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

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Date:   Tue, 15 Feb 2022 15:43:31 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Brijesh Singh <brijesh.singh@....com>, 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 Sun, Feb 13, 2022 at 01:15:23PM +0100, Borislav Petkov wrote:
> 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.

I don't think it works. TDX and SME/SEV has opposite polarity of the mask.
SME/SEV has to clear the mask to share the page. TDX has to set it.

Making a single global mask only increases confusion.

-- 
 Kirill A. Shutemov