powerpc: Fix unrecoverable SLB miss during restore_math()

cyrilbur-ibm · mpe · commit 6e669f085d59 · 2016-03-16T15:23:02.000+11:00
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: 70fe3d9 ("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>
diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
@@ -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
