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powerpc: Fix unrecoverable SLB miss during restore_math()

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Commit 70fe3d9 "powerpc: Restore FPU/VEC/VSX if previously used" introduces a
call to restore_math() late in the syscall return path, after MSR_RI has been
cleared. The MSR_RI flag is used to indicate whether the kernel can take
another exception or not. A cleared MSR_RI flag indicates that the kernel
cannot.

Unfortunately when a machine is under SLB pressure an SLB miss can occur
in restore_math() which (with MSR_RI cleared) leads to an unrecoverable
exception.

  Unrecoverable exception 4100 at c0000000000088d8
  cpu 0x0: Vector: 4100  at [c0000003fa473b20]
      pc: c0000000000088d8: .load_vr_state+0x70/0x110
      lr: c00000000000f710: .restore_math+0x130/0x188
      sp: c0000003fa473da0
     msr: 9000000002003030
    current = 0xc0000007f876f180
    paca    = 0xc00000000fff0000	 softe: 0	 irq_happened: 0x01
      pid   = 1944, comm = K08umountfs
  [link register   ] c00000000000f710 .restore_math+0x130/0x188
  [c0000003fa473da0] c0000003fa473e30 (unreliable)
  [c0000003fa473e30] c000000000007b6c system_call+0x84/0xfc

The clearing of MSR_RI is actually an optimisation to avoid multiple MSR
writes, what must be disabled are interrupts. See comment in entry_64.S:

  /*
   * For performance reasons we clear RI the same time that we
   * clear EE. We only need to clear RI just before we restore r13
   * below, but batching it with EE saves us one expensive mtmsrd call.
   * We have to be careful to restore RI if we branch anywhere from
   * here (eg syscall_exit_work).
   */

At the point of calling restore_math() r13 has not been restored, as such, the
quick fix of turning MSR_RI back on for the call to restore_math() will
eliminate the occurrence of an unrecoverable exception.

We'd like to do a better fix in future.

Fixes: 70fe3d980f ("powerpc: Restore FPU/VEC/VSX if previously used")
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Cyril Bur 2016-03-16 13:29:30 +11:00 committed by Michael Ellerman
parent 2e098dcee6
commit 6e669f085d
1 changed files with 9 additions and 0 deletions
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9
arch/powerpc/kernel/entry_64.S
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@ -218,7 +218,16 @@ system_call: /* label this so stack traces look sane */
bne 3f
#endif
2: addi r3,r1,STACK_FRAME_OVERHEAD
#ifdef CONFIG_PPC_BOOK3S
mtmsrd r10,1 /* Restore RI */
#endif
bl restore_math
#ifdef CONFIG_PPC_BOOK3S
ld r10,PACAKMSR(r13)
li r9,MSR_RI
andc r11,r10,r9 /* Re-clear RI */
mtmsrd r11,1
#endif
ld r8,_MSR(r1)
ld r3,RESULT(r1)
li r11,-MAX_ERRNO