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Date:   Thu, 21 Apr 2022 19:14:20 +0200
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Subject: Re: [PATCH] KVM: x86: add HC_VMM_CUSTOM hypercall
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To:     Peter Oskolkov <posk@google.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>
Cc:     kvm@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        Paul Turner <pjt@google.com>, Peter Oskolkov <posk@posk.io>
References: <20220421165137.306101-1-posk@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <20220421165137.306101-1-posk@google.com>
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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: