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

[Jarkko Sakkinen]

On Tue, Sep 06, 2022 at 06:44:23PM +0300, Jarkko Sakkinen wrote:
> 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).

I created a small test program:

$ cat test.c
#include <stdio.h>
int main(void)
{
        unsigned long arr[] = {0x8, 0x1000, 0x200000, 0x40000000, 0x8000000000};
        int i;

        for (i = 1; i < sizeof(arr)/sizeof(unsigned long); i++) {
                printf("%048b\n", arr[i] - arr[i - 1]);
                printf("%048b\n", (arr[i] - 1) ^ (arr[i - 1] - 1));
        }
}

kultaheltta in linux on  host-snp-v7 [?]
$ gcc -o test test.c

kultaheltta in linux on  host-snp-v7 [?]
$ ./test
000000000000000000000000000000000000111111111000
000000000000000000000000000000000000111111111000
000000000000000000000000000111111111000000000000
000000000000000000000000000111111111000000000000
000000000000000000111111111000000000000000000000
000000000000000000111111111000000000000000000000
000000000000000011000000000000000000000000000000
000000000000000011000000000000000000000000000000

So the operation could be described as:

        pfn = PFN_DOWN(address & (~page_level_mask(level) ^ ~page_level_mask(level - 1)));

Which IMHO already documents itself quite well: index
with the granularity of PGD by removing bits used for
PGD's below it.

BR, Jarkko