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[Xen-devel] [PATCH v2] x86/mm/pat: Fix BUG_ON in mmap_mem on QEMU/i386

The following BUG_ON error was reported on QEMU/i386:

  kernel BUG at arch/x86/mm/physaddr.c:79!
  Call Trace:
  ? mmap_region

after commit edfe63ec97ed ("x86/mtrr: Fix Xorg crashes in
Qemu sessions").

PAT is now set to disabled state when MTRRs are disabled.
Thus, reactivating the __pa(high_memory) check in

When CONFIG_DEBUG_VIRTUAL is set, __pa() calls __phys_addr(),
which in turn calls slow_virt_to_phys() for 'high_memory'.
Because 'high_memory' is set to (the max direct mapped virt
addr + 1), it is not a valid virtual address.  Hence,
slow_virt_to_phys() returns 0 and hit the BUG_ON.  Using
__pa_nodebug() instead of __pa() will fix this BUG_ON.

However, this code block, originally written for Pentiums and
earlier, is no longer adequate since a 32-bit Xen guest has
MTRRs disabled and supports ZONE_HIGHMEM.  In this setup,
this code sets UC attribute for accessing RAM in high memory

Delete this code block as it has been unused for a long time.

Reported-by: kernel test robot <ying.huang@xxxxxxxxxxxxxxx>
Link: https://lkml.org/lkml/2016/4/1/608
Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: David Vrabel <david.vrabel@xxxxxxxxxx>
This patch is based on -tip.
v2: Change to remove the code block.
 arch/x86/mm/pat.c |   19 -------------------
 1 file changed, 19 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index c4c3ddc..fb0604f 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -778,25 +778,6 @@ int phys_mem_access_prot_allowed(struct file *file, 
unsigned long pfn,
        if (file->f_flags & O_DSYNC)
                pcm = _PAGE_CACHE_MODE_UC_MINUS;
-#ifdef CONFIG_X86_32
-       /*
-        * On the PPro and successors, the MTRRs are used to set
-        * memory types for physical addresses outside main memory,
-        * so blindly setting UC or PWT on those pages is wrong.
-        * For Pentiums and earlier, the surround logic should disable
-        * caching for the high addresses through the KEN pin, but
-        * we maintain the tradition of paranoia in this code.
-        */
-       if (!pat_enabled() &&
-           !(boot_cpu_has(X86_FEATURE_MTRR) ||
-             boot_cpu_has(X86_FEATURE_K6_MTRR) ||
-             boot_cpu_has(X86_FEATURE_CYRIX_ARR) ||
-             boot_cpu_has(X86_FEATURE_CENTAUR_MCR)) &&
-           (pfn << PAGE_SHIFT) >= __pa(high_memory)) {
-               pcm = _PAGE_CACHE_MODE_UC;
-       }
        *vma_prot = __pgprot((pgprot_val(*vma_prot) & ~_PAGE_CACHE_MASK) |
        return 1;

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