linux-kernel - Re: [PATCH v8 13/40] x86/kernel: Make the bss.decrypted section shared in RMP table

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Message-ID: <444b6c78-c4d3-f49c-6579-bd28ae32ca3c@amd.com>
Date:   Wed, 5 Jan 2022 15:39:22 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Dave Hansen <dave.hansen@...el.com>,
        Venu Busireddy <venu.busireddy@...cle.com>
Cc:     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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 13/40] x86/kernel: Make the bss.decrypted section
 shared in RMP table



On 1/5/22 2:27 PM, Dave Hansen wrote:
> On 1/5/22 11:52, Brijesh Singh wrote:
>>>>           for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
>>>> +            /*
>>>> +             * When SEV-SNP is active then transition the page to 
>>>> shared in the RMP
>>>> +             * table so that it is consistent with the page table 
>>>> attribute change.
>>>> +             */
>>>> +            early_snp_set_memory_shared(__pa(vaddr), __pa(vaddr), 
>>>> PTRS_PER_PMD);
>>>
>>> Shouldn't the first argument be vaddr as below?
>>
>> Nope, sme_postprocess_startup() is called while we are fixing the 
>> initial page table and running with identity mapping (so va == pa).
> 
> I'm not sure I've ever seen a line of code that wanted a comment so badly.

The early_snp_set_memory_shared() call the PVALIDATE instruction to 
clear the validated bit from the BSS region. The PVALIDATE instruction 
needs a virtual address, so we need to use the identity mapped virtual 
address so that PVALIDATE can clear the validated bit. I will add more 
comments to clarify it.

-Brijesh