Thank you for your inputs. On Wed, Oct 18, 2017 at 2:53 PM, Alex Williamson <alex.william...@redhat.com> wrote: > On Wed, 18 Oct 2017 13:52:17 -0400 > Spoorti Doddamani <spoortidoddamani1...@gmail.com> wrote: > >> Hi, >> >> We are using pci-assign option for device passthrough; we observe that >> the interrupts are still going through host and cpu utilization is >> very high. >> >> We unbind the NIC card from host driver and use the following command >> for device passthrough: >> qemu-system-x86_64 -m 512 -net none -hda xxx.img -device >> pci-assign,host=01:00.0 >> >> Could you please let us know why cpu utilization is very high even >> though we do direct device assignment? Also, could you please let us >> know where to find more documentation on pci-assign. > > As of kernel v4.12, Legacy KVM device assignment has been removed from > the kernel (it had previously been deprecated), so I suggest to start > with migrating your configuration to using vfio-pci instead. You can > find vfio documentation in the Linux kernel tree: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/vfio.txt > > In my blog: > > http://vfio.blogspot.com/ > > And various other places. > > Regarding interrupt behavior, without additional hardware assistance, > assigned device interrupts are handled by the host kernel and injected > into the guest. This does not generally incur much load on the host > since the interrupt is simply forwarded and not serviced at the device > in the host. Usually the problem here is the additional latency, which > is one of the more notable overheads when using device assignment. > > Various processor offloads can help to alleviate this. Intel first > introduced APICv, which among other things, allows interrupts to be > injected into the VM via IPI without incurring a vm-exit. To take > advantage of this, you'd want a) hardware supporting APICv and b) > redirect the physical interrupt to a host processor not running a vCPU > such that the interrupt itself doesn't trigger a world switch. The > kvm_intel module has an enable_apicv option, if you can't enable it, as > seen in /sys/module/kvm_intel/parameters/enable_apicv, then your > hardware doesn't support it. > > Newer yet Intel hardware supports posted interrupts, which allows > interrupts to be injected directly into the VM, bypassing the host > entirely. This one is even trickier to setup AIUI the interrupt needs > to arrive to the pCPU while it's running the vCPU to which the > interrupt is directed in the VM. I'm not entirely sure how to test for > this support, but unless you're running pretty recent Xeon E5+ > processors, it's probably safe to assume it's not there. > > The above optimizations also assume that you're using a modern device > which supports nicely aligned MMIO mapped registers and MSI or MSI/X > interrupts. If you're using a device that requires INTx or makes use > of I/O port registers, or interacts with PCI config space of the device > on each interrupt, or perhaps even if you're using an old OS in the VM > that flips MSI mask bits regularly, most of those actions require > hypervisor interaction and will reduce the performance and increase the > overhead of the device. Thanks, > > Alex
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