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

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Date:   Tue, 6 Sep 2022 18:44:23 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     "Kalra, Ashish" <Ashish.Kalra@....com>
Cc:     Marc Orr <marcorr@...gle.com>, 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>,
        "Roth, Michael" <Michael.Roth@....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

On Tue, Sep 06, 2022 at 02:17:15PM +0000, Kalra, Ashish wrote:
> [AMD Official Use Only - General]
> 
> >> On Tue, Aug 09, 2022 at 06:55:43PM +0200, Borislav Petkov wrote:
> >> > On Mon, Jun 20, 2022 at 11:03:43PM +0000, Ashish Kalra wrote:
> >> > > +   pfn = pte_pfn(*pte);
> >> > > +
> >> > > +   /* If its large page then calculte the fault pfn */
> >> > > +   if (level > PG_LEVEL_4K) {
> >> > > +           unsigned long mask;
> >> > > +
> >> > > +           mask = pages_per_hpage(level) - pages_per_hpage(level - 1);
> >> > > +           pfn |= (address >> PAGE_SHIFT) & mask;
> >> >
> >> > Oh boy, this is unnecessarily complicated. Isn't this
> >> >
> >> >       pfn |= pud_index(address);
> >> >
> >> > or
> >> >       pfn |= pmd_index(address);
> >>
> >> I played with this a bit and ended up with
> >>
> >>         pfn = pte_pfn(*pte) | PFN_DOWN(address & page_level_mask(level 
> >> - 1));
> >>
> >> Unless I got something terribly wrong, this should do the same (see 
> >> the attached patch) as the existing calculations.
> 
> >Actually, I don't think they're the same. I think Jarkko's version is correct. Specifically:
> >- For level = PG_LEVEL_2M they're the same.
> >- For level = PG_LEVEL_1G:
> >The current code calculates a garbage mask:
> >mask = pages_per_hpage(level) - pages_per_hpage(level - 1); translates to:
> >>> hex(262144 - 512)
> >'0x3fe00'
> 
> No actually this is not a garbage mask, as I explained in earlier responses we need to capture the address bits 
> to get to the correct 4K index into the RMP table.
> Therefore, for level = PG_LEVEL_1G:
> mask = pages_per_hpage(level) - pages_per_hpage(level - 1) => 0x3fe00 (which is the correct mask).
> 
> >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.

I think you're correct, so I'll retry:

(address / PAGE_SIZE) & (pages_per_hpage(level) - pages_per_hpage(level - 1)) =

(address / PAGE_SIZE) & ((page_level_size(level) / PAGE_SIZE) - (page_level_size(level - 1) / PAGE_SIZE)) =

[ factor out 1 / PAGE_SIZE ]

(address & (page_level_size(level) - page_level_size(level - 1))) / PAGE_SIZE  =

[ Substitute with PFN_DOWN() ] 

PFN_DOWN(address & (page_level_size(level) - page_level_size(level - 1)))

So you can just:

pfn = pte_pfn(*pte) | PFN_DOWN(address & (page_level_size(level) - page_level_size(level - 1)));

Which is IMHO way better still what it is now because no branching
and no ad-hoc helpers (the current is essentially just page_level_size
wrapper).

BR, Jarkko