On Thu, Feb 09, 2017 at 08:01:14AM -0500, Jintack Lim wrote: > On Wed, Feb 8, 2017 at 10:52 PM, Peter Xu <pet...@redhat.com> wrote: > > (cc qemu-devel and Alex) > > > > On Wed, Feb 08, 2017 at 09:14:03PM -0500, Jintack Lim wrote: > >> On Wed, Feb 8, 2017 at 10:49 AM, Jintack Lim <jint...@cs.columbia.edu> > >> wrote: > >> > Hi Peter, > >> > > >> > On Tue, Feb 7, 2017 at 10:12 PM, Peter Xu <pet...@redhat.com> wrote: > >> >> On Tue, Feb 07, 2017 at 02:16:29PM -0500, Jintack Lim wrote: > >> >>> Hi Peter and Michael, > >> >> > >> >> Hi, Jintack, > >> >> > >> >>> > >> >>> I would like to get some help to run a VM with the emulated iommu. I > >> >>> have tried for a few days to make it work, but I couldn't. > >> >>> > >> >>> What I want to do eventually is to assign a network device to the > >> >>> nested VM so that I can measure the performance of applications > >> >>> running in the nested VM. > >> >> > >> >> Good to know that you are going to use [4] to do something useful. :-) > >> >> > >> >> However, could I ask why you want to measure the performance of > >> >> application inside nested VM rather than host? That's something I am > >> >> just curious about, considering that virtualization stack will > >> >> definitely introduce overhead along the way, and I don't know whether > >> >> that'll affect your measurement to the application. > >> > > >> > I have added nested virtualization support to KVM/ARM, which is under > >> > review now. I found that application performance running inside the > >> > nested VM is really bad both on ARM and x86, and I'm trying to figure > >> > out what's the real overhead. I think one way to figure that out is to > >> > see if the direct device assignment to L2 helps to reduce the overhead > >> > or not. > > > > I see. IIUC you are trying to use an assigned device to replace your > > old emulated device in L2 guest to see whether performance will drop > > as well, right? Then at least I can know that you won't need a nested > > VT-d here (so we should not need a vIOMMU in L2 guest). > > That's right. > > > > > In that case, I think we can give it a shot, considering that L1 guest > > will use vfio-pci for that assigned device as well, and when L2 guest > > QEMU uses this assigned device, it'll use a static mapping (just to > > map the whole GPA for L2 guest) there, so even if you are using a > > kernel driver in L2 guest with your to-be-tested application, we > > should still be having a static mapping in vIOMMU in L1 guest, which > > is IMHO fine from performance POV. > > > > I cced Alex in case I missed anything here. > > > >> > > >> >> > >> >> Another thing to mention is that (in case you don't know that), device > >> >> assignment with VT-d protection would be even slower than generic VMs > >> >> (without Intel IOMMU protection) if you are using generic kernel > >> >> drivers in the guest, since we may need real-time DMA translation on > >> >> data path. > >> >> > >> > > >> > So, this is the comparison between using virtio and using the device > >> > assignment for L1? I have tested application performance running > >> > inside L1 with and without iommu, and I found that the performance is > >> > better with iommu.
Here iiuc you mean that "L1 guest with vIOMMU performs better than when without vIOMMU", while ... > >> > I thought whether the device is assigned to L1 or > >> > L2, the DMA translation is done by iommu, which is pretty fast? Maybe > >> > I misunderstood what you said? > > > > I failed to understand why an vIOMMU could help boost performance. :( > > Could you provide your command line here so that I can try to > > reproduce? > > Sure. This is the command line to launch L1 VM > > qemu-system-x86_64 -M q35,accel=kvm,kernel-irqchip=split \ > -m 12G -device intel-iommu,intremap=on,eim=off,caching-mode=on \ > -drive file=/mydata/guest0.img,format=raw --nographic -cpu host \ > -smp 4,sockets=4,cores=1,threads=1 \ > -device vfio-pci,host=08:00.0,id=net0 > > And this is for L2 VM. > > ./qemu-system-x86_64 -M q35,accel=kvm \ > -m 8G \ > -drive file=/vm/l2guest.img,format=raw --nographic -cpu host \ > -device vfio-pci,host=00:03.0,id=net0 ... here looks like these are command lines for L1/L2 guest, rather than L1 guest with/without vIOMMU? > > > > > Besides, what I mentioned above is just in case you don't know that > > vIOMMU will drag down the performance in most cases. > > > > I think here to be more explicit, the overhead of vIOMMU is different > > for assigned devices and emulated ones. > > > > (1) For emulated devices, the overhead is when we do the > > translation, or say when we do the DMA operation. We need > > real-time translation which should drag down the performance. > > > > (2) For assigned devices (our case), the overhead is when we setup > > the pages (since we are trapping the setup procedures via CM > > bit). However, after it's setup, we should have no much > > performance drag when we really do the data transfer (during > > DMA) since that'll all be done in the hardware IOMMU (no matter > > whether the device is assigned to L1/L2 guest). > > > > Now, after I know your use case now (use vIOMMU in L1 guest, don't use > > vIOMMU in L2 guest, only use assigned devices), I suspect we would > > have no big problem according to (2). > > > >> > > >> >>> > >> >>> First, I am having trouble to boot a VM with the emulated iommu. I > >> >>> have posted my problem to the qemu user mailing list[1], > >> >> > >> >> Here I would suggest that you cc qemu-devel as well next time: > >> >> > >> >> qemu-devel@nongnu.org > >> >> > >> >> Since I guess not all people are registered to qemu-discuss, at least > >> >> I am not in that loop. Imho cc qemu-devel could let the question > >> >> spread to more people, and it'll get a higher chance to be answered. > >> > > >> > Thanks. I'll cc qemu-devel next time. > >> > > >> >> > >> >>> but to put it > >> >>> in a nutshell, I'd like to know the setting I can reuse to boot a VM > >> >>> with the emulated iommu. (e.g. how to create a VM with q35 chipset > >> >>> and/or libvirt xml if you use virsh). > >> >> > >> >> IIUC you are looking for device assignment for the nested VM case. So, > >> >> firstly, you may need my tree to run this (see below). Then, maybe you > >> >> can try to boot a L1 guest with assigned device (under VT-d > >> >> protection), with command: > >> >> > >> >> $qemu -M q35,accel=kvm,kernel-irqchip=split -m 1G \ > >> >> -device intel-iommu,intremap=on,eim=off,caching-mode=on \ > >> >> -device vfio-pci,host=$HOST_PCI_ADDR \ > >> >> $YOUR_IMAGE_PATH > >> >> > >> > > >> > Thanks! I'll try this right away. > >> > > >> >> Here $HOST_PCI_ADDR should be something like 05:00.0, which is the > >> >> host PCI address of the device to be assigned to guest. > >> >> > >> >> (If you go over the cover letter in [4], you'll see similar command > >> >> line there, though with some more devices assigned, and with traces) > >> >> > >> >> If you are playing with nested VM, you'll also need a L2 guest, which > >> >> will be run inside the L1 guest. It'll require similar command line, > >> >> but I would suggest you first try a L2 guest without intel-iommu > >> >> device. Frankly speaking I haven't played with that yet, so just let > >> >> me know if you got any problem, which is possible. :-) > >> >> > >> > >> I was able to boot L2 guest without assigning a network device > >> successfully. (host iommu was on, L1 iommu was on, and the network > >> device was assigned to L1) > >> > >> Then, I unbound the network device in L1 and bound it to vfio-pci. > >> When I try to run L2 with the following command, I got an assertion. > >> > >> # ./qemu-system-x86_64 -M q35,accel=kvm \ > >> -m 8G \ > >> -drive file=/vm/l2guest.img,format=raw --nographic -cpu host \ > >> -device vfio-pci,host=00:03.0,id=net0 > >> > >> qemu-system-x86_64: hw/pci/pcie.c:686: pcie_add_capability: Assertion > >> `prev >= 0x100' failed. > >> Aborted (core dumped) > >> > >> Thoughts? > > > > I don't know whether it'll has anything to do with how vfio-pci works, > > anyway I cced Alex and the list in case there is quick answer. > > > > I'll reproduce this nested case and update when I got anything. > > Thanks! I tried to reproduce this issue with the following 10g network card: 00:03.0 Ethernet controller: Intel Corporation Ethernet Controller 10-Gigabit X540-AT2 (rev 01) In my case, both L1/L2 guests can boot with the assigned device. I also did a quick netperf TCP STREAM test, the result is (in case you are interested): L1 guest: 1.12Gbps L2 guest: 8.26Gbps First of all, just to confirm that you were using the same qemu binary in both host and L1 guest, right? Then, I *think* above assertion you encountered would fail only if prev == 0 here, but I still don't quite sure why was that happening. Btw, could you paste me your "lspci -vvv -s 00:03.0" result in your L1 guest? Thanks, -- peterx
