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