[PATCH v4] coresight: etm4x: avoid build failure with unrolled loops

[Nick Desaulniers]

When the following configs are enabled:
* CORESIGHT
* CORESIGHT_SOURCE_ETM4X
* UBSAN
* UBSAN_TRAP

Clang fails assemble the kernel with the error:
<instantiation>:1:7: error: expected constant expression in '.inst' directive
.inst (0xd5200000|((((2) << 19) | ((1) << 16) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 7) & 0x7)) << 12) | ((((((((((0x160 + (i * 4))))) >> 2))) & 0xf)) << 8) | (((((((((((0x160 + (i * 4))))) >> 2))) >> 4) & 0x7)) << 5)))|(.L__reg_num_x8))
      ^
drivers/hwtracing/coresight/coresight-etm4x-core.c:702:4: note: while in macro instantiation
                        etm4x_relaxed_read32(csa, TRCCNTVRn(i));
                        ^
drivers/hwtracing/coresight/coresight-etm4x.h:403:4: note: expanded from macro 'etm4x_relaxed_read32'
                 read_etm4x_sysreg_offset((offset), false)))
                 ^
drivers/hwtracing/coresight/coresight-etm4x.h:383:12: note: expanded from macro 'read_etm4x_sysreg_offset'
                        __val = read_etm4x_sysreg_const_offset((offset));       \
                                ^
drivers/hwtracing/coresight/coresight-etm4x.h:149:2: note: expanded from macro 'read_etm4x_sysreg_const_offset'
        READ_ETM4x_REG(ETM4x_OFFSET_TO_REG(offset))
        ^
drivers/hwtracing/coresight/coresight-etm4x.h:144:2: note: expanded from macro 'READ_ETM4x_REG'
        read_sysreg_s(ETM4x_REG_NUM_TO_SYSREG((reg)))
        ^
arch/arm64/include/asm/sysreg.h:1108:15: note: expanded from macro 'read_sysreg_s'
        asm volatile(__mrs_s("%0", r) : "=r" (__val));                  \
                     ^
arch/arm64/include/asm/sysreg.h:1074:2: note: expanded from macro '__mrs_s'
"       mrs_s " v ", " __stringify(r) "\n"                      \
 ^

Consider the definitions of TRCSSCSRn and TRCCNTVRn:
drivers/hwtracing/coresight/coresight-etm4x.h:56
 #define TRCCNTVRn(n)      (0x160 + (n * 4))
drivers/hwtracing/coresight/coresight-etm4x.h:81
 #define TRCSSCSRn(n)      (0x2A0 + (n * 4))

Where the macro parameter is expanded to i; a loop induction variable
from etm4_disable_hw.

When any compiler can determine that loops may be unrolled, then the
__builtin_constant_p check in read_etm4x_sysreg_offset() defined in
drivers/hwtracing/coresight/coresight-etm4x.h may evaluate to true. This
can lead to the expression `(0x160 + (i * 4))` being passed to
read_etm4x_sysreg_const_offset. Via the trace above, this is passed
through READ_ETM4x_REG, read_sysreg_s, and finally to __mrs_s where it
is string-ified and used directly in inline asm.

Regardless of compiler or compiler options determine whether a loop can
or can't be unrolled, which determines whether __builtin_constant_p
evaluates to true when passed an expression using a loop induction
variable, it is NEVER safe to allow the preprocessor to construction
inline asm like:
  asm volatile (".inst (0x160 + (i * 4))" : "=r"(__val));
                                 ^ expected constant expression

Replace unsafe uses of calls to etm4x_relaxed_read32 with
csdev_access_read32 when the parameter is an expression that would be
invalid inline asm so that it does not depend on the ability of the
compiler to optimize __builtin_constant_p of the expression to true.
Only when the second parameter of etm4x_relaxed_read32 expands to an
expression dependent on a loop induction variable do we need to fix
this.

This is not a bug in clang; it's a potentially unsafe use of the macro
arguments in read_etm4x_sysreg_offset dependent on __builtin_constant_p.

Link: https://github.com/ClangBuiltLinux/linux/issues/1310
Suggested-by: Arnd Bergmann <arnd@xxxxxxxxxx>
Suggested-by: Tao Zhang <quic_taozha@xxxxxxxxxxx>
Suggested-by: Suzuki K Poulose <suzuki.poulose@xxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
V1 (Arnd):
https://lore.kernel.org/lkml/20210225094324.3542511-1-arnd@xxxxxxxxxx/

V2 (Arnd):
https://lore.kernel.org/lkml/20210429145752.3218324-1-arnd@xxxxxxxxxx/

V3 (Tao):
https://lore.kernel.org/lkml/1632652550-26048-1-git-send-email-quic_taozha@xxxxxxxxxxx/

 drivers/hwtracing/coresight/coresight-etm4x-core.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/hwtracing/coresight/coresight-etm4x-core.c b/drivers/hwtracing/coresight/coresight-etm4x-core.c
index 87299e99dabb..7c6bd85e36d4 100644
--- a/drivers/hwtracing/coresight/coresight-etm4x-core.c
+++ b/drivers/hwtracing/coresight/coresight-etm4x-core.c
@@ -836,13 +836,13 @@ static void etm4_disable_hw(void *info)
 	/* read the status of the single shot comparators */
 	for (i = 0; i < drvdata->nr_ss_cmp; i++) {
 		config->ss_status[i] =
-			etm4x_relaxed_read32(csa, TRCSSCSRn(i));
+			csdev_access_read32(csa, TRCSSCSRn(i));
 	}
 
 	/* read back the current counter values */
 	for (i = 0; i < drvdata->nr_cntr; i++) {
 		config->cntr_val[i] =
-			etm4x_relaxed_read32(csa, TRCCNTVRn(i));
+			csdev_access_read32(csa, TRCCNTVRn(i));
 	}
 
 	coresight_disclaim_device_unlocked(csdev);
@@ -1177,7 +1177,7 @@ static void etm4_init_arch_data(void *info)
 	drvdata->nr_ss_cmp = FIELD_GET(TRCIDR4_NUMSSCC_MASK, etmidr4);
 	for (i = 0; i < drvdata->nr_ss_cmp; i++) {
 		drvdata->config.ss_status[i] =
-			etm4x_relaxed_read32(csa, TRCSSCSRn(i));
+			csdev_access_read32(csa, TRCSSCSRn(i));
 	}
 	/* NUMCIDC, bits[27:24] number of Context ID comparators for tracing */
 	drvdata->numcidc = FIELD_GET(TRCIDR4_NUMCIDC_MASK, etmidr4);
-- 
2.36.1.476.g0c4daa206d-goog