linux-kernel - Re: [RFC PATCH v3 01/11] KVM: Capture VM start

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Date:   Mon, 10 Jan 2022 15:57:00 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Reiji Watanabe <reijiw@...gle.com>, Marc Zyngier <maz@...nel.org>,
        Andrew Jones <drjones@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

On Mon, Jan 10, 2022 at 3:07 PM Raghavendra Rao Ananta
<rananta@...gle.com> wrote:
>
> On Fri, Jan 7, 2022 at 4:05 PM Jim Mattson <jmattson@...gle.com> wrote:
> >
> > On Fri, Jan 7, 2022 at 3:43 PM Raghavendra Rao Ananta
> > <rananta@...gle.com> wrote:
> > >
> > > Hi Reiji,
> > >
> > > On Thu, Jan 6, 2022 at 10:07 PM Reiji Watanabe <reijiw@...gle.com> wrote:
> > > >
> > > > Hi Raghu,
> > > >
> > > > On Tue, Jan 4, 2022 at 11:49 AM Raghavendra Rao Ananta
> > > > <rananta@...gle.com> wrote:
> > > > >
> > > > > Capture the start of the KVM VM, which is basically the
> > > > > start of any vCPU run. This state of the VM is helpful
> > > > > in the upcoming patches to prevent user-space from
> > > > > configuring certain VM features after the VM has started
> > > > > running.
> >
> > What about live migration, where the VM has already technically been
> > started before the first call to KVM_RUN?
>
> My understanding is that a new 'struct kvm' is created on the target
> machine and this flag should be reset, which would allow the VMM to
> restore the firmware registers. However, we would be running KVM_RUN
> for the first time on the target machine, thus setting the flag.
> So, you are right; It's more of a resume operation from the guest's
> point of view. I guess the name of the variable is what's confusing
> here.

I was actually thinking that live migration gives userspace an easy
way to circumvent your restriction. You said, "This state of the VM is
helpful in the upcoming patches to prevent user-space from configuring
certain VM features after the VM has started running." However, if you
don't ensure that these VM features are configured the same way on the
target machine as they were on the source machine, you have not
actually accomplished your stated goal.

> Thanks,
> Raghavendra