This is a workaround for a ppc64le host kernel bug. For the test case linux-test, we have an instruction trace
IN: sig_alarm ... IN: 0x400080ed28: 380000ac li r0, 0xac 0x400080ed2c: 44000002 sc IN: __libc_nanosleep 0x1003bb4c: 7c0802a6 mflr r0 0x1003bb50: f8010010 std r0, 0x10(r1) Our signal return trampoline has, rightly, changed the guest stack page read-only. Which, rightly, faults on the store of a return address into a stack frame. Checking the host /proc/pid/maps, we see the expected state: 4000800000-4000810000 r--p 00000000 00:00 0 However, the host kernel has supplied si_code == SEGV_MAPERR, which is obviously incorrect. By dropping this check, we may have an extra walk of the page tables, but this should be inexpensive. Signed-off-by: Richard Henderson <richard.hender...@linaro.org> --- FWIW, filed as https://bugzilla.redhat.com/show_bug.cgi?id=1757189 out of habit and then https://bugs.centos.org/view.php?id=16499 when I remembered that the system is running Centos not RHEL. --- accel/tcg/user-exec.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/accel/tcg/user-exec.c b/accel/tcg/user-exec.c index 71c4bf6477..31ef091a70 100644 --- a/accel/tcg/user-exec.c +++ b/accel/tcg/user-exec.c @@ -143,9 +143,12 @@ static inline int handle_cpu_signal(uintptr_t pc, siginfo_t *info, * for some other kind of fault that should really be passed to the * guest, we'd end up in an infinite loop of retrying the faulting * access. + * + * XXX: At least one host kernel, ppc64le w/Centos 7 4.14.0-115.6.1, + * incorrectly reports SEGV_MAPERR for a STDX write to a read-only page. + * Therefore, do not test info->si_code. */ - if (is_write && info->si_signo == SIGSEGV && info->si_code == SEGV_ACCERR && - h2g_valid(address)) { + if (is_write && info->si_signo == SIGSEGV && h2g_valid(address)) { switch (page_unprotect(h2g(address), pc)) { case 0: /* Fault not caused by a page marked unwritable to protect -- 2.17.1
