Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

[Jason Gunthorpe]

On Wed, Feb 15, 2023 at 01:51:14PM +0800, Baolu Lu wrote:
> On 2/13/23 10:19 PM, Jason Gunthorpe wrote:
> > On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
> > > The iommu_group_store_type() requires the devices in the iommu group are
> > > not bound to any device driver during the whole operation. The existing
> > > code locks the device with device_lock(dev) and use device_is_bound() to
> > > check whether any driver is bound to device.
> > > 
> > > In fact, this can be achieved through the DMA ownership helpers. Replace
> > > them with iommu_group_claim/release_dma_owner() helpers.
> > > 
> > > Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
> > > ---
> > >   drivers/iommu/iommu.c | 27 +++++++++++++--------------
> > >   1 file changed, 13 insertions(+), 14 deletions(-)
> > > 
> > > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > > index 4f71dcd2621b..6547cb38480c 100644
> > > --- a/drivers/iommu/iommu.c
> > > +++ b/drivers/iommu/iommu.c
> > > @@ -2807,12 +2807,6 @@ static int iommu_change_dev_def_domain(struct iommu_group *group,
> > >   	mutex_lock(&group->mutex);
> > > -	if (group->default_domain != group->domain) {
> > > -		dev_err_ratelimited(prev_dev, "Group not assigned to default domain\n");
> > > -		ret = -EBUSY;
> > > -		goto out;
> > > -	}
> > > -
> > >   	/*
> > >   	 * iommu group wasn't locked while acquiring device lock in
> > >   	 * iommu_group_store_type(). So, make sure that the device count hasn't
> > > @@ -2971,6 +2965,7 @@ static void iommu_group_unfreeze_dev_ops(struct iommu_group *group)
> > >   static ssize_t iommu_group_store_type(struct iommu_group *group,
> > >   				      const char *buf, size_t count)
> > >   {
> > > +	bool group_owner_claimed = false;
> > >   	struct group_device *grp_dev;
> > >   	struct device *dev;
> > >   	int ret, req_type;
> > > @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct iommu_group *group,
> > >   	else
> > >   		return -EINVAL;
> > > +	if (req_type != IOMMU_DOMAIN_DMA_FQ ||
> > > +	    group->default_domain->type != IOMMU_DOMAIN_DMA) {
> > > +		ret = iommu_group_claim_dma_owner(group, (void *)buf);
> > > +		if (ret)
> > > +			return ret;
> > > +		group_owner_claimed = true;
> > > +	}
> > 
> > I don't get it, this should be done unconditionally. If we couldn't
> > take ownership then we simply can't progress.
> 
> The existing code allows the user to switch the default domain from
> strict to lazy invalidation mode. The default domain is not changed,
> hence it should be seamless and transparent to the device driver.

So make that a special case, get the group lock check if it is this
case and then just adjust it and exit, otherwise get ownership under
the group lock as discussed.
> 
> > which also means this needs to be
> > an externally version of iommu_group_claim_dma_owner()
> 
> Sorry! What does "an externally version of
> iommu_group_claim_dma_owner()" mean?
> 

Oops "externally locked"

> My understanding is that we should limit iommu_group_claim_dma_owner()
> use in the driver context. For this non-driver context, we should not
> use iommu_group_claim_dma_owner() directly, but hold the group->mutex
> and check the group->owner_cnt directly:
> 
>         mutex_lock(&group->mutex);
>         if (group->owner_cnt) {
>                 ret = -EPERM;
>                 goto unlock_out;
>         }
> 
> the group->mutex should be held until everything is done.

Yes, that would be fine as long as we can hold the group mutex
throughout

Jason