On Thu, 2016-01-07 at 15:48 +0530, Shyam wrote: > Hi All, > > We are using Linux Kernel 3.18.19 for running KVM VM's with > pci-assign'ed SRIOV VF interfaces. > > We understand that VFIO is the new recommended way, but unfortunately > it reduces performance significantly on our IO workloads (upto the > order of 40-50%) when compared to pci-passthrough. We run trusted > VM's > & expose services to the external world. Since we control the VM's, > IOMMU security with VFIO is not exactly mandatory, but performance is > much more important that we get with pci-assign. > > We observe a strange behaviour that has already been discussed in > this > forum which is upon a VM spawn it causes the following fault > resulting > in qemu-kvm crashing > > Jan 7 09:41:57 q6-s1 kernel: [90037.228477] intel_iommu_map: iommu > width (48) is not sufficient for the mapped address > (fffffffffe001000) > Jan 7 09:41:57 q6-s1 kernel: [90037.308229] > kvm_iommu_map_address:iommu failed to map pfn=95000 > > We observe that this problem happens only if guest linux running 3.5 > kernel is spun up & this problem doesnt happen when running guest > linux with 3.6 kernel (i.e. all guest with kernels like 3.2 etc up > till 3.5 causes the above crash whereas any guest kernel >=3.6 doesnt > cause this issue). > > So something changed between kernel 3.5 to 3.6 in the guest that > doesnt expose this problem. We have two questions: > 1 - we understand that VFIO suffered a similar problem & it was fixed > with https://github.com/qemu/qemu/commit/d3a2fd9b29e43e202315d5e99399 > b99622469c4a. > Alex Williamson suggested that KVM driver needs an equivalent version > of the fix. Can anybody suggest hints on where this fix should be > made? > 2 - Any insights on what changes in linux kernel between 3.5 to 3.6 > on > the guest that avoids this problem? > > Any helps/input greatly appreciated. Thanks! Legacy KVM device assignment is deprecated, so I'd suggest your efforts are better spent reporting and trying to fix any performance difference you're seeing between pci-assign and vfio-pci. I have a really hard time believing there's anywhere close to a 40-50% difference. What's the device? What's the workload? At some point you're likely to find that pci-assign is no longer even present. Thanks, Alex -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
