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Diffstat (limited to 'gitlab/issues_text/target_missing/host_missing/accel_TCG/1454')
| -rw-r--r-- | gitlab/issues_text/target_missing/host_missing/accel_TCG/1454 | 62 |
1 files changed, 62 insertions, 0 deletions
diff --git a/gitlab/issues_text/target_missing/host_missing/accel_TCG/1454 b/gitlab/issues_text/target_missing/host_missing/accel_TCG/1454 new file mode 100644 index 000000000..f15dfebf6 --- /dev/null +++ b/gitlab/issues_text/target_missing/host_missing/accel_TCG/1454 @@ -0,0 +1,62 @@ +QEMU TCG s390x fails an assertion while dispatching an FIXPT_DIVIDE exception on DR when compiled with LTO +Description of problem: +When running the attached minimal reproducer, with qemu-system-s390x version 7.2.0 compiled with LTO (`--enable-lto`) with GCC v12.2.1, QEMU fails an assertion and crashes: +``` +qemu-system-s390x: ../target/s390x/tcg/excp_helper.c:215: do_program_interrupt: Assertion `ilen == 2 || ilen == 4 || ilen == 6' failed. +Aborted (core dumped) +``` +Steps to reproduce: +1. Compile QEMU v7.2.0 for s390x with LTO enabled: + ``` + ../configure --target-list=s390x-softmmu --enable-lto + ``` +2. Compile the given reproducer assembler [lpswe-to-pgm.S](/uploads/200fb0e777ddd0ed26f51009e81c26ea/lpswe-to-pgm.S): + ``` + s390x-linux-gnu-gcc -march=z13 -m64 -nostdlib -nostartfiles -static -Wl,-Ttext=0 -Wl,--build-id=none lpswe-to-pgm.S -o lpswe-to-pgm + ``` +3. Execute QEMU on the reproducer: + ``` + ./qemu-system-s390x -kernel lpswe-to-pgm + ``` +Additional information: +I have debugged QEMU to try to find the root cause, and I believe I found it, but I'm not sure what the most appropriate way to fix it would be: + +QEMU executes the `DR` instruction by executing the `divs32` helper. + +When the helper sees that the final division result does not fit in 32 bits, it generates a program interrupt for fixed point divide by calling the `tcg_s390_program_interrupt` function, with the final parameter being the TCG host PC, which is found by calling `GETPC`. + +`tcg_s390_program_interrupt` then calls `cpu_restore_state`, and then as long as the host PC is valid, `cpu_restore_state` eventually calls `s390x_restore_state_to_opc` through a long chain of calls, which sets `CPUS390XState::int_pgm_ilen` to a valid value. + +Unfortunately when compiling with LTO, the host PC is not valid, which means we don't update `int_pgm_ilen`, resulting in the failed assertion. + +The reason the host PC is not valid when compiling with LTO, is that GCC decides to split `helper_divs32` into 2 parts, the actual div logic being the first part, and the call to `GETPC` & `tcg_s390_program_interrupt` being the second part. The way GCC implements it is by turning the second part into a separate function, which the first part calls - see disassembly below. (GCC then re-uses the second part in other similar TCG helpers) + +Because we now called the second part before calling `GETPC`, we have a new return address, and `GETPC` returns the address of the first part, instead of the TCG host PC. + +``` +000000000022c870 <helper_divs32>: + 22c870: 48 83 ec 08 sub rsp,0x8 + 22c874: 85 d2 test edx,edx + 22c876: 74 22 je 22c89a <helper_divs32+0x2a> + 22c878: 48 89 f0 mov rax,rsi + 22c87b: 48 63 ca movsxd rcx,edx + 22c87e: 48 99 cqo + 22c880: 48 f7 f9 idiv rcx + 22c883: 4c 63 c0 movsxd r8,eax + 22c886: 48 89 97 10 03 00 00 mov QWORD PTR [rdi+0x310],rdx + 22c88d: 49 39 c0 cmp r8,rax + 22c890: 75 17 jne 22c8a9 <helper_divs32+0x39> + 22c892: 4c 89 c0 mov rax,r8 + 22c895: 48 83 c4 08 add rsp,0x8 + 22c899: c3 ret + 22c89a: 48 8b 54 24 08 mov rdx,QWORD PTR [rsp+0x8] + 22c89f: be 09 00 00 00 mov esi,0x9 + 22c8a4: e8 47 e5 ff ff call 22adf0 <tcg_s390_program_interrupt> + 22c8a9: e8 b2 fe ff ff call 22c760 <helper_divs32.part.0> + +000000000022c760 <helper_divs32.part.0>: + 22c760: 48 83 ec 08 sub rsp,0x8 + 22c764: be 09 00 00 00 mov esi,0x9 + 22c769: 48 8b 54 24 08 mov rdx,QWORD PTR [rsp+0x8] + 22c76e: e8 7d e6 ff ff call 22adf0 <tcg_s390_program_interrupt> +``` |