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From:   Steve Rutherford <srutherford@google.com>
Date:   Wed, 19 Feb 2020 17:58:00 -0800
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Subject: Re: [PATCH 10/12] mm: x86: Invoke hypercall when page encryption
 status is changed
To:     Ashish Kalra <Ashish.Kalra@amd.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
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        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        David Rientjes <rientjes@google.com>, x86@kernel.org,
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On Wed, Feb 12, 2020 at 5:18 PM Ashish Kalra <Ashish.Kalra@amd.com> wrote:
>
> From: Brijesh Singh <brijesh.singh@amd.com>
>
> Invoke a hypercall when a memory region is changed from encrypted ->
> decrypted and vice versa. Hypervisor need to know the page encryption
> status during the guest migration.

One messy aspect, which I think is fine in practice, is that this
presumes that pages are either treated as encrypted or decrypted. If
also done on SEV, the in-place re-encryption supported by SME would
break SEV migration. Linux doesn't do this now on SEV, and I don't
have an intuition for why Linux might want this, but we will need to
ensure it is never done in order to ensure that migration works down
the line. I don't believe the AMD manual promises this will work
anyway.

Something feels a bit wasteful about having all future kernels
universally announce c-bit status when SEV is enabled, even if KVM
isn't listening, since it may be too old (or just not want to know).
Might be worth eliding the hypercalls if you get ENOSYS back? There
might be a better way of passing paravirt config metadata across than
just trying and seeing if the hypercall succeeds, but I'm not super
familiar with it.