linux-kernel - Re: [Part1 PATCH v5 02/17] x86/mm: Add Secure Encrypted Virtualization (SEV) support

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Date:   Thu, 28 Sep 2017 21:23:47 +0200
From:   Borislav Petkov <bp@...e.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [Part1 PATCH v5 02/17] x86/mm: Add Secure Encrypted
 Virtualization (SEV) support

Hi,

On Thu, Sep 28, 2017 at 01:48:48PM -0500, Brijesh Singh wrote:
> Let me understand the ask, are you saying that we need a method to disable the SEV
> feature from the host OS so that Hypervisor will not be able to create a SEV guest?
> Because once a guest is booted with SEV feature, there is no way to disable the SEV
> feature from the guest.
> 
> i.e if "mem_encrypt=smeonly" is set then we clear X86_FEATURE_SEV capability flag
> defined in [1].

So actually we need chicken bits to be able to *enable* both when
CONFIG_AMD_MEM_ENCRYPT_ACTIVE_BY_DEFAULT is not set.

I.e.,

* mem_encrypt=on - both SME and SEV enabled

* mem_encrypt=smeonly - only SME, no SEV on the host. This option will
basically prevent from using any SEV guests and make the SEV part of the
code inactive. I.e., sev_active() and sev_enabled should be false. As
you say above, we should clear X86_FEATURE_SEV, yes.

* mem_encrypt=off - neither SME/SEV are enabled.

And =on and =off we already have.

How does that sound?

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--