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

[Jacob Pan]

Hi Jean-Philippe,

On Mon, 19 Oct 2020 16:08:24 +0200, Jean-Philippe Brucker
<jean-philippe at linaro.org> 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?

> Thanks,
> Jean
> 
> > 
> > How does the driver know that everything host received has been
> > responded back to device?
> >   
> > >   
> > > > I'm not sure about other IOMMU's how they behave, When there is no
> > > > space in the PRQ, IOMMU auto-responds to the device. This puts the
> > > > device in a while (1) loop. The fake successful response will let
> > > > the device do a ATS lookup, and that would fail forcing the device
> > > > to do another PRQ.  
> > > 
> > > But in the sequence above, step 1 should ensure that the device will
> > > not send another PR for any successful response coming back at step
> > > 3.  
> > 
> > True, but there could be some page-request in flight on its way to the
> > IOMMU. By draining and getting that round trip back to IOMMU we
> > gaurantee things in flight are flushed to PRQ after that Drain
> > completes.  
> > > 
> > > So I agree with the below if we suspect there could be pending PR, but
> > > given that pending PR are a stop marker thing and we don't know any
> > > device using stop markers, I wondered why I bothered implementing
> > > PRIq flush at all for SMMUv3, hence this RFC.
> > >   
> > 
> > Cheers,
> > Ashok  


Thanks,

Jacob