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Tsirkin" , Juan Quintela , qemu-devel@nongnu.org, Eric Auger , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020/6/27 上午5:29, Peter Xu wrote: > Hi, Eugenio, > > (CCing Eric, Yan and Michael too) > > On Fri, Jun 26, 2020 at 08:41:22AM +0200, Eugenio Pérez wrote: >> diff --git a/memory.c b/memory.c >> index 2f15a4b250..7f789710d2 100644 >> --- a/memory.c >> +++ b/memory.c >> @@ -1915,8 +1915,6 @@ void memory_region_notify_one(IOMMUNotifier *notifier, >> return; >> } >> >> - assert(entry->iova >= notifier->start && entry_end <= notifier->end); > I can understand removing the assertion should solve the issue, however imho > the major issue is not about this single assertion but the whole addr_mask > issue behind with virtio... I don't get here, it looks to the the range was from guest IOMMU drivers. > > For normal IOTLB invalidations, we were trying our best to always make > IOMMUTLBEntry contain a valid addr_mask to be 2**N-1. E.g., that's what we're > doing with the loop in vtd_address_space_unmap(). I'm sure such such assumption can work for any type of IOMMU. > > But this is not the first time that we may want to break this assumption for > virtio so that we make the IOTLB a tuple of (start, len), then that len can be > not a address mask any more. That seems to be more efficient for things like > vhost because iotlbs there are not page based, so it'll be inefficient if we > always guarantee the addr_mask because it'll be quite a lot more roundtrips of > the same range of invalidation. Here we've encountered another issue of > triggering the assertion with virtio-net, but only with the old RHEL7 guest. > > I'm thinking whether we can make the IOTLB invalidation configurable by > specifying whether the backend of the notifier can handle arbitary address > range in some way. So we still have the guaranteed addr_masks by default > (since I still don't think totally break the addr_mask restriction is wise...), > however we can allow the special backends to take adavantage of using arbitary > (start, len) ranges for reasons like performance. > > To do that, a quick idea is to introduce a flag IOMMU_NOTIFIER_ARBITRARY_MASK > to IOMMUNotifierFlag, to declare that the iommu notifier (and its backend) can > take arbitrary address mask, then it can be any value and finally becomes a > length rather than an addr_mask. Then for every iommu notify() we can directly > deliver whatever we've got from the upper layer to this notifier. With the new > flag, vhost can do iommu_notifier_init() with UNMAP|ARBITRARY_MASK so it > declares this capability. Then no matter for device iotlb or normal iotlb, we > skip the complicated procedure to split a big range into small ranges that are > with strict addr_mask, but directly deliver the message to the iommu notifier. > E.g., we can skip the loop in vtd_address_space_unmap() if the notifier is with > ARBITRARY flag set. I'm not sure coupling IOMMU capability to notifier is the best choice. How about just convert to use a range [start, end] for any notifier and move the checks (e.g the assert) into the actual notifier implemented (vhost or vfio)? Thanks > > Then, the assert() is not accurate either, and may become something like: > > diff --git a/memory.c b/memory.c > index 2f15a4b250..99d0492509 100644 > --- a/memory.c > +++ b/memory.c > @@ -1906,6 +1906,7 @@ void memory_region_notify_one(IOMMUNotifier *notifier, > { > IOMMUNotifierFlag request_flags; > hwaddr entry_end = entry->iova + entry->addr_mask; > + IOMMUTLBEntry tmp = *entry; > > /* > * Skip the notification if the notification does not overlap > @@ -1915,7 +1916,13 @@ void memory_region_notify_one(IOMMUNotifier *notifier, > return; > } > > - assert(entry->iova >= notifier->start && entry_end <= notifier->end); > + if (notifier->notifier_flags & IOMMU_NOTIFIER_ARBITRARY_MASK) { > + tmp.iova = MAX(tmp.iova, notifier->start); > + tmp.addr_mask = MIN(tmp.addr_mask, notifier->end); > + assert(tmp.iova <= tmp.addr_mask); > + } else { > + assert(entry->iova >= notifier->start && entry_end <= notifier->end); > + } > > if (entry->perm & IOMMU_RW) { > request_flags = IOMMU_NOTIFIER_MAP; > @@ -1924,7 +1931,7 @@ void memory_region_notify_one(IOMMUNotifier *notifier, > } > > if (notifier->notifier_flags & request_flags) { > - notifier->notify(notifier, entry); > + notifier->notify(notifier, &tmp); > } > } > > Then we can keep the assert() for e.g. vfio, however vhost can skip it and even > get some further performance boosts.. Does that make sense? > > Thanks, >