linux-kernel - Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption status is changed

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Message-ID: <70951773-fe39-c694-abeb-69052934b76b@redhat.com>
Date:   Fri, 14 May 2021 11:57:27 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ashish Kalra <ashish.kalra@....com>
Cc:     Borislav Petkov <bp@...en8.de>, seanjc@...gle.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        joro@...tes.org, thomas.lendacky@....com, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        srutherford@...gle.com, venu.busireddy@...cle.com,
        brijesh.singh@....com
Subject: Re: [PATCH v2 2/4] mm: x86: Invoke hypercall when page encryption
 status is changed

On 14/05/21 11:05, Ashish Kalra wrote:
> I absolutely agree with both of your point of view. But what's the
> alternative ?
> 
> Ideally we should fail/stop migration even if a single guest page
> encryption status cannot be notified and that should be the way to
> proceed in this case, the guest kernel should notify the source
> userspace VMM to block/stop migration in this case.
> 
>  From a practical side, i do see Qemu's migrate_add_blocker() interface
> but that looks to be a static interface and also i don't think it will
> force stop an ongoing migration, is there an existing mechanism
> to inform userspace VMM from kernel about blocking/stopping migration ?

On the Linux side, all you need to do is WARN and write 0 to the 
MIGRATION_CONTROL MSR.

QEMU can check the MSR value when migrating the CPU registers at the 
end, and fail migration if the MSR value is 0.

Paolo