linux-kernel - Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when changing C-bit

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Date:   Sun, 13 Feb 2022 13:15:23 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>,
        "Kirill A . Shutemov" <kirill@...temov.name>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v10 21/45] x86/mm: Add support to validate memory when
 changing C-bit

On Fri, Feb 11, 2022 at 11:27:54AM -0600, Brijesh Singh wrote:
> > Simply have them always present. They will have !0 values on the
> > respective guest types and 0 otherwise. This should simplify a lot of
> > code and another unconditionally present u64 won't be the end of the
> > world.
> >
> > Any other aspect I'm missing?
> 
> I think that's mostly about it. IIUC, the recommendation is to define a
> new callback in x86_platform_op. The callback will be invoked
> unconditionally; The default implementation for this callback is NOP;
> The TDX and SEV will override with the platform specific implementation.
> I think we may able to handle everything in one callback hook but having
> pre and post will be a more desirable. Here is why I am thinking so:
> 
> * On SNP, the page must be invalidated before clearing the _PAGE_ENC
> from the page table attribute
> 
> * On SNP, the page must be validated after setting the _PAGE_ENC in the
> page table attribute.

Right, we could have a pre- and post- callback, if that would make
things simpler/clearer.

Also, in thinking further about the encryption mask, we could make it a
*single*, *global* variable called cc_mask which each guest type sets it
as it wants to.

Then, it would use it in the vendor-specific encrypt/decrypt helpers
accordingly and that would simplify a lot of code. And we can get rid of
all the ifdeffery around it too.

So I think the way to go should be we do the common functionality, I
queue it on the common tip:x86/cc branch and then SNP and TDX will be
both based ontop of it.

Thoughts?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette