Re: [PATCH] intel-iommu: Document iova_tree

[Peter Xu]

On Thu, Dec 01, 2022 at 02:22:27PM -0500, Peter Xu wrote:
> On Thu, Dec 01, 2022 at 07:17:41PM +0100, Eric Auger wrote:
> > Hi Peter
> 
> Hi, Eric,
> 
> > 
> > On 12/1/22 17:25, Peter Xu wrote:
> > > It seems not super clear on when iova_tree is used, and why.  Add a rich
> > > comment above iova_tree to track why we needed the iova_tree, and when we
> > > need it.
> > >
> > > Suggested-by: Jason Wang <jasowang@redhat.com>
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > >  include/hw/i386/intel_iommu.h | 30 +++++++++++++++++++++++++++++-
> > >  1 file changed, 29 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/hw/i386/intel_iommu.h b/include/hw/i386/intel_iommu.h
> > > index 46d973e629..8d130ab2e3 100644
> > > --- a/include/hw/i386/intel_iommu.h
> > > +++ b/include/hw/i386/intel_iommu.h
> > > @@ -109,7 +109,35 @@ struct VTDAddressSpace {
> > >      QLIST_ENTRY(VTDAddressSpace) next;
> > >      /* Superset of notifier flags that this address space has */
> > >      IOMMUNotifierFlag notifier_flags;
> > > -    IOVATree *iova_tree;          /* Traces mapped IOVA ranges */
> > > +    /*
> > > +     * @iova_tree traces mapped IOVA ranges.
> > > +     *
> > > +     * The tree is not needed if no MAP notifiers is registered with
> > 
> > s/no MAP notifiers/no MAP notifier
> 
> Will fix.
> 
> > > +     * current VTD address space, because all UNMAP (including iotlb or
> > > +     * dev-iotlb) events can be transparently delivered to !MAP iommu
> > > +     * notifiers.
> > because all UNMAP notifications (iotlb or dev-iotlb) can be triggered
> > directly, as opposed to MAP notifications. (?)
> 
> What I wanted to say is any PSI or DSI messages we got from the guest can
> be transparently delivered to QEMU's iommu notifiers.  I'm not sure
> "triggered directly" best describe the case here.
> 
> PSI: Page Selective Invalidations
> DSI: Domain Selective Invalidations
> 
> Sorry to mention these terms again, but that's really what the "transparent
> delivery" means here - we get the PSI/DSI messages, then we notify with the
> same ranges in IOMMU notifiers.  They're not the same concept but we do
> that transparently without changing the core of the messages.
> 
> Maybe I should spell out "!MAP" as "UNMAP-only"?  Would that help?
> 
> > > +     *
> > > +     * The tree OTOH is required for MAP typed iommu notifiers for a few
> > > +     * reasons.
> > > +     *
> > > +     * Firstly, there's no way to identify whether an PSI event is MAP or
> > maybe give the decryption of the 'PSI' and 'DSI" acronyms once ;-)
> 
> Please see above. :)
> 
> These are VT-d terms used in multiple places in the .[ch] files, I assume
> I'll just keep using them because otherwise I'll need to comment them
> everytime we use any PSI/DSI terms.  It might become an overkill I'm afraid.
> 
> > > +     * UNMAP within the PSI message itself.  Without having prior 
> > > knowledge
> > > +     * of existing state vIOMMU doesn't know whether it should notify MAP
> > > +     * or UNMAP for a PSI message it received.
> > > +     *
> > > +     * Secondly, PSI received from guest driver (or even a large PSI can
> > > +     * grow into a DSI at least with Linux intel-iommu driver) can be
> > > +     * larger in range than the newly mapped ranges for either MAP or 
> > > UNMAP
> > > +     * events. If it directly pass-throughs any such event it may confuse
> > 
> > If it directly notifies the registered device with the unmodified range, it 
> > may confuse the drivers ../..
> 
> Will fix.
> 
> > 
> > So the range of the MAP notification can be adapted based on the existing 
> > IOVA mappings.  
> 
> Yes, e.g. the iova tree makes sure we don't map something again if it's 
> mapped.

I figured maybe simpler I just attach a v2..

Thanks,

-- 
Peter Xu

0001-intel-iommu-Document-iova_tree.patch
Description: Text document