linux-kernel - Re: [PATCH RFC v8 15/56] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

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Date:   Wed, 1 Mar 2023 08:15:46 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Michael Roth <michael.roth@....com>, kvm@...r.kernel.org
Cc:     linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, x86@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
        rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
        bp@...en8.de, vbabka@...e.cz, kirill@...temov.name,
        ak@...ux.intel.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH RFC v8 15/56] x86/sev: Invalidate pages from the direct
 map when adding them to the RMP table

On 2/20/23 10:38, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@....com>
> 
> The integrity guarantee of SEV-SNP is enforced through the RMP table.
> The RMP is used with standard x86 and IOMMU page tables to enforce
> memory restrictions and page access rights. The RMP check is enforced as
> soon as SEV-SNP is enabled globally in the system. When hardware
> encounters an RMP-check failure, it raises a page-fault exception.
> 
> The rmp_make_private() and rmp_make_shared() helpers are used to add
> or remove the pages from the RMP table. Improve the rmp_make_private()
> to invalidate state so that pages cannot be used in the direct-map after
> they are added the RMP table, and restored to their default valid
> permission after the pages are removed from the RMP table.

This is a purely "what" changelog.  It doesn't explain the "why" at all.

Could you please elaborate on why this unmapping operation is necessary?