Re: [PATCH qemu-web] Add a blog post on "Micro-Optimizing KVM VM-Exits"

[Thomas Huth]

On 08/11/2019 10.22, Kashyap Chamarthy wrote:
> This blog post summarizes the talk "Micro-Optimizing KVM VM-Exits"[1],
> given by Andrea Arcangeli at the recently concluded KVM Forum 2019.
> 

 Hi Kashyap,

first thanks for writing up this article! It's a really nice summary of
the presentation, I think.

But before we include it, let me ask a meta-question: Is an article
about the KVM *kernel* code suitable for the *QEMU* blog? Or is there
maybe a better place for this, like an article on www.linux-kvm.org ?

Opinions? Ideas?

 Thomas


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> 
> diff --git a/_posts/2019-11-06-micro-optimizing-kvm-vmexits.txt 
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> +---
> +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.
>