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Tsirkin" , qemu-devel@nongnu.org, Eric Auger , Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Fri, Jun 26, 2020 at 11:29 PM Peter Xu wrote: > > Hi, Eugenio, > > (CCing Eric, Yan and Michael too) > > On Fri, Jun 26, 2020 at 08:41:22AM +0200, Eugenio P=C3=A9rez 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 *noti= fier, > > return; > > } > > > > - assert(entry->iova >=3D notifier->start && entry_end <=3D notifier= ->end); > > I can understand removing the assertion should solve the issue, however i= mho > the major issue is not about this single assertion but the whole addr_mas= k > issue behind with virtio... > > 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(). > > 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 c= an 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 roundtri= ps 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 gue= st. > > I'm thinking whether we can make the IOTLB invalidation configurable by > specifying whether the backend of the notifier can handle arbitary addres= s > 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 wis= e...), > however we can allow the special backends to take adavantage of using arb= itary > (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 di= rectly > deliver whatever we've got from the upper layer to this notifier. With t= he 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 iotl= b, we > skip the complicated procedure to split a big range into small ranges tha= t are > with strict addr_mask, but directly deliver the message to the iommu noti= fier. > E.g., we can skip the loop in vtd_address_space_unmap() if the notifier i= s with > ARBITRARY flag set. > > 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 *notifi= er, > { > IOMMUNotifierFlag request_flags; > hwaddr entry_end =3D entry->iova + entry->addr_mask; > + IOMMUTLBEntry tmp =3D *entry; > > /* > * Skip the notification if the notification does not overlap > @@ -1915,7 +1916,13 @@ void memory_region_notify_one(IOMMUNotifier *notif= ier, > return; > } > > - assert(entry->iova >=3D notifier->start && entry_end <=3D notifier->= end); > + if (notifier->notifier_flags & IOMMU_NOTIFIER_ARBITRARY_MASK) { > + tmp.iova =3D MAX(tmp.iova, notifier->start); Hi! If I modify the tmp.iova, the guest will complain (in dmesg): [ 154.426828] DMAR: DRHD: handling fault status reg 2 [ 154.427700] DMAR: [DMA Read] Request device [01:00.0] fault addr ffff90d53fada000 [fault reason 04] Access beyond MGAW And will not forward packets anymore on that interface. Guests are totally ok if I only modify addr_mask. Still investigating the issue. Thanks! > + tmp.addr_mask =3D MIN(tmp.addr_mask, notifier->end); > + assert(tmp.iova <=3D tmp.addr_mask); > + } else { > + assert(entry->iova >=3D notifier->start && entry_end <=3D notifi= er->end); > + } > > if (entry->perm & IOMMU_RW) { > request_flags =3D IOMMU_NOTIFIER_MAP; > @@ -1924,7 +1931,7 @@ void memory_region_notify_one(IOMMUNotifier *notifi= er, > } > > 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 an= d even > get some further performance boosts.. Does that make sense? > > Thanks, > > -- > Peter Xu >