linux-kernel - Re: [PATCH v8 18/18] KVM: SVM: Enable SEV live migration feature implicitly on Incoming VM(s).

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Date:   Fri, 4 Dec 2020 12:11:02 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ashish Kalra <Ashish.Kalra@....com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, bp@...e.de, Thomas.Lendacky@....com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        srutherford@...gle.com, rientjes@...gle.com,
        venu.busireddy@...cle.com, brijesh.singh@....com
Subject: Re: [PATCH v8 18/18] KVM: SVM: Enable SEV live migration feature
 implicitly on Incoming VM(s).

On 05/05/20 23:22, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> For source VM, live migration feature is enabled explicitly
> when the guest is booting, for the incoming VM(s) it is implied.
> This is required for handling A->B->C->... VM migrations case.
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> ---
>   arch/x86/kvm/svm/sev.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 6f69c3a47583..ba7c0ebfa1f3 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1592,6 +1592,13 @@ int svm_set_page_enc_bitmap(struct kvm *kvm,
>   	if (ret)
>   		goto unlock;
>   
> +	/*
> +	 * For source VM, live migration feature is enabled
> +	 * explicitly when the guest is booting, for the
> +	 * incoming VM(s) it is implied.
> +	 */
> +	sev_update_migration_flags(kvm, KVM_SEV_LIVE_MIGRATION_ENABLED);
> +
>   	bitmap_copy(sev->page_enc_bmap + BIT_WORD(gfn_start), bitmap,
>   		    (gfn_end - gfn_start));

Why?  I'd prefer the host to do this manually using a KVM_ENABLE_CAP. 
The hook in patch 12 would also be enabled/disabled using KVM_ENABLE_CAP.

Paolo