Re: [PATCH v3 4/4] vfio/type1: Use iommu_aux_at(de)tach_group() APIs

Lu Baolu
2020-07-31T01:42:57Z

Hi Alex,

On 7/31/20 5:17 AM, Alex Williamson wrote:
> On Thu, 30 Jul 2020 10:41:32 +0800
> Lu Baolu <baolu.lu@linux.intel.com> wrote:
>
>> Hi Alex,
>>
>> On 7/30/20 4:32 AM, Alex Williamson wrote:
>>> On Tue, 14 Jul 2020 13:57:03 +0800
>>> Lu Baolu <baolu.lu@linux.intel.com> wrote:
>>>
>>>> Replace iommu_aux_at(de)tach_device() with iommu_aux_at(de)tach_group().
>>>> It also saves the IOMMU_DEV_FEAT_AUX-capable physcail device in the
>>>> vfio_group data structure so that it could be reused in other places.
>>>>
>>>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>>>> ---
>>>> drivers/vfio/vfio_iommu_type1.c | 44 ++++++---------------------------
>>>> 1 file changed, 7 insertions(+), 37 deletions(-)
>>>>
>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>> index 5e556ac9102a..f8812e68de77 100644
>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>> @@ -100,6 +100,7 @@ struct vfio_dma {
>>>> struct vfio_group {
>>>> struct iommu_group *iommu_group;
>>>> struct list_head next;
>>>> + struct device *iommu_device;
>>>> bool mdev_group; /* An mdev group */
>>>> bool pinned_page_dirty_scope;
>>>> };
>>>> @@ -1627,45 +1628,13 @@ static struct device *vfio_mdev_get_iommu_device(struct device *dev)
>>>> return NULL;
>>>> }
>>>>
>>>> -static int vfio_mdev_attach_domain(struct device *dev, void *data)
>>>> -{
>>>> - struct iommu_domain *domain = data;
>>>> - struct device *iommu_device;
>>>> -
>>>> - iommu_device = vfio_mdev_get_iommu_device(dev);
>>>> - if (iommu_device) {
>>>> - if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
>>>> - return iommu_aux_attach_device(domain, iommu_device);
>>>> - else
>>>> - return iommu_attach_device(domain, iommu_device);
>>>> - }
>>>> -
>>>> - return -EINVAL;
>>>> -}
>>>> -
>>>> -static int vfio_mdev_detach_domain(struct device *dev, void *data)
>>>> -{
>>>> - struct iommu_domain *domain = data;
>>>> - struct device *iommu_device;
>>>> -
>>>> - iommu_device = vfio_mdev_get_iommu_device(dev);
>>>> - if (iommu_device) {
>>>> - if (iommu_dev_feature_enabled(iommu_device, IOMMU_DEV_FEAT_AUX))
>>>> - iommu_aux_detach_device(domain, iommu_device);
>>>> - else
>>>> - iommu_detach_device(domain, iommu_device);
>>>> - }
>>>> -
>>>> - return 0;
>>>> -}
>>>> -
>>>> static int vfio_iommu_attach_group(struct vfio_domain *domain,
>>>> struct vfio_group *group)
>>>> {
>>>> if (group->mdev_group)
>>>> - return iommu_group_for_each_dev(group->iommu_group,
>>>> - domain->domain,
>>>> - vfio_mdev_attach_domain);
>>>> + return iommu_aux_attach_group(domain->domain,
>>>> + group->iommu_group,
>>>> + group->iommu_device);
>>>
>>> No, we previously iterated all devices in the group and used the aux
>>> interface only when we have an iommu_device supporting aux. If we
>>> simply assume an mdev group only uses an aux domain we break existing
>>> users, ex. SR-IOV VF backed mdevs. Thanks,
>>
>> Oh, yes. Sorry! I didn't consider the physical device backed mdevs
>> cases.
>>
>> Looked into this part of code, it seems that there's a lock issue here.
>> The group->mutex is held in iommu_group_for_each_dev() and will be
>> acquired again in iommu_attach_device().
>
> These are two different groups. We walk the devices in the mdev's
> group with iommu_group_for_each_dev(), holding the mdev's group lock,
> but we call iommu_attach_device() with iommu_device, which results in
> acquiring the lock for the iommu_device's group.

You are right. Sorry for the noise. Please ignore it.

>
>> How about making it like:
>>
>> static int vfio_iommu_attach_group(struct vfio_domain *domain,
>> struct vfio_group *group)
>> {
>> if (group->mdev_group) {
>> struct device *iommu_device = group->iommu_device;
>>
>> if (WARN_ON(!iommu_device))
>> return -EINVAL;
>>
>> if (iommu_dev_feature_enabled(iommu_device,
>> IOMMU_DEV_FEAT_AUX))
>> return iommu_aux_attach_device(domain->domain,
>> iommu_device);
>> else
>> return iommu_attach_device(domain->domain,
>> iommu_device);
>> } else {
>> return iommu_attach_group(domain->domain,
>> group->iommu_group);
>> }
>> }
>>
>> The caller (vfio_iommu_type1_attach_group) has guaranteed that all mdevs
>> in an iommu group should be derived from a same physical device.
>
> Have we?

We have done this with below.

static int vfio_mdev_iommu_device(struct device *dev, void *data)
{
struct device **old = data, *new;

new = vfio_mdev_get_iommu_device(dev);
if (!new || (*old && *old != new))
return -EINVAL;

*old = new;

return 0;
}

But I agree that as a generic iommu aux-domain api, we shouldn't put
this limited assumption in it.

> iommu_attach_device() will fail if the group is not
> singleton, but that's just encouraging us to use the _attach_group()
> interface where the _attach_device() interface is relegated to special
> cases. Ideally we'd get out of those special cases and create an
> _attach_group() for aux that doesn't further promote these notions.

Yes. Fair enough.

>
>> Any thoughts?
>
> See my reply to Kevin, I'm thinking we need to provide a callback that
> can enlighten the IOMMU layer to be able to do _attach_group() with
> aux or separate IOMMU backed devices.

Thanks for the guide. I will check your reply.

Best regards,
baolu