[Xen-devel] [PATCH v2 3/6] x86/mtrr: Fix Xorg crashes in Qemu sessions

[Toshi Kani <toshi.kani@xxxxxxx>]

A Xorg failure on qemu32 was reported as a regression caused
by 'commit 9cd25aac1f44 ("x86/mm/pat: Emulate PAT when it is
disabled")'. [1]  This patch fixes the regression.

Negative effects of this regression were two failures in Xorg
on qemu32 env, which were triggered by the fact that its virtual
CPU does not support MTRR. [2]

 #1. copy_process() failed in the check in reserve_pfn_range()

    copy_process
     copy_mm
      dup_mm
       dup_mmap
        copy_page_range
         track_pfn_copy
          reserve_pfn_range

 #2. error path in copy_process() then hit WARN_ON_ONCE in
     untrack_pfn().

     x86/PAT: Xorg:509 map pfn expected mapping type uncached-
     minus for [mem 0xfd000000-0xfdffffff], got write-combining
      Call Trace:
     dump_stack+0x58/0x79
     warn_slowpath_common+0x8b/0xc0
     ? untrack_pfn+0x9f/0xb0
     ? untrack_pfn+0x9f/0xb0
     warn_slowpath_null+0x22/0x30
     untrack_pfn+0x9f/0xb0
     ? __kunmap_atomic+0x54/0x110
     unmap_single_vma+0x56f/0x580
     ? pagevec_move_tail_fn+0xa0/0xa0
     unmap_vmas+0x43/0x60
     exit_mmap+0x5f/0xf0
     mmput+0x2d/0xa0
     copy_process.part.47+0x1229/0x1430
     _do_fork+0xb4/0x3b0
     SyS_clone+0x2c/0x30
     do_syscall_32_irqs_on+0x54/0xb0
     entry_INT80_32+0x2a/0x2a

These negative effects are caused by two separate bugs, but they
can be dealt in lower priority.  Fixing the pat_init() issue below
avoids Xorg to hit these cases.

When the CPU does not support MTRR, MTRR does not call pat_init(),
which leaves PAT enabled without initializing PAT.  This pat_init()
issue is a long-standing issue, but manifested as issue #1 (and then
hit issue #2) with the commit because the memtype now tracks cache
attribute with 'page_cache_mode'.  A WC map request is tracked as WC
in memtype, but sets a PTE as UC (pgprot) per __cachemode2pte_tbl[].
This caused the error in reserve_pfn_range() when it was called from
track_pfn_copy(), which obtained pgprot from a PTE.  It converts
pgprot to page_cache_mode, which does not necessarily result in
the original page_cache_mode since __cachemode2pte_tbl[] redirects
multiple types to UC.  This is a separate issue in reserve_pfn_range().

This pat_init() issue existed before the commit, but we used pgprot
in memtype.  Hence, we did not have issue #1 before.  But WC request
resulted in WT in effect because WC pgrot is actually WT when PAT
is not initialized.  This is not how it was designed to work.  When
PAT is set to disable properly, WC is converted to UC.  The use of
WT can result in a system crash if the target range does not support
WT.  Fortunately, nobody ran into such issue before.

To fix this pat_init() issue, PAT code has been enhanced to provide
pat_disable() interface, which disables the OS to initialize PAT MSR,
and sets PAT table to the BIOS handoff state.  This patch changes
MTRR code to call pat_disable() when MTRR is disabled as PAT cannot
be initialized in this case.  This sets PAT to disable properly, and
makes PAT code to bypass the memtype check.  This avoids issue #1
(which can be dealt in lower priority).

[1]: https://lkml.org/lkml/2016/3/3/828
[2]: https://lkml.org/lkml/2016/3/4/775
Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Luis R. Rodriguez <mcgrof@xxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 arch/x86/include/asm/mtrr.h     |    6 +++++-
 arch/x86/kernel/cpu/mtrr/main.c |   10 +++++++++-
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/mtrr.h b/arch/x86/include/asm/mtrr.h
index b94f6f6..634c593 100644
--- a/arch/x86/include/asm/mtrr.h
+++ b/arch/x86/include/asm/mtrr.h
@@ -24,6 +24,7 @@
 #define _ASM_X86_MTRR_H
 
 #include <uapi/asm/mtrr.h>
+#include <asm/pat.h>
 
 
 /*
@@ -83,9 +84,12 @@ static inline int mtrr_trim_uncached_memory(unsigned long 
end_pfn)
 static inline void mtrr_centaur_report_mcr(int mcr, u32 lo, u32 hi)
 {
 }
+static inline void mtrr_bp_init(void)
+{
+       pat_disable("Skip PAT initialization");
+}
 
 #define mtrr_ap_init() do {} while (0)
-#define mtrr_bp_init() do {} while (0)
 #define set_mtrr_aps_delayed_init() do {} while (0)
 #define mtrr_aps_init() do {} while (0)
 #define mtrr_bp_restore() do {} while (0)
diff --git a/arch/x86/kernel/cpu/mtrr/main.c b/arch/x86/kernel/cpu/mtrr/main.c
index 10f8d47..2d7d8d7 100644
--- a/arch/x86/kernel/cpu/mtrr/main.c
+++ b/arch/x86/kernel/cpu/mtrr/main.c
@@ -759,8 +759,16 @@ void __init mtrr_bp_init(void)
                }
        }
 
-       if (!mtrr_enabled())
+       if (!mtrr_enabled()) {
                pr_info("MTRR: Disabled\n");
+
+               /*
+                * PAT initialization relies on MTRR's rendezvous handler.
+                * Skip PAT init until the handler can initialize both
+                * features independently.
+                */
+               pat_disable("Skip PAT initialization");
+       }
 }
 
 void mtrr_ap_init(void)

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