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From: Peter Maydell <peter.maydell@linaro.org>
To: qemu-devel@nongnu.org
Subject: [Bug 1859021] Re: [PATCH v1 1/2] target/arm: detect 64 bit overflow caused by high cval + voff
Date: Fri, 17 Jan 2020 11:50:33 -0000
Message-ID: <CAFEAcA_0hHjkhKtVsQEsN=Xow=W6vPvGJPJdzHh=O6RG1=_mAQ@mail.gmail.com> (raw)
Message-ID: <20200117115033.wIF0a3wjL3YvRf8G4q6E-RJmoi7COrNjFkGiM5shN0g@z> (raw)
In-Reply-To: <CAFEAcA9nNH9pu+8E_YYkiNtzePjZdrEBjK_9zJv+XJaSvcnhmA@mail.gmail.com>

On Thu, 16 Jan 2020 at 18:45, Peter Maydell <peter.maydell@linaro.org> wrote:
> There's something odd going on with this code. Adding a bit of context:
>
>         uint64_t offset = timeridx == GTIMER_VIRT ?
>                                       cpu->env.cp15.cntvoff_el2 : 0;
>         uint64_t count = gt_get_countervalue(&cpu->env);
>         /* Note that this must be unsigned 64 bit arithmetic: */
>         int istatus = count - offset >= gt->cval;
>         [...]
>         if (istatus) {
>             /* Next transition is when count rolls back over to zero */
>             nexttick = UINT64_MAX;
>         } else {
>             /* Next transition is when we hit cval */
>             nexttick = gt->cval + offset;
>         }
>
> I think this patch is correct, in that the 'nexttick' values
> are all absolute and this cval/offset combination implies
> that the next timer interrupt is going to be in a future
> so distant we can't even fit the duration in a uint64_t.
>
> But the other half of the 'if' also looks wrong: that's
> for the case of "timer has fired, how long until the
> wraparound causes the interrupt line to go low again?".
> UINT64_MAX is right for the EL1 case where offset is 0,
> but the offset might actually be set such that the wrap
> around happens fairly soon. We want to calculate the
> tick when (count - offset) hits 0, saturated to
> UINT64_MAX. It's getting late here and I couldn't figure
> out what that expression should be with 15 minutes of
> fiddling around with pen and paper diagrams. I'll have another
> go tomorrow if nobody else gets there first...

With a fresher brain:

For the if (istatus) branch we want the absolute tick
when (count - offset) wraps round to 0, saturated to UINT64_MAX.
I think this is:
    if (offset <= count) {
        nexttick = UINT64_MAX;
    } else {
        nexttick = offset;
    }

Should we consider this a separate bugfix to go in its own patch?

thanks
-- PMM

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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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  parent reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 16:16 [PATCH v1 0/2] fix for bug 1859021 Alex Bennée
2020-01-10 16:16 ` [PATCH v1 1/2] target/arm: detect 64 bit overflow caused by high cval + voff Alex Bennée
2020-01-10 16:16   ` [Bug 1859021] " Alex Bennée
2020-01-16 18:45   ` Peter Maydell
2020-01-16 18:45     ` [Bug 1859021] " Peter Maydell
2020-01-17 11:50     ` Peter Maydell [this message]
2020-01-17 11:50       ` Peter Maydell
2020-01-10 16:16 ` [PATCH v1 2/2] tests/tcg: add a vtimer test for aarch64 Alex Bennée
2020-01-17 14:07   ` Peter Maydell
2020-02-06 17:00     ` Alex Bennée
  -- strict thread matches above, loose matches on Subject: below --
2020-01-09 13:24 [Bug 1859021] [NEW] qemu-system-aarch64 (tcg): cval + voff overflow not handled, causes qemu to hang Alex Longwall
2020-01-09 14:44 ` [Bug 1859021] " Alex Bennée
2020-01-09 16:25 ` [RFC PATCH] tests/tcg: add a vtimer test for aarch64 Alex Bennée
2020-01-09 16:25   ` [Bug 1859021] Re: qemu-system-aarch64 (tcg): cval + voff overflow not handled, causes qemu to hang Alex Bennée

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