arm64: kernel: enforce pmuserenr_el0 initialization and restore

The pmuserenr_el0 register value is architecturally UNKNOWN on reset.
Current kernel code resets that register value iff the core pmu device is
correctly probed in the kernel. On platforms with missing DT pmu nodes (or
disabled perf events in the kernel), the pmu is not probed, therefore the
pmuserenr_el0 register is not reset in the kernel, which means that its
value retains the reset value that is architecturally UNKNOWN (system
may run with eg pmuserenr_el0 == 0x1, which means that PMU counters access
is available at EL0, which must be disallowed).

This patch adds code that resets pmuserenr_el0 on cold boot and restores
it on core resume from shutdown, so that the pmuserenr_el0 setup is
always enforced in the kernel.

Cc: <stable@vger.kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/perf_event.c

@@ -705,9 +705,6 @@ static void armv8pmu_reset(void *info)
 
 	/* Initialize & Reset PMNC: C and P bits. */
 	armv8pmu_pmcr_write(ARMV8_PMCR_P | ARMV8_PMCR_C);
-
-	/* Disable access from userspace. */
-	asm volatile("msr pmuserenr_el0, %0" :: "r" (0));
 }
 
 static int armv8_pmuv3_map_event(struct perf_event *event)

arch/arm64/mm/proc.S

@@ -117,6 +117,7 @@ ENTRY(cpu_do_resume)
 	 */
 	ubfx	x11, x11, #1, #1
 	msr	oslar_el1, x11
+	msr	pmuserenr_el0, xzr		// Disable PMU access from EL0
 	mov	x0, x12
 	dsb	nsh		// Make sure local tlb invalidation completed
 	isb
@@ -155,6 +156,7 @@ ENTRY(__cpu_setup)
 	msr	cpacr_el1, x0			// Enable FP/ASIMD
 	mov	x0, #1 << 12			// Reset mdscr_el1 and disable
 	msr	mdscr_el1, x0			// access to the DCC from EL0
+	msr	pmuserenr_el0, xzr		// Disable PMU access from EL0
 	/*
 	 * Memory region attributes for LPAE:
 	 *