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

[Alex Bennée]

** Tags added: arm tcg

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

Status in QEMU:
  New

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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