This blog post summarizes the talk "Micro-Optimizing KVM VM-Exits"[1], given by Andrea Arcangeli at the recently concluded KVM Forum 2019.
[1] https://kvmforum2019.sched.com/event/Tmwr/micro-optimizing-kvm-vm-exits-andrea-arcangeli-red-hat-inc Signed-off-by: Kashyap Chamarthy <kcham...@redhat.com> --- v2: Address Rich W.M Jones' feedback --- ...019-11-06-micro-optimizing-kvm-vmexits.txt | 115 ++++++++++++++++++ 1 file changed, 115 insertions(+) create mode 100644 _posts/2019-11-06-micro-optimizing-kvm-vmexits.txt diff --git a/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt b/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt new file mode 100644 index 0000000000000000000000000000000000000000..f4a28d58ddb40103dd599fdfd861eeb4c41ed976 --- /dev/null +++ b/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt @@ -0,0 +1,115 @@ +--- +layout: post +title: "Micro-Optimizing KVM VM-Exits" +date: 2019-11-08 +categories: [kvm, optimization] +--- + +Background on VM-Exits +---------------------- + +KVM (Kernel-based Virtual Machine) is the Linux kernel module that +allows a host to run virtualized guests (Linux, Windows, etc). The KVM +"guest execution loop", with QEMU (the open source emulator and +virtualizer) as its user space, is roughly as follows: QEMU issues the +ioctl(), KVM_RUN, to tell KVM to prepare to enter the CPU's "Guest Mode" +-- a special processor mode which allows guest code to safely run +directly on the physical CPU. The guest code, which is inside a "jail" +and thus cannot interfere with the rest of the system, keeps running on +the hardware until it encounters a request it cannot handle. Then the +processor gives the control back (referred to as "VM-Exit") either to +kernel space, or to the user space to handle the request. Once the +request is handled, native execution of guest code on the processor +resumes again. And the loop goes on. + +There are dozens of reasons for VM-Exits (Intel's Software Developer +Manual outlines 64 "Basic Exit Reasons"). For example, when a guest +needs to emulate the CPUID instruction, it causes a "light-weight exit" +to kernel space, because CPUID (among a few others) is emulated in the +kernel itself, for performance reasons. But when the kernel _cannot_ +handle a request, e.g. to emulate certain hardware, it results in a +"heavy-weight exit" to QEMU, to perform the emulation. These VM-Exits +and subsequent re-entries ("VM-Enters"), even the light-weight ones, can +be expensive. What can be done about it? + +Guest workloads that are hard to virtualize +------------------------------------------- + +At the 2019 edition of the KVM Forum in Lyon, kernel developer, Andrea +Arcangeli, attempted to address the kernel part of minimizing VM-Exits. + +His talk touched on the cost of VM-Exits into the kernel, especially for +guest workloads (e.g. enterprise databases) that are sensitive to their +performance penalty. However, these workloads cannot avoid triggering +VM-Exits with a high frequency. Andrea then outlined some of the +optimizations he's been working on to improve the VM-Exit performance in +the KVM code path -- especially in light of applying mitigations for +speculative execution flaws (Spectre v2, MDS, L1TF). + +Andrea gave a brief recap of the different kinds of speculative +execution attacks (retpolines, IBPB, PTI, SSBD, etc). Followed by that +he outlined the performance impact of Spectre-v2 mitigations in context +of KVM. + +The microbechmark: CPUID in a one million loop +---------------------------------------------- + +The synthetic microbenchmark (meaning, focus on measuring the +performance of a specific area of code) Andrea used was to run the CPUID +instruction one million times, without any GCC optimizations or caching. +This was done to test the latency of VM-Exits. + +While stressing that the results of these microbenchmarks do not +represent real-world workloads, he had two goals in mind with it: (a) +explain how the software mitigation works; and (b) to justify to the +broader community the value of the software optimizations he's working +on in KVM. + +Andrea then reasoned through several interesting graphs that show how +CPU computation time gets impacted when you disable or enable the +various kernel-space mitigations for Spectre v2, L1TF, MDS, et al. + +The proposal: "KVM Monolithic" +------------------------------ + +Based on his investigation, Andrea proposed a patch series, ["KVM +monolithc"](https://lwn.net/Articles/800870/), to get rid of the KVM +common module, 'kvm.ko'. Instead the KVM common code gets linked twice +into each of the vendor-specific KVM modules, 'kvm-intel.ko' and +'kvm-amd.ko'. + +The reason for doing this is that the 'kvm.ko' module indirectly calls +(via the "retpoline" technique) the vendor-specific KVM modules at every +VM-Exit, several times. These indirect calls were not optimal before, +but the "retpoline" mitigation (which isolates indirect branches, that +allow a CPU to execute code from arbitrary locations, from speculative +execution) for Spectre v2 compounds the problem, as it degrades +performance. + +This approach will result in a few MiB of increased disk space for +'kvm-intel.ko' and 'kvm-amd.ko', but the upside in saved indirect calls, +and the elimination of "retpoline" overhead at run-time more than +compensate for it. + +With the "KVM Monolithic" patch series applied, Andrea's microbenchmarks +show a double-digit improvement in performance with default mitigations +(for Spectre v2, et al) enabled on both Intel 'VMX' and AMD 'SVM'. And +with 'spectre_v2=off' or for CPUs with IBRS_ALL in ARCH_CAPABILITIES +"KVM monolithic" still improve[s] performance, albiet it's on the order +of 1%. + +Conclusion +---------- + +Removal of the common KVM module has a non-negligible positive +performance impact. And the "KVM Monolitic" patch series is still +actively being reviewed, modulo some pending clean-ups. Based on the +upstream review discussion, KVM Maintainer, Paolo Bonzini, and other +reviewers seemed amenable to merge the series. + +Although, we still have to deal with mitigations for 'indirect branch +prediction' for a long time, reducing the VM-Exit latency is important +in general; and more specifically, for guest workloads that happen to +trigger frequent VM-Exits, without having to disable Spectre v2 +mitigations on the host, as Andrea stated in the cover letter of his +patch series. -- 2.21.0
