[Bug 1859021] Re: qemu-system-aarch64 (tcg): cval + voff overflow not handled, causes qemu to hang

["Alex Bennée" <alex.bennee@linaro.org>]

Bug: https://bugs.launchpad.net/bugs/1859021

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 tests/tcg/aarch64/system/vtimer.c         | 48 +++++++++++++++++++++++
 tests/tcg/aarch64/Makefile.softmmu-target |  4 ++
 2 files changed, 52 insertions(+)
 create mode 100644 tests/tcg/aarch64/system/vtimer.c

diff --git a/tests/tcg/aarch64/system/vtimer.c b/tests/tcg/aarch64/system/vtimer.c
new file mode 100644
index 00000000000..42f2f7796c7
--- /dev/null
+++ b/tests/tcg/aarch64/system/vtimer.c
@@ -0,0 +1,48 @@
+/*
+ * Simple Virtual Timer Test
+ *
+ * Copyright (c) 2020 Linaro Ltd
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include <inttypes.h>
+#include <minilib.h>
+
+/* grabbed from Linux */
+#define __stringify_1(x...) #x
+#define __stringify(x...)   __stringify_1(x)
+
+#define read_sysreg(r) ({                                           \
+            uint64_t __val;                                         \
+            asm volatile("mrs %0, " __stringify(r) : "=r" (__val)); \
+            __val;                                                  \
+})
+
+#define write_sysreg(r, v) do {                     \
+        uint64_t __val = (uint64_t)(v);             \
+        asm volatile("msr " __stringify(r) ", %x0"  \
+                 : : "rZ" (__val));                 \
+} while (0)
+
+int main(void)
+{
+    int i;
+
+    ml_printf("VTimer Test\n");
+
+    write_sysreg(cntvoff_el2, 1);
+    write_sysreg(cntv_cval_el0, -1);
+    write_sysreg(cntv_ctl_el0, 1);
+
+    ml_printf("cntvoff_el2=%lx\n", read_sysreg(cntvoff_el2));
+    ml_printf("cntv_cval_el0=%lx\n", read_sysreg(cntv_cval_el0));
+    ml_printf("cntv_ctl_el0=%lx\n", read_sysreg(cntv_ctl_el0));
+
+    /* Now read cval a few times */
+    for (i = 0; i < 10; i++) {
+        ml_printf("%d: cntv_cval_el0=%lx\n", i, read_sysreg(cntv_cval_el0));
+    }
+
+    return 0;
+}
diff --git a/tests/tcg/aarch64/Makefile.softmmu-target b/tests/tcg/aarch64/Makefile.softmmu-target
index 7b4eede3f07..62cdddbb215 100644
--- a/tests/tcg/aarch64/Makefile.softmmu-target
+++ b/tests/tcg/aarch64/Makefile.softmmu-target
@@ -62,3 +62,7 @@ run-memory-replay: memory-replay run-memory-record
 	  "$< on $(TARGET_NAME)")
 
 EXTRA_TESTS+=memory-record memory-replay
+
+# vtimer test
+QEMU_EL2_MACHINE=-machine virt,virtualization=on,gic-version=2 -cpu cortex-a57 -smp 4
+run-vtimer: QEMU_OPTS=$(QEMU_EL2_MACHINE) $(QEMU_SEMIHOST)  -kernel
-- 
2.20.1


** Changed in: qemu
       Status: New => Confirmed

** Changed in: qemu
     Assignee: (unassigned) => Alex Bennée (ajbennee)

** Tags added: testcase

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https://bugs.launchpad.net/bugs/1859021

Title:
  qemu-system-aarch64 (tcg):  cval + voff overflow not handled, causes
  qemu to hang

Status in QEMU:
  Confirmed

Bug description:
  The Armv8 architecture reference manual states that for any timer set
  (e.g. CNTP* and CNTV*), the condition for such timer to generate an
  interrupt (if enabled & unmasked) is:

  CVAL <= CNT(P/V)CT

  Although this is arguably sloppy coding, I have seen code that is
  therefore assuming it can set CVAL to a very high value (e.g.
  UINT64_MAX) and leave the interrupt enabled in CTL, and never get the
  interrupt.

  On latest master commit as the time of writing, there is an integer
  overflow in target/arm/helper.c gt_recalc_timer affecting the virtual
  timer when the interrupt is enabled in CTL:

      /* Next transition is when we hit cval */
      nexttick = gt->cval + offset;

  When this overflow happens, I notice that qemu is no longer responsive and that I have to SIGKILL the process:
      - qemu takes nearly all the cpu time of the cores it is running on (e.g. 50% cpu usage if running on half the cores) and is completely unresponsive
      - no guest interrupt (reported via -d int) is generated

  Here the minimal code example to reproduce the issue:

      mov     x0, #1
      msr     cntvoff_el2, x0
      mov     x0, #-1
      msr     cntv_cval_el0, x0
      mov     x0, #1
      msr     cntv_ctl_el0, x0 // interrupt generation enabled, not masked; qemu will start to hang here

  Options used:
  -nographic -machine virt,virtualization=on,gic-version=2,accel=tcg -cpu cortex-a57
  -smp 4 -m 1024 -kernel whatever.elf -d unimp,guest_errors,int -semihosting-config enable,target=native
  -serial mon:stdio

  Version used: 4.2

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