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plugins: fix optimization in plugin_gen_disable_mem_helpers

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We were mistakenly checking tcg_ctx->plugin_insn as a canary to know
whether the TB had emitted helpers that might have accessed memory.

The problem is that tcg_ctx->plugin_insn gets updated on every
instruction in the TB, which results in us wrongly performing the
optimization (i.e. not clearing cpu->plugin_mem_cbs) way too often,
since it's not rare that the last instruction in the TB doesn't
use helpers.

Fix it by tracking a per-TB canary.

While at it, expand documentation.

Related: #1381

Signed-off-by: Emilio Cota <cota@braap.org>
Message-Id: <20230108164731.61469-2-cota@braap.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230124180127.1881110-32-alex.bennee@linaro.org>
This commit is contained in:
Emilio Cota 2023-01-24 18:01:23 +00:00 committed by Alex Bennée
parent 2bbbc1be8d
commit 3fd62e73ad
2 changed files with 25 additions and 8 deletions
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26
accel/tcg/plugin-gen.c
View File

@ -579,7 +579,8 @@ static void inject_mem_helper(TCGOp *begin_op, GArray *arr)
* is possible that the code we generate after the instruction is
* dead, we also add checks before generating tb_exit etc.
*/
static void inject_mem_enable_helper(struct qemu_plugin_insn *plugin_insn,
static void inject_mem_enable_helper(struct qemu_plugin_tb *ptb,
struct qemu_plugin_insn *plugin_insn,
TCGOp *begin_op)
{
GArray *cbs[2];
@ -599,6 +600,7 @@ static void inject_mem_enable_helper(struct qemu_plugin_insn *plugin_insn,
rm_ops(begin_op);
return;
}
ptb->mem_helper = true;
arr = g_array_sized_new(false, false,
sizeof(struct qemu_plugin_dyn_cb), n_cbs);
@ -626,15 +628,22 @@ void plugin_gen_disable_mem_helpers(void)
{
TCGv_ptr ptr;
if (likely(tcg_ctx->plugin_insn == NULL ||
!tcg_ctx->plugin_insn->mem_helper)) {
/*
* We could emit the clearing unconditionally and be done. However, this can
* be wasteful if for instance plugins don't track memory accesses, or if
* most TBs don't use helpers. Instead, emit the clearing iff the TB calls
* helpers that might access guest memory.
*
* Note: we do not reset plugin_tb->mem_helper here; a TB might have several
* exit points, and we want to emit the clearing from all of them.
*/
if (!tcg_ctx->plugin_tb->mem_helper) {
return;
}
ptr = tcg_const_ptr(NULL);
tcg_gen_st_ptr(ptr, cpu_env, offsetof(CPUState, plugin_mem_cbs) -
offsetof(ArchCPU, env));
tcg_temp_free_ptr(ptr);
tcg_ctx->plugin_insn->mem_helper = false;
}
static void plugin_gen_tb_udata(const struct qemu_plugin_tb *ptb,
@ -682,14 +691,14 @@ static void plugin_gen_mem_inline(const struct qemu_plugin_tb *ptb,
inject_inline_cb(cbs, begin_op, op_rw);
}
static void plugin_gen_enable_mem_helper(const struct qemu_plugin_tb *ptb,
static void plugin_gen_enable_mem_helper(struct qemu_plugin_tb *ptb,
TCGOp *begin_op, int insn_idx)
{
struct qemu_plugin_insn *insn = g_ptr_array_index(ptb->insns, insn_idx);
inject_mem_enable_helper(insn, begin_op);
inject_mem_enable_helper(ptb, insn, begin_op);
}
static void plugin_gen_disable_mem_helper(const struct qemu_plugin_tb *ptb,
static void plugin_gen_disable_mem_helper(struct qemu_plugin_tb *ptb,
TCGOp *begin_op, int insn_idx)
{
struct qemu_plugin_insn *insn = g_ptr_array_index(ptb->insns, insn_idx);
@ -750,7 +759,7 @@ static void pr_ops(void)
#endif
}
static void plugin_gen_inject(const struct qemu_plugin_tb *plugin_tb)
static void plugin_gen_inject(struct qemu_plugin_tb *plugin_tb)
{
TCGOp *op;
int insn_idx = -1;
@ -870,6 +879,7 @@ bool plugin_gen_tb_start(CPUState *cpu, const DisasContextBase *db,
ptb->haddr1 = db->host_addr[0];
ptb->haddr2 = NULL;
ptb->mem_only = mem_only;
ptb->mem_helper = false;
plugin_gen_empty_callback(PLUGIN_GEN_FROM_TB);
}

7
include/qemu/plugin.h
View File

@ -118,7 +118,10 @@ struct qemu_plugin_insn {
void *haddr;
GArray *cbs[PLUGIN_N_CB_TYPES][PLUGIN_N_CB_SUBTYPES];
bool calls_helpers;
/* if set, the instruction calls helpers that might access guest memory */
bool mem_helper;
bool mem_only;
};
@ -158,6 +161,10 @@ struct qemu_plugin_tb {
void *haddr1;
void *haddr2;
bool mem_only;
/* if set, the TB calls helpers that might access guest memory */
bool mem_helper;
GArray *cbs[PLUGIN_N_CB_SUBTYPES];
};