linux-kernel - Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a vm

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Message-ID: <a2e16531-5522-a334-40a1-2b0e17663800@linux.ibm.com>
Date:   Mon, 24 Oct 2022 09:56:31 +0200
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] KVM: API to block and resume all running vcpus in a
 vm

Am 22.10.22 um 17:48 schrieb Emanuele Giuseppe Esposito:
> This new API allows the userspace to stop all running
> vcpus using KVM_KICK_ALL_RUNNING_VCPUS ioctl, and resume them with
> KVM_RESUME_ALL_KICKED_VCPUS.
> A "running" vcpu is a vcpu that is executing the KVM_RUN ioctl.
> 
> This serie is especially helpful to userspace hypervisors like
> QEMU when they need to perform operations on memslots without the
> risk of having a vcpu reading them in the meanwhile.
> With "memslots operations" we mean grow, shrink, merge and split
> memslots, which are not "atomic" because there is a time window
> between the DELETE memslot operation and the CREATE one.
> Currently, each memslot operation is performed with one or more
> ioctls.
> For example, merging two memslots into one would imply:
> DELETE(m1)
> DELETE(m2)
> CREATE(m1+m2)
> 
> And a vcpu could attempt to read m2 right after it is deleted, but
> before the new one is created.
> 
> Therefore the simplest solution is to pause all vcpus in the kvm
> side, so that:
> - userspace just needs to call the new API before making memslots
> changes, keeping modifications to the minimum
> - dirty page updates are also performed when vcpus are blocked, so
> there is no time window between the dirty page ioctl and memslots
> modifications, since vcpus are all stopped.
> - no need to modify the existing memslots API
Isnt QEMU able to achieve the same goal today by forcing all vCPUs
into userspace with a signal? Can you provide some rationale why this
is better in the cover letter or patch description?