In user-mode emulation Translation Block can consist of 2 guest pages.
In that case QEMU also mprotects 2 host pages that are dedicated for
guest memory, containing instructions. QEMU detects self-modifying code
with SEGFAULT signal processing.
In case if instruction in 1st page is modifying memory of 2nd
page (or vice versa) QEMU will mark 2nd page with PAGE_WRITE,
invalidate TB, generate new TB contatining 1 guest instruction and
exit to CPU loop. QEMU won't call mprotect, and new TB will cause
same SEGFAULT. Page will have both PAGE_WRITE_ORG and PAGE_WRITE
flags, so QEMU will handle the signal as guest binary problem,
and exit with guest SEGFAULT.
Solution is to do following: In case if current TB was invalidated
continue to invalidate TBs from remaining guest pages and mark pages
as PAGE_WRITE. After that disable host page protection with mprotect.
If current tb was invalidated longjmp to main loop. That is more
efficient, since we won't get SEGFAULT when executing new TB.
Backports commit 7399a337e4126f7c8c8af3336726f001378c4798 from qemu
6f20d35cd1