linux-kernel - Re: [PATCH] KVM: x86: add HC_VMM

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Date:   Thu, 21 Apr 2022 19:14:20 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Oskolkov <posk@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Cc:     kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Paul Turner <pjt@...gle.com>, Peter Oskolkov <posk@...k.io>
Subject: Re: [PATCH] KVM: x86: add HC_VMM_CUSTOM hypercall

On 4/21/22 18:51, Peter Oskolkov wrote:
> Allow kvm-based VMMs to request KVM to pass a custom vmcall
> from the guest to the VMM in the host.
> 
> Quite often, operating systems research projects and/or specialized
> paravirtualized workloads would benefit from a extra-low-overhead,
> extra-low-latency guest-host communication channel.

You can use a memory page and an I/O port.  It should be as fast as a 
hypercall.  You can even change it to use ioeventfd if an asynchronous 
channel is enough, and then it's going to be less than 1 us latency.

Paolo

> With cloud-hypervisor modified to handle the new hypercall (simply
> return the sum of the received arguments), the following function in
> guest_userspace_  completes, on average, in 2.5 microseconds (walltime)
> on a relatively modern Intel Xeon processor: