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/d3a2fd9b29e43e202315d5e99399b99622469c4a. 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! --Shyam -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
