On Wed, Apr 01, 2020 at 04:38:42PM -0700, Jacob Pan wrote: > On Wed, 1 Apr 2020 16:03:01 +0200 > Jean-Philippe Brucker <jean-phili...@linaro.org> wrote: > > > Hi Jacob, > > > > On Wed, Mar 25, 2020 at 10:55:21AM -0700, Jacob Pan wrote: > > > IOASID was introduced in v5.5 as a generic kernel allocator service > > > for both PCIe Process Address Space ID (PASID) and ARM SMMU's Sub > > > Stream ID. In addition to basic ID allocation, ioasid_set was > > > introduced as a token that is shared by a group of IOASIDs. This > > > set token can be used for permission checking but lack of some > > > features needed by guest Shared Virtual Address (SVA). In addition, > > > IOASID support for life cycle management is needed among multiple > > > users. > > > > > > This patchset introduces two extensions to the IOASID code, > > > 1. IOASID set operations > > > 2. Notifications for IOASID state synchronization > > > > My main concern with this series is patch 7 changing the spinlock to a > > mutex, which prevents SVA from calling ioasid_free() from the RCU > > callback of MMU notifiers. Could we use atomic notifiers, or do the > > FREE notification another way? > > > Maybe I am looking at the wrong code, I thought > mmu_notifier_ops.free_notifier() is called outside spinlock with > call_srcu(), which will be invoked in the thread context. > in mmu_notifier.c mmu_notifier_put() > spin_unlock(&mm->notifier_subscriptions->lock); > > call_srcu(&srcu, &subscription->rcu, mmu_notifier_free_rcu);
free_notifier() is called from RCU callback, and according to Documentation/RCU/checklist.txt: 5. If call_rcu() or call_srcu() is used, the callback function will be called from softirq context. In particular, it cannot block. When applying the patch I get the sleep-in-atomic warning: [ 87.861793] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:935 [ 87.863293] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 74, name: kworker/6:1 [ 87.863993] 2 locks held by kworker/6:1/74: [ 87.864493] #0: ffffff885ac12538 ((wq_completion)rcu_gp){+.+.}-{0:0}, at: process_one_work+0x740/0x1880 [ 87.865593] #1: ffffff88591efd30 ((work_completion)(&sdp->work)){+.+.}-{0:0}, at: process_one_work+0x740/0x1880 [ 87.866993] CPU: 6 PID: 74 Comm: kworker/6:1 Not tainted 5.6.0-next-20200331+ #121 [ 87.867393] Hardware name: FVP Base (DT) [ 87.867893] Workqueue: rcu_gp srcu_invoke_callbacks [ 87.868393] Call trace: [ 87.868793] dump_backtrace+0x0/0x310 [ 87.869293] show_stack+0x14/0x20 [ 87.869693] dump_stack+0x124/0x180 [ 87.870193] ___might_sleep+0x2ac/0x428 [ 87.870693] __might_sleep+0x88/0x168 [ 87.871094] __mutex_lock+0xa0/0x1270 [ 87.871593] mutex_lock_nested+0x1c/0x28 [ 87.872093] ioasid_free+0x28/0x48 [ 87.872493] io_mm_free+0x1d0/0x608 [ 87.872993] mmu_notifier_free_rcu+0x74/0xe8 [ 87.873393] srcu_invoke_callbacks+0x1d0/0x2c8 [ 87.873893] process_one_work+0x858/0x1880 [ 87.874393] worker_thread+0x314/0xcd0 [ 87.874793] kthread+0x318/0x400 [ 87.875293] ret_from_fork+0x10/0x18 > > Anyway, if we have to use atomic. I tried atomic notifier first, there > are two subscribers to the free event on x86. > 1. IOMMU > 2. KVM > > For #1, the problem is that in the free operation, VT-d driver > needs to do a lot of clean up in thread context. > - hold a mutex to traverse a list of devices > - clear PASID entry and flush cache > > For #2, KVM might be able to deal with spinlocks for updating VMCS > PASID translation table. +Hao > > Perhaps two solutions I can think of: > 1. Use a cyclic IOASID allocator. The main reason of clean up at free > is to prevent race with IOASID alloc. Similar to PID, 2M IOASID > will take long time overflow. Then we can use atomic notifier and a > deferred workqueue to do IOMMU cleanup. The downside is a large and > growing PASID table, may not be a performance issue since it has TLB. That might be a problem for SMMU, which has 1024 * 64kB leaf PASID tables, for a total of 64MB per endpoint if there is too much fragmentation in the IOASID space. > 2. Let VFIO ensure free always happen after unbind. Then there is no > need to do cleanup. But that requires VFIO to keep track of all the > PASIDs within each VM. When the VM terminates, VFIO is responsible for > the clean up. That was Yi's original proposal. I also tried to provide > an IOASID set iterator for VFIO to free the IOASIDs within each VM/set, > but the private data belongs to IOMMU driver. Not really my place to comment on this, but I find it nicer to use the same gpasid_unbind() path when VFIO frees a PASID as when the guest explicitly unbinds before freeing. Thanks, Jean _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu