Delay between the counter hitting zero and the ISR firing is architecturally permitted (the interrupt must be recognized in finite time or at a context synchronizing event, but not necessarily at the same 'clock tick' that the counter hits zero). If you want to ensure that an interrupt has been taken before you read the counter value, you need to use an ISB instruction in your loop.
(There are also some buggy behaviours in our systick device implementation, but in this case I don't think you're running into them.) -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/1754038 Title: ARM M: Systick first wrap delayed (qemu-timers/icount prb?) Status in QEMU: New Bug description: When running this kind of code with qemu: static void SysTickISR(void) { printf("SysTick\n"); } void main() { volatile int i, j; printf("setup timer\n"); *(uint32_t*) 0xE000E014 = 0x8FFFFF; //reload value *(uint32_t*) 0xE000E018 = 0; //force reload *(uint32_t*) 0xE000E010 = 7; //cpu clk + ISR + enable for (j = 0; j < 0x100; j++) { for (i = 0; i < 0x100000; i++) ; printf("cnt %08x -- %8x\n", *(uint32_t*) 0xE000E018, *(uint32_t*)0xE000E010); } } I get the following output (comments added after '#'): setup timer cnt 007cccca -- 7 cnt 006998a2 -- 7 cnt 00566479 -- 7 cnt 0043304f -- 7 cnt 002ffc26 -- 7 cnt 001cc7fd -- 7 cnt 000993d5 -- 7 cnt 00000000 -- 7 <--- problem here, systick should wrap and raise isr cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 cnt 00000000 -- 7 SysTick <--- delayed isr occuring here cnt 000986e0 -- 10007 SysTick cnt 00865290 -- 10007 <---- then running fine as long as regs not modified cnt 00731e51 -- 7 cnt 005fea27 -- 7 cnt 004cb5ff -- 7 cnt 003981d6 -- 7 cnt 00264dad -- 7 cnt 00131984 -- 7 SysTick cnt 008fe545 -- 10007 cnt 007cb106 -- 7 cnt 00697cdd -- 7 cnt 005648b4 -- 7 cnt 0043148b -- 7 cnt 002fe061 -- 7 cnt 001cac38 -- 7 cnt 00097810 -- 7 SysTick cnt 008643d6 -- 10007 cnt 00730f97 -- 7 cnt 005fdb6d -- 7 cnt 004ca745 -- 7 cnt 0039731c -- 7 cnt 00263ef3 -- 7 cnt 00130aca -- 7 SysTick cnt 008fd68b -- 10007 cnt 007ca24c -- 7 cnt 00696e23 -- 7 cnt 005639fa -- 7 cnt 004305d1 -- 7 cnt 002fd1a8 -- 7 cnt 001c9d7f -- 7 cnt 00096956 -- 7 SysTick cnt 0086351d -- 10007 cnt 007300dd -- 7 cnt 005fccb4 -- 7 cnt 004c988c -- 7 cnt 00396463 -- 7 cnt 00263039 -- 7 cnt 0012fc10 -- 7 [...] Command line and version: qemu-system-arm -M lm3s6965evb -nographic -kernel hello.bin -monitor stdio -serial file:/dev/pts/6 -icount 4 -cpu cortex-m4 QEMU 2.11.50 I am compiling from git repo, head is: commit f32408f3b472a088467474ab152be3b6285b2d7b Author: Daniel P. Berrangé <[email protected]> Date: Tue Mar 6 13:43:17 2018 +0000 Config options: ./configure --target-list=arm-softmmu --enable-debug --disable-slirp --enable-tcg-interpreter --disable-blobs --disable-docs --disable-guest-agent --disable-gnutls --disable-nettle --disable-gcrypt --disable-sdl --disable-gtk --disable-vnc --disable-virtfs --disable-mpath --disable-xen --disable-brlapi --disable-curl --disable-bluez --disable-kvm --disable-hax --disable-hvf --disable-whpx --disable-rdma --disable-vde --disable-netmap --disable-linux-aio --disable-cap-ng --disable-attr --disable-vhost-net --disable-spice --disable-rbd --disable-libiscsi --disable-libnfs --disable-smartcard --disable-libusb --disable-live-block-migration --disable-usb-redir --disable-lzo --disable-snappy --disable-bzip2 --disable-seccomp --disable-glusterfs --disable-tpm --disable-libssh2 --disable-numa --disable-libxml2 --disable-tcmalloc --disable-jemalloc --disable-replication --disable-vhost-vsock --disable-opengl --disable-virglrenderer --disable-xfsctl --disable-qom-cast-debug --disable-vxhs --disable-crypto-afalg --disable-vhost-user --disable-capstone --disable-pie --extra-cflags=-mtune=native Not working with git tag 2.10.0 (almost same config) Working with stock qemu-arm 2.5.0 from Ubuntu 16.04. I started investigating, though I am not familiar with qemu code and I could see that the execution is not geting out of qemu_tcg_rr_cpu_thread_fn() 'while' loop and timers are not triggered because the values in cpu->icount_extra or cpu->icount_budget are not to modified accordingly after the timer is set (host side) when the systick register is written (target side). To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/1754038/+subscriptions
