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

[Thomas Huth]

This is an automated cleanup. This bug report has been moved
to QEMU's new bug tracker on gitlab.com and thus gets marked
as 'expired' now. Please continue with the discussion here:

 https://gitlab.com/qemu-project/qemu/-/issues/60


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

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

** Bug watch added: gitlab.com/qemu-project/qemu/-/issues #60
   https://gitlab.com/qemu-project/qemu/-/issues/60

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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:
  Expired

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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