powerpc: Fix context switch DSCR on POWER8

POWER8 allows the DSCR to be accessed directly from userspace via a new SPR
number 0x3 (Rather than 0x11.  DSCR SPR number 0x11 is still used on POWER8 but
like POWER7, is only accessible in HV and OS modes).  Currently, we allow this
by setting H/FSCR DSCR bit on boot.

Unfortunately this doesn't work, as the kernel needs to see the DSCR change so
that it knows to no longer restore the system wide version of DSCR on context
switch (ie. to set thread.dscr_inherit).

This clears the H/FSCR DSCR bit initially.  If a process then accesses the DSCR
(via SPR 0x3), it'll trap into the kernel where we set thread.dscr_inherit in
facility_unavailable_exception().

We also change _switch() so that we set or clear the H/FSCR DSCR bit based on
the thread.dscr_inherit.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Cc: <stable@vger.kernel.org> [v3.10]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 34 insertions, 24 deletions

diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index bf33c22e38a4..e435bc089ea3 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -44,9 +44,7 @@
 #include <asm/machdep.h>
 #include <asm/rtas.h>
 #include <asm/pmc.h>
-#ifdef CONFIG_PPC32
 #include <asm/reg.h>
-#endif
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>
 #endif
@@ -1296,43 +1294,54 @@ void vsx_unavailable_exception(struct pt_regs *regs)
         die("Unrecoverable VSX Unavailable Exception", regs, SIGABRT);
 }
 
+#ifdef CONFIG_PPC64
 void facility_unavailable_exception(struct pt_regs *regs)
 {
         static char *facility_strings[] = {
-                "FPU",
-                "VMX/VSX",
-                "DSCR",
-                "PMU SPRs",
-                "BHRB",
-                "TM",
-                "AT",
-                "EBB",
-                "TAR",
+                [FSCR_FP_LG] = "FPU",
+                [FSCR_VECVSX_LG] = "VMX/VSX",
+                [FSCR_DSCR_LG] = "DSCR",
+                [FSCR_PM_LG] = "PMU SPRs",
+                [FSCR_BHRB_LG] = "BHRB",
+                [FSCR_TM_LG] = "TM",
+                [FSCR_EBB_LG] = "EBB",
+                [FSCR_TAR_LG] = "TAR",
         };
-        char *facility, *prefix;
+        char *facility = "unknown";
         u64 value;
+        u8 status;
+        bool hv;
 
-        if (regs->trap == 0xf60) {
-                value = mfspr(SPRN_FSCR);
-                prefix = "";
-        } else {
+        hv = (regs->trap == 0xf80);
+        if (hv)
                 value = mfspr(SPRN_HFSCR);
-                prefix = "Hypervisor ";
+        else
+                value = mfspr(SPRN_FSCR);
+
+        status = value >> 56;
+        if (status == FSCR_DSCR_LG) {
+                /* User is acessing the DSCR.  Set the inherit bit and allow
+                 * the user to set it directly in future by setting via the
+                 * H/FSCR DSCR bit.
+                 */
+                current->thread.dscr_inherit = 1;
+                if (hv)
+                        mtspr(SPRN_HFSCR, value | HFSCR_DSCR);
+                else
+                        mtspr(SPRN_FSCR,  value | FSCR_DSCR);
+                return;
         }
 
-        value = value >> 56;
+        if ((status < ARRAY_SIZE(facility_strings)) &&
+            facility_strings[status])
+                facility = facility_strings[status];
 
         /* We restore the interrupt state now */
         if (!arch_irq_disabled_regs(regs))
                 local_irq_enable();
 
-        if (value < ARRAY_SIZE(facility_strings))
-                facility = facility_strings[value];
-        else
-                facility = "unknown";
-
         pr_err("%sFacility '%s' unavailable, exception at 0x%lx, MSR=%lx\n",
-                prefix, facility, regs->nip, regs->msr);
+               hv ? "Hypervisor " : "", facility, regs->nip, regs->msr);
 
         if (user_mode(regs)) {
                 _exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
@@ -1341,6 +1350,7 @@ void facility_unavailable_exception(struct pt_regs *regs)
 
         die("Unexpected facility unavailable exception", regs, SIGABRT);
 }
+#endif
 
 #ifdef CONFIG_PPC_TRANSACTIONAL_MEM