		group->pinned_page_dirty_scope = true;
>> -		update_pinned_page_dirty_scope(iommu);
>> +		promote_pinned_page_dirty_scope(iommu);
>>  	}
>>  
>>  	goto pin_done;
>> @@ -1622,27 +1622,26 @@ static struct vfio_group *vfio_iommu_find_iommu_group(struct vfio_iommu *iommu,
>>  	return group;
>>  }
>>  
>> -static void update_pinned_page_dirty_scope(struct vfio_iommu *iommu)
>> +static void promote_pinned_page_dirty_scope(struct vfio_iommu *iommu)
>>  {
>>  	struct vfio_domain *domain;
>>  	struct vfio_group *group;
>>  
>> +	if (iommu->pinned_page_dirty_scope)
>> +		return;
>> +
>>  	list_for_each_entry(domain, &iommu->domain_list, next) {
>>  		list_for_each_entry(group, &domain->group_list, next) {
>> -			if (!group->pinned_page_dirty_scope) {
>> -				iommu->pinned_page_dirty_scope = false;
>> +			if (!group->pinned_page_dirty_scope)
>>  				return;
>> -			}
>>  		}
>>  	}
>>  
>>  	if (iommu->external_domain) {
>>  		domain = iommu->external_domain;
>>  		list_for_each_entry(group, &domain->group_list, next) {
>> -			if (!group->pinned_page_dirty_scope) {
>> -				iommu->pinned_page_dirty_scope = false;
>> +			if (!group->pinned_page_dirty_scope)
>>  				return;
>> -			}
>>  		}
>>  	}
>>  
>> @@ -2057,8 +2056,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data,
>>  			 * addition of a dirty tracking group.
>>  			 */
>>  			group->pinned_page_dirty_scope = true;
>> -			if (!iommu->pinned_page_dirty_scope)
>> -				update_pinned_page_dirty_scope(iommu);
>> +			promote_pinned_page_dirty_scope(iommu);
>>  			mutex_unlock(&iommu->lock);
>>  
>>  			return 0;
>> @@ -2341,7 +2339,7 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>  	struct vfio_iommu *iommu = iommu_data;
>>  	struct vfio_domain *domain;
>>  	struct vfio_group *group;
>> -	bool update_dirty_scope = false;
>> +	bool promote_dirty_scope = false;
>>  	LIST_HEAD(iova_copy);
>>  
>>  	mutex_lock(&iommu->lock);
>> @@ -2349,7 +2347,7 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>  	if (iommu->external_domain) {
>>  		group = find_iommu_group(iommu->external_domain, iommu_group);
>>  		if (group) {
>> -			update_dirty_scope = !group->pinned_page_dirty_scope;
>> +			promote_dirty_scope = !group->pinned_page_dirty_scope;
>>  			list_del(&group->next);
>>  			kfree(group);
>>  
>> @@ -2379,7 +2377,7 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>  			continue;
>>  
>>  		vfio_iommu_detach_group(domain, group);
>> -		update_dirty_scope = !group->pinned_page_dirty_scope;
>> +		promote_dirty_scope = !group->pinned_page_dirty_scope;
>>  		list_del(&group->next);
>>  		kfree(group);
>>  		/*
>> @@ -2415,8 +2413,8 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>  	 * Removal of a group without dirty tracking may allow the iommu scope
>>  	 * to be promoted.
>>  	 */
>> -	if (update_dirty_scope)
>> -		update_pinned_page_dirty_scope(iommu);
>> +	if (promote_dirty_scope)
>> +		promote_pinned_page_dirty_scope(iommu);
>>  	mutex_unlock(&iommu->lock);
>>  }
>>  
> 
> .
>