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

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Message-ID: <f077234c-ea74-faaf-422a-fd5d2c1c6923@redhat.com>
Date:   Mon, 18 May 2020 13:51:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>,
        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 13:34, Maxim Levitsky wrote:
>> 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.
>
> This can be used to run kernel drivers inside a very thin VM IMHO to break up the stigma,
> that kernel driver is always a bad thing to and should be by all means replaced by a userspace driver,
> something I see a lot lately, and what was the ground for rejection of my nvme-mdev proposal.

It's a tought design decision between speeding up a kernel driver with
something like eBPF or wanting to move everything to userspace.

Networking has moved more towards the first because there are many more
opportunities for NIC-based acceleration, while storage has moved
towards the latter with things such as io_uring.  That said, I don't see
why in-kernel NVMeoF drivers would be acceptable for anything but Fibre
Channel (and that's only because FC HBAs try hard to hide most of the
SAN layers).

Paolo