target/xtensa: fix timers test

The value of CCOUNT special register is calculated as time elapsed since CCOUNT == 0 multiplied by the core frequency. In icount mode time increment between consecutive instructions that don't involve time warps is constant, but unless the result of multiplication of this constant by the core frequency is a whole number the CCOUNT increment between these instructions may not be constant. E.g. with icount=7 each instruction takes 128ns, with core clock of 10MHz CCOUNT values for consecutive instructions are:

502: (128 * 502 * 10000000) / 1000000000 = 642.56 503: (128 * 503 * 10000000) / 1000000000 = 643.84 504: (128 * 504 * 10000000) / 1000000000 = 645.12

I.e.the CCOUNT increments depend on the absolute time. This results in varying CCOUNT differences for consecutive instructions in tests that involve time warps and don't set CCOUNT explicitly.

Change frequency of the core used in tests so that clock cycle takes exactly 64ns. Change icount power used in tests to 6, so that each instruction takes exactly 1 clock cycle. With these changes CCOUNT increments only depend on the number of executed instructions and that's what timer tests expect, so they work correctly.

Longer story:
http://lists.nongnu.org/archive/html/qemu-devel/2018-03/msg04326.html

Cc Pavel Dovgaluk <[email protected]>

Cc Philippe Mathieu-Daudé <[email protected]>

Signed-off-by Max Filippov <[email protected]>

---
Changes v1->v2:
- expand changelog;
- replace clock_freq_khz initializer constant with a more meaningful expression.

 target/xtensa/core-dc232b.c | 2 +-
 tests/tcg/xtensa/Makefile   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

target/xtensa/core-dc232b.c +1 -1

index aa07018af4e7..7331eeea2fb6 100644
@@ -47,7 +47,7 @@ static XtensaConfig dc232b __attribute__((unused)) = {
         }
     },
     .isa_internal = &xtensa_modules,
-    .clock_freq_khz = 10000,
+    .clock_freq_khz = (NANOSECONDS_PER_SECOND / 64) / 1000,
     DEFAULT_SECTIONS
 };
 

tests/tcg/xtensa/Makefile +1 -1

index 2882c431e4a9..091518c05583 100644
@@ -5,7 +5,7 @@ CROSS=xtensa-$(CORE)-elf-
 
 ifndef XT
 SIM = ../../../xtensa-softmmu/qemu-system-xtensa
-SIMFLAGS = -M sim -cpu $(CORE) -nographic -semihosting -icount 7 $(EXTFLAGS) -kernel
+SIMFLAGS = -M sim -cpu $(CORE) -nographic -semihosting -icount 6 $(EXTFLAGS) -kernel
 SIMDEBUG = -s -S
 else
 SIM = xt-run