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tcg/loongarch64: Reorg goto_tb implementation 

The old implementation replaces two insns, swapping between

        b       <dest>
        nop
and
        pcaddu18i tmp, <dest>
        jirl      zero, tmp, <dest> & 0xffff

There is a race condition in which a thread could be stopped at
the jirl, i.e. with the top of the address loaded, and when
restarted we have re-linked to a different TB, so that the top
half no longer matches the bottom half.

Note that while we never directly re-link to a different TB, we
can link, unlink, and link again all while the stopped thread
remains stopped.

The new implementation replaces only one insn, swapping between

        b       <dest>
and
        pcadd   tmp, <jmp_addr>

falling through to load the address from tmp, and branch.

Reviewed-by: WANG Xuerui <git@xen0n.name>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
This commit is contained in:
Richard Henderson 2022-11-29 11:02:20 -08:00
parent 21199bfbef
commit 709bcd7da3
2 changed files with 33 additions and 46 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

72
tcg/loongarch64/tcg-target.c.inc
View File

@ -1151,37 +1151,6 @@ static void tcg_out_qemu_st(TCGContext *s, const TCGArg *args)
#endif
}
/* LoongArch uses `andi zero, zero, 0` as NOP. */
#define NOP OPC_ANDI
static void tcg_out_nop(TCGContext *s)
{
tcg_out32(s, NOP);
}
void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
uintptr_t jmp_rx, uintptr_t jmp_rw)
{
tcg_insn_unit i1, i2;
ptrdiff_t upper, lower;
uintptr_t addr = tb->jmp_target_addr[n];
ptrdiff_t offset = (ptrdiff_t)(addr - jmp_rx) >> 2;
if (offset == sextreg(offset, 0, 26)) {
i1 = encode_sd10k16_insn(OPC_B, offset);
i2 = NOP;
} else {
tcg_debug_assert(offset == sextreg(offset, 0, 36));
lower = (int16_t)offset;
upper = (offset - lower) >> 16;
i1 = encode_dsj20_insn(OPC_PCADDU18I, TCG_REG_TMP0, upper);
i2 = encode_djsk16_insn(OPC_JIRL, TCG_REG_ZERO, TCG_REG_TMP0, lower);
}
uint64_t pair = ((uint64_t)i2 << 32) | i1;
qatomic_set((uint64_t *)jmp_rw, pair);
flush_idcache_range(jmp_rx, jmp_rw, 8);
}
/*
* Entry-points
*/
@ -1202,22 +1171,43 @@ static void tcg_out_exit_tb(TCGContext *s, uintptr_t a0)
static void tcg_out_goto_tb(TCGContext *s, int which)
{
/*
* Ensure that patch area is 8-byte aligned so that an
* atomic write can be used to patch the target address.
* Direct branch, or load indirect address, to be patched
* by tb_target_set_jmp_target. Check indirect load offset
* in range early, regardless of direct branch distance,
* via assert within tcg_out_opc_pcaddu2i.
*/
if ((uintptr_t)s->code_ptr & 7) {
tcg_out_nop(s);
}
uintptr_t i_addr = get_jmp_target_addr(s, which);
intptr_t i_disp = tcg_pcrel_diff(s, (void *)i_addr);
set_jmp_insn_offset(s, which);
/*
* actual branch destination will be patched by
* tb_target_set_jmp_target later
*/
tcg_out_opc_pcaddu18i(s, TCG_REG_TMP0, 0);
tcg_out_opc_pcaddu2i(s, TCG_REG_TMP0, i_disp >> 2);
/* Finish the load and indirect branch. */
tcg_out_ld(s, TCG_TYPE_PTR, TCG_REG_TMP0, TCG_REG_TMP0, 0);
tcg_out_opc_jirl(s, TCG_REG_ZERO, TCG_REG_TMP0, 0);
set_jmp_reset_offset(s, which);
}
void tb_target_set_jmp_target(const TranslationBlock *tb, int n,
uintptr_t jmp_rx, uintptr_t jmp_rw)
{
uintptr_t d_addr = tb->jmp_target_addr[n];
ptrdiff_t d_disp = (ptrdiff_t)(d_addr - jmp_rx) >> 2;
tcg_insn_unit insn;
/* Either directly branch, or load slot address for indirect branch. */
if (d_disp == sextreg(d_disp, 0, 26)) {
insn = encode_sd10k16_insn(OPC_B, d_disp);
} else {
uintptr_t i_addr = (uintptr_t)&tb->jmp_target_addr[n];
intptr_t i_disp = i_addr - jmp_rx;
insn = encode_dsj20_insn(OPC_PCADDU2I, TCG_REG_TMP0, i_disp >> 2);
}
qatomic_set((tcg_insn_unit *)jmp_rw, insn);
flush_idcache_range(jmp_rx, jmp_rw, 4);
}
static void tcg_out_op(TCGContext *s, TCGOpcode opc,
const TCGArg args[TCG_MAX_OP_ARGS],
const int const_args[TCG_MAX_OP_ARGS])

7
tcg/loongarch64/tcg-target.h
View File

@ -42,11 +42,8 @@
#define TCG_TARGET_INSN_UNIT_SIZE 4
#define TCG_TARGET_NB_REGS 32
/*
* PCADDU18I + JIRL sequence can give 20 + 16 + 2 = 38 bits
* signed offset, which is +/- 128 GiB.
*/
#define MAX_CODE_GEN_BUFFER_SIZE (128 * GiB)
#define MAX_CODE_GEN_BUFFER_SIZE ((size_t)-1)
typedef enum {
TCG_REG_ZERO,