> From: Alex Williamson [mailto:alex.william...@redhat.com] > Sent: Wednesday, September 25, 2019 2:03 AM > > On Tue, 24 Sep 2019 02:19:15 +0000 > "Tian, Kevin" <kevin.t...@intel.com> wrote: > > > > From: Tian, Kevin > > > Sent: Friday, September 13, 2019 7:00 AM > > > > > > > From: Alex Williamson [mailto:alex.william...@redhat.com] > > > > Sent: Thursday, September 12, 2019 10:41 PM > > > > > > > > On Tue, 3 Sep 2019 06:57:27 +0000 > > > > "Tian, Kevin" <kevin.t...@intel.com> wrote: > > > > > > > > > > From: Alex Williamson [mailto:alex.william...@redhat.com] > > > > > > Sent: Saturday, August 31, 2019 12:33 AM > > > > > > > > > > > > On Fri, 30 Aug 2019 08:06:32 +0000 > > > > > > "Tian, Kevin" <kevin.t...@intel.com> wrote: > > > > > > > > > > > > > > From: Tian, Kevin > > > > > > > > Sent: Friday, August 30, 2019 3:26 PM > > > > > > > > > > > > > > > [...] > > > > > > > > > How does QEMU handle the fact that IOVAs are potentially > > > > dynamic > > > > > > while > > > > > > > > > performing the live portion of a migration? For example, > each > > > > time a > > > > > > > > > guest driver calls dma_map_page() or dma_unmap_page(), a > > > > > > > > > MemoryRegionSection pops in or out of the AddressSpace for > > > the > > > > device > > > > > > > > > (I'm assuming a vIOMMU where the device AddressSpace is > not > > > > > > > > > system_memory). I don't see any QEMU code that intercepts > > > that > > > > > > change > > > > > > > > > in the AddressSpace such that the IOVA dirty pfns could be > > > > recorded and > > > > > > > > > translated to GFNs. The vendor driver can't track these > beyond > > > > getting > > > > > > > > > an unmap notification since it only knows the IOVA pfns, > which > > > > can be > > > > > > > > > re-used with different GFN backing. Once the DMA mapping > is > > > > torn > > > > > > down, > > > > > > > > > it seems those dirty pfns are lost in the ether. If this > > > > > > > > > works in > > > > QEMU, > > > > > > > > > please help me find the code that handles it. > > > > > > > > > > > > > > > > I'm curious about this part too. Interestingly, I didn't find > > > > > > > > any > > > > log_sync > > > > > > > > callback registered by emulated devices in Qemu. Looks dirty > > > pages > > > > > > > > by emulated DMAs are recorded in some implicit way. But KVM > > > > always > > > > > > > > reports dirty page in GFN instead of IOVA, regardless of the > > > > presence of > > > > > > > > vIOMMU. If Qemu also tracks dirty pages in GFN for emulated > > > DMAs > > > > > > > > (translation can be done when DMA happens), then we don't > > > need > > > > > > > > worry about transient mapping from IOVA to GFN. Along this > way > > > > we > > > > > > > > also want GFN-based dirty bitmap being reported through VFIO, > > > > > > > > similar to what KVM does. For vendor drivers, it needs to > translate > > > > > > > > from IOVA to HVA to GFN when tracking DMA activities on > VFIO > > > > > > > > devices. IOVA->HVA is provided by VFIO. for HVA->GFN, it can > be > > > > > > > > provided by KVM but I'm not sure whether it's exposed now. > > > > > > > > > > > > > > > > > > > > > > HVA->GFN can be done through hva_to_gfn_memslot in > kvm_host.h. > > > > > > > > > > > > I thought it was bad enough that we have vendor drivers that > depend > > > > on > > > > > > KVM, but designing a vfio interface that only supports a KVM > interface > > > > > > is more undesirable. I also note without comment that > > > > gfn_to_memslot() > > > > > > is a GPL symbol. Thanks, > > > > > > > > > > yes it is bad, but sometimes inevitable. If you recall our discussions > > > > > back to 3yrs (when discussing the 1st mdev framework), there were > > > > similar > > > > > hypervisor dependencies in GVT-g, e.g. querying gpa->hpa when > > > > > creating some shadow structures. gpa->hpa is definitely hypervisor > > > > > specific knowledge, which is easy in KVM (gpa->hva->hpa), but > needs > > > > > hypercall in Xen. but VFIO already makes assumption based on > KVM- > > > > > only flavor when implementing vfio_{un}pin_page_external. > > > > > > > > Where's the KVM assumption there? The MAP_DMA ioctl takes an > IOVA > > > > and > > > > HVA. When an mdev vendor driver calls vfio_pin_pages(), we GUP the > > > HVA > > > > to get an HPA and provide an array of HPA pfns back to the caller. The > > > > other vGPU mdev vendor manages to make use of this without KVM... > the > > > > KVM interface used by GVT-g is GPL-only. > > > > > > To be clear it's the assumption on the host-based hypervisors e.g. KVM. > > > GUP is a perfect example, which doesn't work for Xen since DomU's > > > memory doesn't belong to Dom0. VFIO in Dom0 has to find the HPA > > > through Xen specific hypercalls. > > > > > > > > > > > > So GVT-g > > > > > has to maintain an internal abstraction layer to support both Xen > and > > > > > KVM. Maybe someday we will re-consider introducing some > hypervisor > > > > > abstraction layer in VFIO, if this issue starts to hurt other devices > and > > > > > Xen guys are willing to support VFIO. > > > > > > > > Once upon a time, we had a KVM specific device assignment interface, > > > > ie. legacy KVM devie assignment. We developed VFIO specifically to > get > > > > KVM out of the business of being a (bad) device driver. We do have > > > > some awareness and interaction between VFIO and KVM in the vfio- > kvm > > > > pseudo device, but we still try to keep those interfaces generic. In > > > > some cases we're not very successful at that, see > vfio_group_set_kvm(), > > > > but that's largely just a mechanism to associate a cookie with a group > > > > to be consumed by the mdev vendor driver such that it can work with > > > kvm > > > > external to vfio. I don't intend to add further hypervisor awareness > > > > to vfio. > > > > > > > > > Back to this IOVA issue, I discussed with Yan and we found another > > > > > hypervisor-agnostic alternative, by learning from vhost. vhost is very > > > > > similar to VFIO - DMA also happens in the kernel, while it already > > > > > supports vIOMMU. > > > > > > > > > > Generally speaking, there are three paths of dirty page collection > > > > > in Qemu so far (as previously noted, Qemu always tracks the dirty > > > > > bitmap in GFN): > > > > > > > > GFNs or simply PFNs within an AddressSpace? > > > > > > > > > 1) Qemu-tracked memory writes (e.g. emulated DMAs). Dirty > bitmaps > > > > > are updated directly when the guest memory is being updated. For > > > > > example, PCI writes are completed through pci_dma_write, which > > > > > goes through vIOMMU to translate IOVA into GPA and then update > > > > > the bitmap through cpu_physical_memory_set_dirty_range. > > > > > > > > Right, so the IOVA to GPA (GFN) occurs through an explicit translation > > > > on the IOMMU AddressSpace. > > > > > > > > > 2) Memory writes that are not tracked by Qemu are collected by > > > > > registering .log_sync() callback, which is invoked in the dirty > > > > > logging > > > > > process. Now there are two users: kvm and vhost. > > > > > > > > > > 2.1) KVM tracks CPU-side memory writes, through write-protection > > > > > or EPT A/D bits (+PML). This part is always based on GFN and > returned > > > > > to Qemu when kvm_log_sync is invoked; > > > > > > > > > > 2.2) vhost tracks kernel-side DMA writes, by interpreting vring > > > > > data structure. It maintains an internal iotlb which is synced with > > > > > Qemu vIOMMU through a specific interface: > > > > > - new vhost message type (VHOST_IOTLB_UPDATE/INVALIDATE) > > > > > for Qemu to keep vhost iotlb in sync > > > > > - new VHOST_IOTLB_MISS message to notify Qemu in case of > > > > > a miss in vhost iotlb. > > > > > - Qemu registers a log buffer to kernel vhost driver. The latter > > > > > update the buffer (using internal iotlb to get GFN) when serving > vring > > > > > descriptor. > > > > > > > > > > VFIO could also implement an internal iotlb, so vendor drivers can > > > > > utilize the iotlb to update the GFN-based dirty bitmap. Ideally we > > > > > don't need re-invent another iotlb protocol as vhost does. vIOMMU > > > > > already sends map/unmap ioctl cmds upon any change of IOVA > > > > > mapping. We may introduce a v2 map/unmap interface, allowing > > > > > Qemu to pass both {iova, gpa, hva} together to keep internal iotlb > > > > > in-sync. But we may also need a iotlb_miss_upcall interface, if VFIO > > > > > doesn't want to cache full-size vIOMMU mappings. > > > > > > > > > > Definitely this alternative needs more work and possibly less > > > > > performant (if maintaining a small size iotlb) than straightforward > > > > > calling into KVM interface. But the gain is also obvious, since it > > > > > is fully constrained with VFIO. > > > > > > > > > > Thoughts? :-) > > > > > > > > So vhost must then be configuring a listener across system memory > > > > rather than only against the device AddressSpace like we do in vfio, > > > > such that it get's log_sync() callbacks for the actual GPA space rather > > > > than only the IOVA space. OTOH, QEMU could understand that the > > > device > > > > AddressSpace has a translate function and apply the IOVA dirty bits to > > > > the system memory AddressSpace. Wouldn't it make more sense for > > > > QEMU > > > > to perform a log_sync() prior to removing a MemoryRegionSection > within > > > > an AddressSpace and update the GPA rather than pushing GPA > > > awareness > > > > and potentially large tracking structures into the host kernel? Thanks, > > > > > > > > > > > Hi, Alex, > > > > I moved back the VFIO related discussion to this thread, to not mix with > > vhost related discussions here. > > > > https://lists.nongnu.org/archive/html/qemu-devel/2019- > 09/msg03126.html > > > > Your latest reply still prefers to the userspace approach: > > > > > > Same as last time, you're asking VFIO to be aware of an entirely new > > > > address space and implement tracking structures of that address > space > > > > to make life easier for QEMU. Don't we typically push such complexity > > > > to userspace rather than into the kernel? I'm not convinced. Thanks, > > > > > > > > I answered two points but didn't hear your further thoughts. Can you > > take a look and respond? > > > > The first point is about complexity and performance: > > > > > > Is it really complex? No need of a new tracking structure. Just allowing > > > the MAP interface to carry a new parameter and then record it in the > > > existing vfio_dma objects. > > > > > > Note the frequency of guest DMA map/unmap could be very high. We > > > saw >100K invocations per second with a 40G NIC. To do the right > > > translation Qemu requires log_sync for every unmap, before the > > > mapping for logged dirty IOVA becomes stale. In current Kirti's patch, > > > each log_sync requires several system_calls through the migration > > > info, e.g. setting start_pfn/page_size/total_pfns and then reading > > > data_offset/data_size. That design is fine for doing log_sync in every > > > pre-copy round, but too costly if doing so for every IOVA unmap. If > > > small extension in kernel can lead to great overhead reduction, > > > why not? > > You're citing a workload that already performs abysmally with vfio and > vIOMMU, we cannot handle those rates efficiently with the current vfio > DMA API. The current use cases of vIOMMU and vfio are predominantly > for nesting vfio, ex. DPDK/SPDK, where we assume the mappings are > relatively static or else performance problems are already very > apparent. In that sort of model, I don't see that QEMU doing a > log_sync on unmap is really an issue, unmaps should be relatively > rare. Of course I don't want to compound the issue, but the current > vfio DMA mapping interfaces needs to be scrapped to make this remotely > performant even before we look at migration performance, so does it > really make sense to introduce GPAs for a workload the ioctls are > unsuited for? > > > The second point is about write-protection: > > > > > There is another value of recording GPA in VFIO. Vendor drivers > > > (e.g. GVT-g) may need to selectively write-protect guest memory > > > pages when interpreting certain workload descriptors. Those pages > > > are recorded in IOVA when vIOMMU is enabled, however the KVM > > > write-protection API only knows GPA. So currently vIOMMU must > > > be disabled on Intel vGPUs when GVT-g is enabled. To make it working > > > we need a way to translate IOVA into GPA in the vendor drivers. > > > There are two options. One is having KVM export a new API for such > > > translation purpose. But as you explained earlier it's not good to > > > have vendor drivers depend on KVM. The other is having VFIO > > > maintaining such knowledge through extended MAP interface, > > > then providing a uniform API for all vendor drivers to use. > > So the argument is that in order to interact with KVM (write protecting > guest memory) there's a missing feature (IOVA to GPA translation), but > we don't want to add an API to KVM for this feature because that would > create a dependency on KVM (for interacting with KVM), so lets add an > API to vfio instead. That makes no sense to me. What am I missing? > Thanks, >
Then do you have a recommendation how such feature can be implemented cleanly in vendor driver, without introducing direct dependency on KVM? Thanks Kevin
