arm64: Rework ARM_ERRATUM_1414080 handling

The current handling of erratum 1414080 has the side effect that
cntkctl_el1 can get changed for both 32 and 64bit tasks.

This isn't a problem so far, but if we ever need to mitigate another
of these errata on the 64bit side, we'd better keep the messing with
cntkctl_el1 local to 32bit tasks.

For that, make sure that on entering the kernel from a 32bit tasks,
userspace access to cntvct gets enabled, and disabled returning to
userspace, while it never gets changed for 64bit tasks.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20200706163802.1836732-5-maz@kernel.org
[will: removed branch instructions per Mark's review comments]
Signed-off-by: Will Deacon <will@kernel.org>

diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
index 5304d19..9757a8d 100644 (file)
--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -167,6 +167,17 @@ alternative_cb_end
        stp     x28, x29, [sp, #16 * 14]
 
        .if     \el == 0
+       .if     \regsize == 32
+       // If we're returning from a 32-bit task on a system affected by
+       // 1418040 then re-enable userspace access to the virtual counter.
+#ifdef CONFIG_ARM64_ERRATUM_1418040
+alternative_if ARM64_WORKAROUND_1418040
+       mrs     x0, cntkctl_el1
+       orr     x0, x0, #2      // ARCH_TIMER_USR_VCT_ACCESS_EN
+       msr     cntkctl_el1, x0
+alternative_else_nop_endif
+#endif
+       .endif
        clear_gp_regs
        mrs     x21, sp_el0
        ldr_this_cpu    tsk, __entry_task, x20
@@ -320,6 +331,14 @@ alternative_else_nop_endif
        tst     x22, #PSR_MODE32_BIT            // native task?
        b.eq    3f
 
+#ifdef CONFIG_ARM64_ERRATUM_1418040
+alternative_if ARM64_WORKAROUND_1418040
+       mrs     x0, cntkctl_el1
+       bic     x0, x0, #2                      // ARCH_TIMER_USR_VCT_ACCESS_EN
+       msr     cntkctl_el1, x0
+alternative_else_nop_endif
+#endif
+
 #ifdef CONFIG_ARM64_ERRATUM_845719
 alternative_if ARM64_WORKAROUND_845719
 #ifdef CONFIG_PID_IN_CONTEXTIDR
@@ -331,21 +350,6 @@ alternative_if ARM64_WORKAROUND_845719
 alternative_else_nop_endif
 #endif
 3:
-#ifdef CONFIG_ARM64_ERRATUM_1418040
-alternative_if_not ARM64_WORKAROUND_1418040
-       b       4f
-alternative_else_nop_endif
-       /*
-        * if (x22.mode32 == cntkctl_el1.el0vcten)
-        *     cntkctl_el1.el0vcten = ~cntkctl_el1.el0vcten
-        */
-       mrs     x1, cntkctl_el1
-       eon     x0, x1, x22, lsr #3
-       tbz     x0, #1, 4f
-       eor     x1, x1, #2      // ARCH_TIMER_USR_VCT_ACCESS_EN
-       msr     cntkctl_el1, x1
-4:
-#endif
        scs_save tsk, x0
 
        /* No kernel C function calls after this as user keys are set. */