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? > 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
