Re: [Qemu-devel] [PATCH v6 0/3] IOMMU: intel_iommu support map and unmap notifications

[Michael S. Tsirkin]

On Wed, Nov 16, 2016 at 01:03:14PM -0700, Alex Williamson wrote:
> On Wed, 16 Nov 2016 21:50:46 +0200
> "Aviv B.D." <address@hidden> wrote:
> 
> > On Wed, Nov 16, 2016 at 5:34 PM, Alex Williamson <address@hidden
> > > wrote:  
> > 
> > > On Wed, 16 Nov 2016 15:54:56 +0200
> > > "Michael S. Tsirkin" <address@hidden> wrote:
> > >  
> > > > On Thu, Nov 10, 2016 at 12:44:47PM -0700, Alex Williamson wrote:  
> > > > > On Thu, 10 Nov 2016 21:20:36 +0200
> > > > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > > >  
> > > > > > On Thu, Nov 10, 2016 at 09:04:13AM -0700, Alex Williamson wrote:  
> > > > > > > On Thu, 10 Nov 2016 17:54:35 +0200
> > > > > > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > > > > >  
> > > > > > > > On Thu, Nov 10, 2016 at 08:30:21AM -0700, Alex Williamson 
> > > > > > > > wrote:  
> > > > > > > > > On Thu, 10 Nov 2016 17:14:24 +0200
> > > > > > > > > "Michael S. Tsirkin" <address@hidden> wrote:
> > > > > > > > >  
> > > > > > > > > > On Tue, Nov 08, 2016 at 01:04:21PM +0200, Aviv B.D wrote:  
> > > > > > > > > > > From: "Aviv Ben-David" <address@hidden>
> > > > > > > > > > >
> > > > > > > > > > > * Advertize Cache Mode capability in iommu cap register.
> > > > > > > > > > >   This capability is controlled by "cache-mode" property  
> > > of intel-iommu device.  
> > > > > > > > > > >   To enable this option call QEMU with "-device  
> > > intel-iommu,cache-mode=true".  
> > > > > > > > > > >
> > > > > > > > > > > * On page cache invalidation in intel vIOMMU, check if 
> > > > > > > > > > > the  
> > > domain belong to  
> > > > > > > > > > >   registered notifier, and notify accordingly.  
> > > > > > > > > >
> > > > > > > > > > This looks sane I think. Alex, care to comment?
> > > > > > > > > > Merging will have to wait until after the release.
> > > > > > > > > > Pls remember to re-test and re-ping then.  
> > > > > > > > >
> > > > > > > > > I don't think it's suitable for upstream until there's a  
> > > reasonable  
> > > > > > > > > replay mechanism  
> > > > > > > >
> > > > > > > > Could you pls clarify what do you mean by replay?
> > > > > > > > Is this when you attach a device by hotplug to
> > > > > > > > a running system?
> > > > > > > >
> > > > > > > > If yes this can maybe be addressed by disabling hotplug  
> > > temporarily.  
> > > > > > >
> > > > > > > No, hotplug is not required, moving a device between existing  
> > > domains  
> > > > > > > requires replay, ie. actually using it for nested device  
> > > assignment.  
> > > > > >
> > > > > > Good point, that one is a correctness thing. Aviv,
> > > > > > could you add this in TODO list in a cover letter pls?
> > > > > >  
> > > > > > > > > and we straighten out whether it's expected to get
> > > > > > > > > multiple notifies and the notif-ee is responsible for 
> > > > > > > > > filtering
> > > > > > > > > them or if the notif-er should do filtering.  
> > > > > > > >
> > > > > > > > OK this is a documentation thing.  
> > > > > > >
> > > > > > > Well no, it needs to be decided and if necessary implemented.  
> > > > > >
> > > > > > Let's assume it's the notif-ee for now. Less is more and all that.  
> > > > >
> > > > > I think this is opposite of the approach dwg suggested.
> > > > >  
> > > > > > > > >  Without those, this is
> > > > > > > > > effectively just an RFC.  
> > > > > > > >
> > > > > > > > It's infrastructure without users so it doesn't break things,
> > > > > > > > I'm more interested in seeing whether it's broken in
> > > > > > > > some way than whether it's complete.  
> > > > > > >
> > > > > > > If it allows use with vfio but doesn't fully implement the  
> > > complete set  
> > > > > > > of interfaces, it does break things.  We currently prevent viommu 
> > > > > > >  
> > > usage  
> > > > > > > with vfio because it is incomplete.  
> > > > > >
> > > > > > Right - that bit is still in as far as I can see.  
> > > > >
> > > > > Nope, 3/3 changes vtd_iommu_notify_flag_changed() to allow use with
> > > > > vfio even though it's still incomplete.  We would at least need
> > > > > something like a replay callback for VT-d that triggers an abort if 
> > > > > you
> > > > > still want to accept it incomplete.  Thanks,
> > > > >
> > > > > Alex  
> > > >
> > > > IIUC practically things seems to work, right?  
> > >
> > > AFAIK, no.
> > >  
> > > > So how about disabling by default with a flag for people that want to
> > > > experiment with it?
> > > > E.g. x-vfio-allow-broken-translations ?  
> > >
> > > We've already been through one round of "intel-iommu is incomplete for
> > > use with device assignment, how can we prevent it from being used",
> > > which led to the notify_flag_changed callback on MemoryRegionIOMMUOps.
> > > This series now claims to fix that yet still doesn't provide a
> > > mechanism to do memory_region_iommu_replay() given that VT-d has a much
> > > larger address width.  Why is the onus on vfio to resolve this or
> > > provide some sort of workaround?  vfio is using the QEMU iommu
> > > interface correctly, intel-iommu is still incomplete. The least it
> > > could do is add an optional replay callback to MemoryRegionIOMMUOps
> > > that supersedes the existing memory_region_iommu_replay() code and
> > > triggers an abort when it gets called.  I don't know what an
> > > x-vfio-allow-broken-translations option would do, how I'd implement it,
> > > or why I'd bother to implement it.  Thanks,
> > >
> > > Alex
> > >  
> > 
> > I'll implement your suggestion regarding the replay framwork.
> > Probably in another patch set, if it is OK?
> 
> I think it needs to be committed in the same series that enables the
> notifier_flag_changed callback, otherwise we're opening the door to
> make use of vfio devices when we know that support is incomplete.  QEMU
> is in hard freeze anyway, this has missed 2.8.  I would absolutely
> object to including this in 2.8 without the replay provision.  Thanks,
> 
> Alex

I don't think this is 2.8 material but I'd like to start merging
this support first thing afterwards.

-- 
MST