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From:   Nathan Tempelman <natet@google.com>
Date:   Fri, 5 Mar 2021 12:43:07 -0800
Message-ID: <CAKiEG5oicgF=wuYda6RhH_Memc_gnoYSeiimVthkuxckKWN9_w@mail.gmail.com>
Subject: Re: [RFC] KVM: x86: Support KVM VMs sharing SEV context
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Ashish Kalra <ashish.kalra@amd.com>,
        Thomas Lendacky <thomas.lendacky@amd.com>, x86@kernel.org,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Steve Rutherford <srutherford@google.com>,
        Sean Christopherson <seanjc@google.com>,
        David Rientjes <rientjes@google.com>,
        Brijesh Singh <brijesh.singh@amd.com>
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On Fri, Mar 5, 2021 at 7:13 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 05/03/21 15:04, Ashish Kalra wrote:
> >> +    /* Mirrors of mirrors should work, but let's not get silly */
> >> +    if (is_mirroring_enc_context(kvm)) {
> >> +            ret = -ENOTTY;
> >> +            goto failed;
> >> +    }
> > How will A->B->C->... type of live migration work if mirrors of
> > mirrors are not supported ?
>
> Each host would only run one main VM and one mirror, wouldn't it?

That's correct. You could create a second mirror vm of the original
(A->B, A->C) if you needed two in-guest workers, but I don't see a use
for a chain. If anyone can see one I can write it that way, but in the interest
of keeping it simple I've blocked it. Originally I'd built it with
that functionality,
but allowing a chain like that smells like recursion and from what I
understand we don't like recursion in the kernel. There's also the fear as
steve mentioned that we could blow the callstack with a long chain of
destroys starting from the leaf.
Ideally we give userspace one less gun to shoot itself in the foot with.