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

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Date:   Thu, 6 Jan 2022 14:16:14 -0600
From:   Venu Busireddy <venu.busireddy@...cle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Brijesh Singh <brijesh.singh@....com>,
        Dave Hansen <dave.hansen@...el.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>,
        "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 2022-01-06 13:06:13 -0600, Tom Lendacky wrote:
> On 1/6/22 11:40 AM, Venu Busireddy wrote:
> > On 2022-01-05 15:39:22 -0600, Brijesh Singh wrote:
> > > 
> > > 
> > > 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.
> > 
> > Looking forward to see your final comments explaining this. I can't
> > still follow why, when PVALIDATE needs the virtual address, we are doing
> > a __pa() on the vaddr.
> 
> It's because of the phase of booting that the kernel is in. At this point,
> the kernel is running in identity mapped mode (VA == PA). The
> __start_bss_decrypted address is a regular kernel address, e.g. for the
> kernel I'm on it is 0xffffffffa7600000. Since the PVALIDATE instruction
> requires a valid virtual address, the code needs to perform a __pa() against
> __start_bss_decrypted to get the identity mapped virtual address that is
> currently in place.

Perhaps  my confusion stems from the fact that __pa(x) is defined either
as "((unsigned long ) (x))" (for the cases where paddr and vaddr are
same), or as "__phys_addr((unsigned long )(x))", where a vaddr needs to
be converted to a paddr. If the paddr and vaddr are same in our case,
what exactly is the _pa(vaddr) doing to the vaddr?

Venu