Messages in this thread |  | | Date | Tue, 6 Sep 2022 18:44:23 +0300 | From | Jarkko Sakkinen <> | Subject | Re: [PATCH Part2 v6 09/49] x86/fault: Add support to handle the RMP fault for user address |
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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
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