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tcg: Clean up direct block chaining safety checks

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We don't take care of direct jumps when address mapping changes. Thus we
must be sure to generate direct jumps so that they always keep valid
even if address mapping changes. Luckily, we can only allow to execute a
TB if it was generated from the pages which match with current mapping.

Document tcg_gen_goto_tb() declaration and note the reason for
destination PC limitations.

Some targets with variable length instructions allow TB to straddle a
page boundary. However, we make sure that both of TB pages match the
current address mapping when looking up TBs. So it is safe to do direct
jumps into the both pages. Correct the checks for some of those targets.

Given that, we can safely patch a TB which spans two pages. Remove the
unnecessary check in cpu_exec() and allow such TBs to be patched.

Backports commit 5b053a4a28278bca606eeff7d1c0730df1b047e9 from qemu
This commit is contained in:
Sergey Fedorov 2018-02-23 22:25:51 -05:00 committed by Lioncash
parent 39d262f0d2
commit 73c59faad5
No known key found for this signature in database
GPG key ID: 4E3C3CC1031BA9C7
5 changed files with 16 additions and 8 deletions
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7
qemu/cpu-exec.c
View file

@ -198,11 +198,8 @@ int cpu_exec(struct uc_struct *uc, CPUState *cpu)
next_tb = 0;
tcg_ctx->tb_ctx.tb_invalidated_flag = 0;
}
/* see if we can patch the calling TB. When the TB
spans two pages, we cannot safely do a direct
jump. */
if (next_tb != 0 && tb->page_addr[1] == -1
&& !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
/* See if we can patch the calling TB. */
if (next_tb != 0 && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
next_tb & TB_EXIT_MASK, tb);
}

3
qemu/target-arm/translate.c
View file

@ -4155,7 +4155,8 @@ static inline void gen_goto_tb(DisasContext *s, int n, target_ulong dest)
TranslationBlock *tb;
tb = s->tb;
if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK) ||
((s->pc - 1) & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
tcg_gen_goto_tb(tcg_ctx, n);
gen_set_pc_im(s, dest);
tcg_gen_exit_tb(tcg_ctx, (uintptr_t)tb + n);

2
qemu/target-i386/translate.c
View file

@ -2373,7 +2373,7 @@ static inline void gen_goto_tb(DisasContext *s, int tb_num, target_ulong eip)
tb = s->tb;
/* NOTE: we handle the case where the TB spans two pages here */
if ((pc & TARGET_PAGE_MASK) == (tb->pc & TARGET_PAGE_MASK) ||
(pc & TARGET_PAGE_MASK) == ((s->pc - 1) & TARGET_PAGE_MASK)) {
(pc & TARGET_PAGE_MASK) == (s->pc_start & TARGET_PAGE_MASK)) {
/* jump to same page: we can use a direct jump */
tcg_gen_goto_tb(tcg_ctx, tb_num);
gen_jmp_im(s, eip);

2
qemu/target-m68k/translate.c
View file

@ -851,7 +851,7 @@ static void gen_jmp_tb(DisasContext *s, int n, uint32_t dest)
if (unlikely(s->singlestep_enabled)) {
gen_exception(s, dest, EXCP_DEBUG);
} else if ((tb->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK) ||
(s->pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
(s->insn_pc & TARGET_PAGE_MASK) == (dest & TARGET_PAGE_MASK)) {
tcg_gen_goto_tb(tcg_ctx, n);
tcg_gen_movi_i32(tcg_ctx, tcg_ctx->QREG_PC, dest);
tcg_gen_exit_tb(tcg_ctx, (uintptr_t)tb + n);

10
qemu/tcg/tcg-op.h
View file

@ -753,6 +753,16 @@ static inline void tcg_gen_exit_tb(TCGContext *s, uintptr_t val)
tcg_gen_op1i(s, INDEX_op_exit_tb, val);
}
/**
* tcg_gen_goto_tb() - output goto_tb TCG operation
* @idx: Direct jump slot index (0 or 1)
*
* See tcg/README for more info about this TCG operation.
*
* NOTE: Direct jumps with goto_tb are only safe within the pages this TB
* resides in because we don't take care of direct jumps when address mapping
* changes, e.g. in tlb_flush().
*/
void tcg_gen_goto_tb(TCGContext *s, unsigned idx);
#if TARGET_LONG_BITS == 32