Date: Tue, 29 Nov 2016 20:56:18 +0100
From: Borislav Petkov <bp@alien8.de>
To: Tom Lendacky <thomas.lendacky@amd.com>
Cc: linux-arch@vger.kernel.org, linux-efi@vger.kernel.org,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org, x86@kernel.org,
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        linux-mm@kvack.org, iommu@lists.linux-foundation.org,
        Rik van Riel <riel@redhat.com>,
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        Arnd Bergmann <arnd@arndb.de>, Jonathan Corbet <corbet@lwn.net>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        Joerg Roedel <joro@8bytes.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Larry Woodman <lwoodman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Alexander Potapenko <glider@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [RFC PATCH v3 20/20] x86: Add support to make use of Secure
 Memory Encryption
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On Tue, Nov 29, 2016 at 12:48:17PM -0600, Tom Lendacky wrote:
> > One more thing: just like we're adding an =on switch, we'd need an =off
> > switch in case something's wrong with the SME code. IOW, if a user
> > supplies "mem_encrypt=off", we do not encrypt.
> 
> Well, we can document "off", but if the exact string "mem_encrypt=on"
> isn't specified on the command line then the encryption won't occur.

So you have this:

+       /*
+        * Fixups have not been to applied phys_base yet, so we must obtain
+        * the address to the SME command line option in the following way.
+        */
+       asm ("lea sme_cmdline_arg(%%rip), %0"
+            : "=r" (cmdline_arg)
+            : "p" (sme_cmdline_arg));
+       cmdline_ptr = bp->hdr.cmd_line_ptr | ((u64)bp->ext_cmd_line_ptr << 32);
+       if (cmdline_find_option_bool((char *)cmdline_ptr, cmdline_arg))
+               sme_me_mask = 1UL << (ebx & 0x3f);

If I parse this right, we will enable SME *only* if mem_encrypt=on is
explicitly supplied on the command line.

Which means, users will have to *know* about that cmdline switch first.
Which then means, we have to go and tell them. Do you see where I'm
going with this?

I know we talked about this already but I still think we should enable
it by default and people who don't want it will use the =off switch. We
can also do something like CONFIG_AMD_SME_ENABLED_BY_DEFAULT which we
can be selected during build for the different setups.

Hmmm.

-- 
Regards/Gruss,
    Boris.

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