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From: Paolo Bonzini <pbonzini@redhat.com>
To: Anastassios Nanos <ananos@nubificus.co.uk>,
	Marc Zyngier <maz@kernel.org>
Cc: kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-kernel@vger.kernel.org, James Morse <james.morse@arm.com>,
	Julien Thierry <julien.thierry.kdev@gmail.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 0/2] Expose KVM API to Linux Kernel
Date: Mon, 18 May 2020 13:18:23 +0200	[thread overview]
Message-ID: <760e0927-d3a7-a8c6-b769-55f43a65e095@redhat.com> (raw)
In-Reply-To: <CALRTab90UyMq2hMxCdCmC3GwPWFn2tK_uKMYQP2YBRcHwzkEUQ@mail.gmail.com>

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


  parent reply	other threads:[~2020-05-18 11:18 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-18  6:58 [PATCH 0/2] Expose KVM API to Linux Kernel Anastassios Nanos
2020-05-18  6:58 ` [PATCH 1/2] KVMM: export needed symbols Anastassios Nanos
2020-05-18  7:41   ` Marc Zyngier
2020-05-18  7:01 ` [PATCH 2/2] KVMM: Memory and interface related changes Anastassios Nanos
2020-05-18  6:59   ` Anastassios Nanos
2020-05-18  9:13   ` kbuild test robot
2020-05-18  9:28   ` kbuild test robot
2020-05-18 10:16   ` kbuild test robot
2020-05-18  7:50 ` [PATCH 0/2] Expose KVM API to Linux Kernel Marc Zyngier
     [not found]   ` <CALRTab90UyMq2hMxCdCmC3GwPWFn2tK_uKMYQP2YBRcHwzkEUQ@mail.gmail.com>
2020-05-18 11:18     ` Paolo Bonzini [this message]
2020-05-18 11:34       ` Maxim Levitsky
2020-05-18 11:51         ` Paolo Bonzini
2020-05-18 12:12           ` Maxim Levitsky
2020-05-18 20:45     ` Andy Lutomirski
2020-05-18  8:42 ` Thomas Gleixner
     [not found]   ` <CALRTab-mEYtRG4zQbSGoAri+jg8xNL-imODv=MWE330Hkt_t+Q@mail.gmail.com>
2020-05-18  9:18     ` Vitaly Kuznetsov
2020-05-18  9:38     ` Thomas Gleixner

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