linux-kernel - Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address

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Message-ID: <CAA03e5ETjW-apRwDa-27YrcLUPy_QxJXu9zDKWOBHa1ZVJUfwg@mail.gmail.com>
Date:   Tue, 6 Sep 2022 22:14:10 -0700
From:   Marc Orr <marcorr@...gle.com>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
Cc:     "Roth, Michael" <Michael.Roth@....com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Borislav Petkov <bp@...en8.de>, x86 <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        Linux Memory Management List <linux-mm@...ck.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        "Lendacky, Thomas" <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>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Alper Gun <alpergun@...gle.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>
Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address

> >> >But I believe Jarkko's version calculates the correct mask (below), incorporating all 18 offset bits into the 1G page.
> >> >>> hex(262144 -1)
> >> >'0x3ffff'
> >>
> >> We can get this simply by doing (page_per_hpage(level)-1), but as I mentioned above this is not what we need.
>
> >If we actually want the 4K page, I think we would want to use the 0x3ffff mask as Marc suggested to get to the specific 4K RMP entry, which I don't think the current code is trying to do. But maybe that *should* be what we should be doing.
>
> Ok, I agree to get to the specific 4K RMP entry.

Thanks, Michael, for a thorough and complete reply! I have to admit,
there was some nuance I missed in my earlier reply. But after reading
through what you wrote, I agree, always going to the 4k-entry to get
the "assigned" bit and also leveraging the implementation of
snp_lookup_rmpentry() to lookup the size bit in the 2M-aligned entry
seems like an elegant way to code this up. Assuming this suggestion
becomes the consensus, we might consider a comment in the source code
to capture this discussion. Otherwise, I think I'll forget all of this
the next time I'm reading this code :-). Something like:

/*
 * The guest-assigned bit is always propagated to the paddr's respective 4k RMP
 * entry -- regardless of the actual RMP page size. In contrast, the RMP page
 * size, handled in snp_lookup_rmpentry(), is indicated by the 2M-aligned RMP
 * entry.
 */