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From:   Marc Orr <marcorr@google.com>
Date:   Tue, 6 Sep 2022 22:14:10 -0700
Message-ID: <CAA03e5ETjW-apRwDa-27YrcLUPy_QxJXu9zDKWOBHa1ZVJUfwg@mail.gmail.com>
Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address
To:     "Kalra, Ashish" <Ashish.Kalra@amd.com>
Cc:     "Roth, Michael" <Michael.Roth@amd.com>,
        Jarkko Sakkinen <jarkko@kernel.org>,
        Borislav Petkov <bp@alien8.de>, x86 <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kvm list <kvm@vger.kernel.org>,
        "linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        "Lendacky, Thomas" <Thomas.Lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
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> >> >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.
 */