Re: [PATCH 4/5] vfio/iommu_type1: Carefully use unmap_unpin_all during dirty tracking

[Keqian Zhu]

On 2021/1/13 3:53, Alex Williamson wrote:
> On Tue, 12 Jan 2021 20:04:38 +0800
> Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote:
> 
>> On 2021/1/12 5:49, Alex Williamson wrote:
>>> On Thu, 7 Jan 2021 17:29:00 +0800
>>> Keqian Zhu <zhukeqian1@xxxxxxxxxx> wrote:
>>>   
>>>> If we detach group during dirty page tracking, we shouldn't remove
>>>> vfio_dma, because dirty log will lose.
>>>>
>>>> But we don't prevent unmap_unpin_all in vfio_iommu_release, because
>>>> under normal procedure, dirty tracking has been stopped.  
>>>
>>> This looks like it's creating a larger problem than it's fixing, it's
>>> not our job to maintain the dirty bitmap regardless of what the user
>>> does.  If the user detaches the last group in a container causing the
>>> mappings within that container to be deconstructed before the user has
>>> collected dirty pages, that sounds like a user error.  A container with
>>> no groups is de-privileged and therefore loses all state.  Thanks,
>>>
>>> Alex  
>>
>> Hi Alex,
>>
>> This looks good to me ;-). That's a reasonable constraint for user behavior.
>>
>> What about replacing this patch with an addition to the uapi document of
>> VFIO_GROUP_UNSET_CONTAINER? User should pay attention to this when call this
>> ioctl during dirty tracking.
> 
> Here's the current uapi comment:
> 
> /**
>  * VFIO_GROUP_UNSET_CONTAINER - _IO(VFIO_TYPE, VFIO_BASE + 5)
>  *
>  * Remove the group from the attached container.  This is the
>  * opposite of the SET_CONTAINER call and returns the group to
>  * an initial state.  All device file descriptors must be released
>  * prior to calling this interface.  When removing the last group
>  * from a container, the IOMMU will be disabled and all state lost,
>  * effectively also returning the VFIO file descriptor to an initial
>  * state.
>  * Return: 0 on success, -errno on failure.
>  * Availability: When attached to container
>  */
> 
> So we already indicate that "all state" of the container is lost when
> removing the last group, I don't see that it's necessarily to
> explicitly include dirty bitmap state beyond that statement.  Without
> mappings there can be no dirty bitmap to track.
OK :-) .

> 
>  > And any comments on other patches? thanks.
> 
> I had a difficult time mapping the commit log to the actual code
> change, I'll likely have some wording suggestions.  Is patch 5/5 still
> necessary if this patch is dropped?  Thanks,
> 
I think the 5th patch is still necessary. vfio_sanity_check_pfn_list() is used to check
whether pfn_list of vfio_dma is empty. but we apply this check just for external domain.
If the iommu backed domain also pin some pages, then this check fails. So I think we should
use this check only when all domains are about to be removed.

Besides, this patch should extract the "WARN_ON(iommu->notifier.head);" just for external domain.

Thanks,
Keqian

> Alex
> 
>>>> Fixes: d6a4c185660c ("vfio iommu: Implementation of ioctl for dirty pages tracking")
>>>> Signed-off-by: Keqian Zhu <zhukeqian1@xxxxxxxxxx>
>>>> ---
>>>>  drivers/vfio/vfio_iommu_type1.c | 14 ++++++++++++--
>>>>  1 file changed, 12 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 26b7eb2a5cfc..9776a059904d 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -2373,7 +2373,12 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>>>  			if (list_empty(&iommu->external_domain->group_list)) {
>>>>  				vfio_sanity_check_pfn_list(iommu);
>>>>  
>>>> -				if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu))
>>>> +				/*
>>>> +				 * During dirty page tracking, we can't remove
>>>> +				 * vfio_dma because dirty log will lose.
>>>> +				 */
>>>> +				if (!IS_IOMMU_CAP_DOMAIN_IN_CONTAINER(iommu) &&
>>>> +				    !iommu->dirty_page_tracking)
>>>>  					vfio_iommu_unmap_unpin_all(iommu);
>>>>  
>>>>  				kfree(iommu->external_domain);
>>>> @@ -2406,10 +2411,15 @@ static void vfio_iommu_type1_detach_group(void *iommu_data,
>>>>  		 * iommu and external domain doesn't exist, then all the
>>>>  		 * mappings go away too. If it's the last domain with iommu and
>>>>  		 * external domain exist, update accounting
>>>> +		 *
>>>> +		 * Note: During dirty page tracking, we can't remove vfio_dma
>>>> +		 * because dirty log will lose. Just update accounting is a good
>>>> +		 * choice.
>>>>  		 */
>>>>  		if (list_empty(&domain->group_list)) {
>>>>  			if (list_is_singular(&iommu->domain_list)) {
>>>> -				if (!iommu->external_domain)
>>>> +				if (!iommu->external_domain &&
>>>> +				    !iommu->dirty_page_tracking)
>>>>  					vfio_iommu_unmap_unpin_all(iommu);
>>>>  				else
>>>>  					vfio_iommu_unmap_unpin_reaccount(iommu);  
>>>
>>> .
>>>   
>>
> 
> .
>