Re: [RFC v2 1/1] memory: Delete assertion in memory_region_unregister_iommu_notifier

[Jason Wang]

On 2020/6/28 下午10:47, Peter Xu wrote:
> On Sun, Jun 28, 2020 at 03:03:41PM +0800, Jason Wang wrote:
> > 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.
> Yes.  Note that I didn't mean that it's a problem in virtio, it's just the fact
> that virtio is the only one I know that would like to support arbitrary address
> range for the translated region.  I don't know about tcg, but vfio should still
> need some kind of page alignment in both the address and the addr_mask.  We
> have that assumption too across the memory core when we do translations.


Right but it looks to me the issue is not the alignment.


> A further cause of the issue is the MSI region when vIOMMU enabled - currently
> we implemented the interrupt region using another memory region so it split the
> whole DMA region into two parts.  That's really a clean approach to IR
> implementation, however that's also a burden to the invalidation part because
> then we'll need to handle things like this when the listened range is not page
> alighed at all (neither 0-0xfedffff, nor 0xfef0000-MAX).  If without the IR
> region (so the whole iommu address range will be a single FlatRange),


Is this a bug? I remember that at least for vtd, it won't do any DMAR on 
the intrrupt address range


>   I think
> we probably don't need most of the logic in vtd_address_space_unmap() at all,
> then we can directly deliver all the IOTLB invalidations without splitting into
> small page aligned ranges to all the iommu notifiers.  Sadly, so far I still
> don't have ideal solution for it, because we definitely need IR.


Another possible (theoretical) issue (for vhost) is that it can't 
trigger interrupt through the interrupt range.


> > > 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.
> IMHO it's not an IOMMU capability.  The flag I wanted to introduce is a
> capability of the one who listens to the IOMMU TLB updates.  For our case, it's
> virtio/vhost's capability to allow arbitrary length. The IOMMU itself
> definitely has some limitation on the address range to be bound to an IOTLB
> invalidation, e.g., the device-iotlb we're talking here only accept both the
> iova address and addr_mask to be aligned to 2**N-1.


I think this go back to one of our previous discussion of whether to 
introduce a dedicated notifiers for device IOTLB.

For IOMMU, it might have limitation like GAW, but for device IOTLB it 
probably doesn't. That's the reason we hit the assert here.


> > 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)?
> IOMMUTLBEntry itself is the abstraction layer of TLB entry.  Hardware TLB entry
> is definitely not arbitrary range either (because AFAICT the hardware should
> only cache PFN rather than address, so at least PAGE_SIZE aligned).
> Introducing this flag will already make this trickier just to avoid introducing
> another similar struct to IOMMUTLBEntry, but I really don't want to make it a
> default option...  Not to mention I probably have no reason to urge the rest
> iommu notifier users (tcg, vfio) to change their existing good code to suite
> any of the backend who can cooperate with arbitrary address ranges...


Ok, so it looks like we need a dedicated notifiers to device IOTLB.

Thanks


> Thanks,