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target/m68k: Improve ending TB at page boundaries

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Rather than limit total TB size to PAGE-32 bytes, end the TB when
near the end of a page. This should provide proper semantics of
SIGSEGV when executing near the end of a page.

Backports commit 4c7a0f6f34869b3dfe7091d28ff27a8dfbdd8b70 from qemu
This commit is contained in:
Richard Henderson 2018-06-15 11:36:03 -04:00 committed by Lioncash
parent c42ca54685
commit a8540a30d2
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GPG key ID: 4E3C3CC1031BA9C7
1 changed files with 19 additions and 3 deletions
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22
qemu/target/m68k/translate.c
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@ -6361,9 +6361,25 @@ static void m68k_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
disas_m68k_insn(cpu->env_ptr, dc);
dc->base.pc_next = dc->pc;
if (dc->base.is_jmp == DISAS_NEXT
&& dc->pc - dc->base.pc_first >= TARGET_PAGE_SIZE - 32) {
dc->base.is_jmp = DISAS_TOO_MANY;
if (dc->base.is_jmp == DISAS_NEXT) {
/* Stop translation when the next insn might touch a new page.
* This ensures that prefetch aborts at the right place.
*
* We cannot determine the size of the next insn without
* completely decoding it. However, the maximum insn size
* is 32 bytes, so end if we do not have that much remaining.
* This may produce several small TBs at the end of each page,
* but they will all be linked with goto_tb.
*
* ??? ColdFire maximum is 4 bytes; MC68000's maximum is also
* smaller than MC68020's.
*/
target_ulong start_page_offset
= dc->pc - (dc->base.pc_first & TARGET_PAGE_MASK);
if (start_page_offset >= TARGET_PAGE_SIZE - 32) {
dc->base.is_jmp = DISAS_TOO_MANY;
}
}
}