linux-kernel - Re: [PATCH 0/2] Expose KVM API to Linux Kernel

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Message-ID: <760e0927-d3a7-a8c6-b769-55f43a65e095@redhat.com>
Date:   Mon, 18 May 2020 13:18:23 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Anastassios Nanos <ananos@...ificus.co.uk>,
        Marc Zyngier <maz@...nel.org>
Cc:     kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH 0/2] Expose KVM API to Linux Kernel

On 18/05/20 10:45, Anastassios Nanos wrote:
> Being in the kernel saves us from doing unneccessary mode switches.
> Of course there are optimizations for handling I/O on QEMU/KVM VMs
> (virtio/vhost), but essentially what happens is removing mode-switches (and
> exits) for I/O operations -- is there a good reason not to address that
> directly? a guest running in the kernel exits because of an I/O request,
> which gets processed and forwarded directly to the relevant subsystem *in*
> the kernel (net/block etc.).

In high-performance configurations, most of the time virtio devices are
processed in another thread that polls on the virtio rings.  In this
setup, the rings are configured to not cause a vmexit at all; this has
much smaller latency than even a lightweight (kernel-only) vmexit,
basically corresponding to writing an L1 cache line back to L2.

Paolo