Re: [RFC PATCH 0/2] iommu: Avoid unnecessary PRI queue flushes

[Jean-Philippe Brucker]

On Mon, Oct 19, 2020 at 11:33:16AM -0700, Jacob Pan wrote:
> Hi Jean-Philippe,
> 
> On Mon, 19 Oct 2020 16:08:24 +0200, Jean-Philippe Brucker
> <jean-philippe@xxxxxxxxxx> wrote:
> 
> > On Sat, Oct 17, 2020 at 04:25:25AM -0700, Raj, Ashok wrote:
> > > > For devices that *don't* use a stop marker, the PCIe spec says
> > > > (10.4.1.2):
> > > > 
> > > >   To stop [using a PASID] without using a Stop Marker Message, the
> > > >   function shall:
> > > >   1. Stop queueing new Page Request Messages for this PASID.  
> > > 
> > > The device driver would need to tell stop sending any new PR's.
> > >   
> > > >   2. Finish transmitting any multi-page Page Request Messages for this
> > > >      PASID (i.e. send the Page Request Message with the L bit Set).
> > > >   3. Wait for PRG Response Messages associated any outstanding Page
> > > >      Request Messages for the PASID.
> > > > 
> > > > So they have to flush their PR themselves. And since the device driver
> > > > completes this sequence before calling unbind(), then there shouldn't
> > > > be any oustanding PR for the PASID, and unbind() doesn't need to
> > > > flush, right?  
> > > 
> > > I can see how the device can complete #2,3 above. But the device driver
> > > isn't the one managing page-responses right. So in order for the device
> > > to know the above sequence is complete, it would need to get some
> > > assist from IOMMU driver?  
> > 
> > No the device driver just waits for the device to indicate that it has
> > completed the sequence. That's what the magic stop-PASID mechanism
> > described by PCIe does. In 6.20.1 "Managing PASID TLP Prefix Usage" it
> > says:
> > 
> > "A Function must have a mechanism to request that it gracefully stop using
> >  a specific PASID. This mechanism is device specific but must satisfy the
> >  following rules:
> >  [...]
> >  * When the stop request mechanism indicates completion, the Function has:
> >    [...]
> >    * Complied with additional rules described in Address Translation
> >      Services (Chapter 10 [10.4.1.2 quoted above]) if Address Translations
> >      or Page Requests were issued on the behalf of this PASID."
> > 
> > So after the device driver initiates this mechanism in the device, the
> > device must be able to indicate completion of the mechanism, which
> > includes completing all in-flight Page Requests. At that point the device
> > driver can call unbind() knowing there is no pending PR for this PASID.
> > 
> In step #3, I think it is possible that device driver received page response
> as part of the auto page response, so it may not guarantee all the in-flight
> PRQs are completed inside IOMMU. Therefore, drain is _always_ needed to be
> sure?

So I don't think that's a problem, because non-last PR are not handled by
IOPF until the last PR is received. And when the IOMMU driver detects that
the queue has been overflowing and could have auto-responded to last PR,
we discard any pending non-last PR. I couldn't find a leak here.

Thanks,
Jean