[arm] Check PMCCNTR availability before use for profiling

Reading from PMCCNTR causes an undefined instruction exception when
running in PL0 (e.g. as a Linux userspace binary), unless the
PMUSERENR.EN bit is set.

Restructure profile_timestamp() for 32-bit ARM to perform an
availability check on the first invocation, with subsequent
invocations returning zero if PMCCNTR could not be enabled.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/arm32/core/pmccntr.S

@@ -0,0 +1,85 @@
+/*
+ * Copyright (C) 2024 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+	FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
+/** @file
+ *
+ * Performance Monitor Cycle Counter (PMCCNTR)
+ *
+ */
+
+	.section ".note.GNU-stack", "", %progbits
+	.text
+	.arm
+
+/*
+ * PMCCNTR status
+ *
+ * bit 31 	set if PMCCNTR availability is not yet determined
+ * bit 0	set if PMCCNTR is available
+ *
+ */
+	.section ".data.pmccntr_status", "aw", %progbits
+	.globl	pmccntr_status
+pmccntr_status:
+	.word	0x80000000
+
+/*
+ * Check PMCCNTR availability
+ *
+ * Must preserve all registers, and return with either PMCCNTR enabled
+ * or the Z flag set to indicate unavailability.
+ *
+ */
+	.section ".text.pmccntr_check", "ax", %progbits
+	.globl	pmccntr_check
+	.type	pmccntr_check, %function
+pmccntr_check:
+	/* Save registers */
+	stmfd	sp!, { r0, r1 }
+	/* Read CPSR.M (bits 3:0, always permitted in PL0) */
+	mrs	r0, cpsr
+	and	r0, r0, #0x0000000f
+	/* Read PMUSERENR.EN (bit 0, always permitted in PL0) */
+	mrc	p15, 0, r1, c9, c14, 0
+	and	r1, r1, #0x00000001
+	/* Check if we are in PL1+ or in PL0 with PMUSERENR.EN set */
+	orrs	r0, r0, r1
+	/* If PMCCNTR is unavailable, exit with status=0 and ZF set */
+	beq	1f
+	/* Set PMCR.E (bit 0), set exit status=1 and ZF clear */
+	movs	r0, #0x00000001
+	mcr	p15, 0, r0, c9, c12, 0
+	/* Set PMCNTENSET.C (bit 31) */
+	mov	r1, #0x80000000
+	mcr	p15, 0, r1, c9, c12, 1
+1:	/* Store PMCCNTR status */
+	ldr	r1, pmccntr_status_ptr
+	str	r0, [r1]
+	/* Restore registers and return */
+	ldmfd	sp!, { r0, r1 }
+	bx	lr
+pmccntr_status_ptr:
+	.word pmccntr_status
+	.size	pmccntr_check, . - pmccntr_check

src/arch/arm32/include/bits/profile.h

@@ -11,6 +11,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #include <stdint.h>
 
+extern uint32_t pmccntr_status;
+
 /**
  * Get profiling timestamp
  *
@@ -21,9 +23,18 @@ profile_timestamp ( void ) {
 	uint32_t cycles;
 
 	/* Read cycle counter */
-	__asm__ __volatile__ ( "mcr p15, 0, %1, c9, c12, 0\n\t"
+	__asm__ __volatile__ ( /* Check PMCCNTR status */
-			       "mrc p15, 0, %0, c9, c13, 0\n\t"
+			       "tst %0, %0\n\t"
-			       : "=r" ( cycles ) : "r" ( 1 ) );
+			       /* Check availability if not yet known */
+			       "it mi\n\t"
+			       "blxmi pmccntr_check\n\t"
+			       /* Read PMCCNTR if available */
+			       "it ne\n\t"
+			       "mrcne p15, 0, %0, c9, c13, 0\n\t"
+			       "\n1:\n\t"
+			       : "=r" ( cycles )
+			       : "0" ( pmccntr_status )
+			       : "cc", "lr" );
 	return cycles;
 }