linux/arch/x86/mm
Dave Hansen 39114b7a74 x86/pti: Never implicitly clear _PAGE_GLOBAL for kernel image
Summary:

In current kernels, with PTI enabled, no pages are marked Global. This
potentially increases TLB misses.  But, the mechanism by which the Global
bit is set and cleared is rather haphazard.  This patch makes the process
more explicit.  In the end, it leaves us with Global entries in the page
tables for the areas truly shared by userspace and kernel and increases
TLB hit rates.

The place this patch really shines in on systems without PCIDs.  In this
case, we are using an lseek microbenchmark[1] to see how a reasonably
non-trivial syscall behaves.  Higher is better:

  No Global pages (baseline): 6077741 lseeks/sec
  88 Global Pages (this set): 7528609 lseeks/sec (+23.9%)

On a modern Skylake desktop with PCIDs, the benefits are tangible, but not
huge for a kernel compile (lower is better):

  No Global pages (baseline): 186.951 seconds time elapsed  ( +-  0.35% )
  28 Global pages (this set): 185.756 seconds time elapsed  ( +-  0.09% )
                               -1.195 seconds (-0.64%)

I also re-checked everything using the lseek1 test[1]:

  No Global pages (baseline): 15783951 lseeks/sec
  28 Global pages (this set): 16054688 lseeks/sec
			     +270737 lseeks/sec (+1.71%)

The effect is more visible, but still modest.

Details:

The kernel page tables are inherited from head_64.S which rudely marks
them as _PAGE_GLOBAL.  For PTI, we have been relying on the grace of
$DEITY and some insane behavior in pageattr.c to clear _PAGE_GLOBAL.
This patch tries to do better.

First, stop filtering out "unsupported" bits from being cleared in the
pageattr code.  It's fine to filter out *setting* these bits but it
is insane to keep us from clearing them.

Then, *explicitly* go clear _PAGE_GLOBAL from the kernel identity map.
Do not rely on pageattr to do it magically.

After this patch, we can see that "GLB" shows up in each copy of the
page tables, that we have the same number of global entries in each
and that they are the *same* entries.

  /sys/kernel/debug/page_tables/current_kernel:11
  /sys/kernel/debug/page_tables/current_user:11
  /sys/kernel/debug/page_tables/kernel:11

  9caae8ad6a1fb53aca2407ec037f612d  current_kernel.GLB
  9caae8ad6a1fb53aca2407ec037f612d  current_user.GLB
  9caae8ad6a1fb53aca2407ec037f612d  kernel.GLB

A quick visual audit also shows that all the entries make sense.
0xfffffe0000000000 is the cpu_entry_area and 0xffffffff81c00000
is the entry/exit text:

  0xfffffe0000000000-0xfffffe0000002000           8K     ro                 GLB NX pte
  0xfffffe0000002000-0xfffffe0000003000           4K     RW                 GLB NX pte
  0xfffffe0000003000-0xfffffe0000006000          12K     ro                 GLB NX pte
  0xfffffe0000006000-0xfffffe0000007000           4K     ro                 GLB x  pte
  0xfffffe0000007000-0xfffffe000000d000          24K     RW                 GLB NX pte
  0xfffffe000002d000-0xfffffe000002e000           4K     ro                 GLB NX pte
  0xfffffe000002e000-0xfffffe000002f000           4K     RW                 GLB NX pte
  0xfffffe000002f000-0xfffffe0000032000          12K     ro                 GLB NX pte
  0xfffffe0000032000-0xfffffe0000033000           4K     ro                 GLB x  pte
  0xfffffe0000033000-0xfffffe0000039000          24K     RW                 GLB NX pte
  0xffffffff81c00000-0xffffffff81e00000           2M     ro         PSE     GLB x  pmd

[1.] https://github.com/antonblanchard/will-it-scale/blob/master/tests/lseek1.c

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <namit@vmware.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/20180406205517.C80FBE05@viggo.jf.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
2018-04-12 09:06:00 +02:00
..
amdtopology.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
cpu_entry_area.c x86/pti: Enable global pages for shared areas 2018-04-12 09:05:59 +02:00
debug_pagetables.c x86/efi: Replace efi_pgd with efi_mm.pgd 2018-03-12 11:05:05 +01:00
dump_pagetables.c x86/mm: Consider effective protection attributes in W+X check 2018-02-26 08:43:21 +01:00
extable.c x86/extable: Mark exception handler functions visible 2018-01-14 20:04:16 +01:00
fault.c Merge branch 'x86-mm-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2018-04-02 15:45:30 -07:00
highmem_32.c x86/mm: Audit and remove any unnecessary uses of module.h 2016-07-14 13:04:20 +02:00
hugetlbpage.c x86/mm: Prevent non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border 2017-11-16 11:43:11 +01:00
ident_map.c x86/mm: Do not auto-massage page protections 2018-04-12 09:04:22 +02:00
init_32.c x86/mm: Introduce "default" kernel PTE mask 2018-04-09 18:27:32 +02:00
init_64.c x86/mm: Introduce "default" kernel PTE mask 2018-04-09 18:27:32 +02:00
init.c x86/pti: Never implicitly clear _PAGE_GLOBAL for kernel image 2018-04-12 09:06:00 +02:00
iomap_32.c x86/mm: Do not auto-massage page protections 2018-04-12 09:04:22 +02:00
ioremap.c x86/mm: Do not auto-massage page protections 2018-04-12 09:04:22 +02:00
kasan_init_64.c x86/mm: Do not auto-massage page protections 2018-04-12 09:04:22 +02:00
kaslr.c x86/mm: Replace compile-time checks for 5-level paging with runtime-time checks 2018-02-16 10:48:49 +01:00
kmmio.c x86/mm: Rename flush_tlb_single() and flush_tlb_one() to __flush_tlb_one_[user|kernel]() 2018-02-15 01:15:52 +01:00
Makefile x86/mm/sme: Disable stack protection for mem_encrypt_identity.c 2018-02-28 15:24:12 +01:00
mem_encrypt_boot.S x86/mm/sme, objtool: Annotate indirect call in sme_encrypt_execute() 2018-02-21 09:05:04 +01:00
mem_encrypt_identity.c x86/mm/sme: Disable stack protection for mem_encrypt_identity.c 2018-02-28 15:24:12 +01:00
mem_encrypt.c dma/direct: Handle force decryption for DMA coherent buffers in common code 2018-03-20 10:01:59 +01:00
mm_internal.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
mmap.c x86/mm: Limit mmap() of /dev/mem to valid physical addresses 2017-11-16 12:49:48 +01:00
mmio-mod.c x86: do not use print_symbol() 2018-01-05 15:23:01 +01:00
mpx.c Merge branch 'x86/mpx' into x86/asm, to pick up dependent commits 2017-11-08 10:55:48 +01:00
numa_32.c x86/mm/32: Remove unused node_memmap_size_bytes() & CONFIG_NEED_NODE_MEMMAP_SIZE logic 2018-03-27 08:45:02 +02:00
numa_64.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
numa_emulation.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
numa_internal.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
numa.c Merge branch 'x86/boot' into x86/mm, to avoid conflict 2017-04-11 08:56:05 +02:00
pageattr-test.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
pageattr.c x86/pti: Never implicitly clear _PAGE_GLOBAL for kernel image 2018-04-12 09:06:00 +02:00
pat_internal.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
pat_rbtree.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
pat.c x86/mm: add a function to check if a pfn is UC/UC-/WC 2018-01-16 16:40:09 +01:00
pf_in.c x86/mm: Audit and remove any unnecessary uses of module.h 2016-07-14 13:04:20 +02:00
pf_in.h
pgtable_32.c x86/mm: Rename flush_tlb_single() and flush_tlb_one() to __flush_tlb_one_[user|kernel]() 2018-02-15 01:15:52 +01:00
pgtable.c x86/mm: Do not auto-massage page protections 2018-04-12 09:04:22 +02:00
physaddr.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
physaddr.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
pkeys.c x86/fpu: Rename fpu::fpstate_active to fpu::initialized 2017-09-26 09:43:36 +02:00
pti.c x86/pti: Never implicitly clear _PAGE_GLOBAL for kernel image 2018-04-12 09:06:00 +02:00
setup_nx.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
srat.c License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
testmmiotrace.c Annotate hardware config module parameters in arch/x86/mm/ 2017-04-04 16:54:21 +01:00
tlb.c Merge branch 'x86/mm' into efi/core 2018-03-12 10:03:09 +01:00