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From: "Alex Bennée" <alex.bennee@linaro.org>
To: qemu-devel@nongnu.org
Subject: [Bug 1859021] [PATCH v1 1/2] target/arm: detect 64 bit overflow caused by high cval + voff
Date: Fri, 10 Jan 2020 16:16:25 -0000
Message-ID: <20200110161626.31943-2-alex.bennee@linaro.org> (raw)
Message-ID: <20200110161625.ybr2Ub5qeMgp4FVjnZLWsYqQR0YZNkLE0J1-4ioWB9Y@z> (raw)
In-Reply-To: <157857629827.5165.2496570379985305724.malonedeb@gac.canonical.com>

If we don't detect this we will be stuck in a busy loop as we schedule
a timer for before now which will continually trigger gt_recalc_timer
even though we haven't reached the state required to trigger the IRQ.

Bug: https://bugs.launchpad.net/bugs/1859021
Cc: 1859021@bugs.launchpad.net
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 target/arm/helper.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index 19a57a17da5..eb17106f7bd 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -2481,6 +2481,9 @@ static void gt_recalc_timer(ARMCPU *cpu, int timeridx)
         } else {
             /* Next transition is when we hit cval */
             nexttick = gt->cval + offset;
+            if (nexttick < gt->cval) {
+                nexttick = UINT64_MAX;
+            }
         }
         /* Note that the desired next expiry time might be beyond the
          * signed-64-bit range of a QEMUTimer -- in this case we just
-- 
2.20.1

-- 
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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

To manage notifications about this bug go to:
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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 ` Alex Bennée [this message]
2020-01-10 16:16   ` [Bug 1859021] [PATCH v1 1/2] target/arm: detect 64 bit overflow caused by high cval + voff 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
2020-01-17 11:50       ` [Bug 1859021] " 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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