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References: <cover.1655761627.git.ashish.kalra@amd.com> <0ecb0a4781be933fcadeb56a85070818ef3566e7.1655761627.git.ashish.kalra@amd.com>
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From: Marc Orr <marcorr@google.com>
Date: Tue, 6 Sep 2022 06:54:30 -0700
Message-ID: <CAA03e5FgiLoixmqpKtfNOXM_0P5Y7LQzr3_oQe+2Z=GJ6kw32g@mail.gmail.com>
Subject: Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP
 fault for user address
To: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>, Ashish Kalra <Ashish.Kalra@amd.com>, x86 <x86@kernel.org>, 
	LKML <linux-kernel@vger.kernel.org>, kvm list <kvm@vger.kernel.org>, 
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On Tue, Sep 6, 2022 at 3:25 AM Jarkko Sakkinen <jarkko@kernel.org> wrote:
>
> 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'

But I believe Jarkko's version calculates the correct mask (below),
incorporating all 18 offset bits into the 1G page.
>>> hex(262144 -1)
'0x3ffff'