linux-kernel - Re: [PATCH v5 31/32] x86: Add sysfs support for Secure Memory Encryption

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Date:   Fri, 28 Apr 2017 13:32:33 +0800
From:   Dave Young <dyoung@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kasan-dev@...glegroups.com, linux-mm@...ck.org,
        iommu@...ts.linux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Rik van Riel <riel@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Jonathan Corbet <corbet@....net>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Joerg Roedel <joro@...tes.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Ingo Molnar <mingo@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Alexander Potapenko <glider@...gle.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH v5 31/32] x86: Add sysfs support for Secure Memory
 Encryption

On 04/27/17 at 08:52am, Dave Hansen wrote:
> On 04/27/2017 12:25 AM, Dave Young wrote:
> > On 04/21/17 at 02:55pm, Dave Hansen wrote:
> >> On 04/18/2017 02:22 PM, Tom Lendacky wrote:
> >>> Add sysfs support for SME so that user-space utilities (kdump, etc.) can
> >>> determine if SME is active.
> >>>
> >>> A new directory will be created:
> >>>   /sys/kernel/mm/sme/
> >>>
> >>> And two entries within the new directory:
> >>>   /sys/kernel/mm/sme/active
> >>>   /sys/kernel/mm/sme/encryption_mask
> >>
> >> Why do they care, and what will they be doing with this information?
> > 
> > Since kdump will copy old memory but need this to know if the old memory
> > was encrypted or not. With this sysfs file we can know the previous SME
> > status and pass to kdump kernel as like a kernel param.
> > 
> > Tom, have you got chance to try if it works or not?
> 
> What will the kdump kernel do with it though?  We kexec() into that
> kernel so the SME keys will all be the same, right?  So, will the kdump
> kernel be just setting the encryption bit in the PTE so it can copy the
> old plaintext out?

I assume it is for active -> non active case, the new boot need to know
the old memory is encrypted. But I think I did not read all the patches
I may miss things.

> 
> Why do we need both 'active' and 'encryption_mask'?  How could it be
> that the hardware-enumerated 'encryption_mask' changes across a kexec()?

Leave this question to Tom..

Thanks
Dave