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Re: plugins: Missing Store Exclusive Memory Accesses
From: |
Aaron Lindsay |
Subject: |
Re: plugins: Missing Store Exclusive Memory Accesses |
Date: |
Wed, 20 Oct 2021 16:49:47 -0400 |
On Oct 20 18:54, Alex Bennée wrote:
> Have you got a test case you are using so I can try and replicate the
> failure you are seeing? So far by inspection everything looks OK to me.
I took some time today to put together a minimal(ish) reproducer using
usermode. The source files used are below, I compiled the test binary on an
AArch64 system using:
$ gcc -g -o stxp stxp.s stxp.c
Then built the plugin from stxp_plugin.cc, and ran it all like:
qemu-aarch64 \
-cpu cortex-a57 \
-D stxp_plugin.log \
-d plugin \
-plugin 'stxp_plugin.so' \
./stxp
I observe that, for me, the objdump of stxp contains:
000000000040070c <loop>:
40070c: f9800011 prfm pstl1strm, [x0]
400710: c87f4410 ldxp x16, x17, [x0]
400714: c8300c02 stxp w16, x2, x3, [x0]
400718: f1000652 subs x18, x18, #0x1
40071c: 54000040 b.eq 400724 <done> // b.none
400720: 17fffffb b 40070c <loop>
But the output in stxp_plugin.log looks something like:
Executing PC: 0x40070c
Executing PC: 0x400710
PC 0x400710 accessed memory at 0x550080ec70
PC 0x400710 accessed memory at 0x550080ec78
Executing PC: 0x400714
Executing PC: 0x400718
Executing PC: 0x40071c
Executing PC: 0x400720
>From this, I believe the ldxp instruction at PC 0x400710 is reporting two
memory accesses but the stxp instruction at 0x400714 is not.
-Aaron
--- stxp.c ---
void stxp_issue_demo();
int main() {
char arr[16];
stxp_issue_demo(&arr);
}
--- stxp.s ---
.align 8
stxp_issue_demo:
mov x18, 0x1000
mov x2, 0x0
mov x3, 0x0
loop:
prfm pstl1strm, [x0]
ldxp x16, x17, [x0]
stxp w16, x2, x3, [x0]
subs x18, x18, 1
beq done
b loop
done:
ret
.global stxp_issue_demo
--- stxp_plugin.cc ---
#include <stdio.h>
extern "C" {
#include <qemu-plugin.h>
QEMU_PLUGIN_EXPORT int qemu_plugin_version = QEMU_PLUGIN_VERSION;
void qemu_logf(const char *str, ...)
{
char message[1024];
va_list args;
va_start(args, str);
vsnprintf(message, 1023, str, args);
qemu_plugin_outs(message);
va_end(args);
}
void before_insn_cb(unsigned int cpu_index, void *udata)
{
uint64_t pc = (uint64_t)udata;
qemu_logf("Executing PC: 0x%" PRIx64 "\n", pc);
}
static void mem_cb(unsigned int cpu_index, qemu_plugin_meminfo_t meminfo,
uint64_t va, void *udata)
{
uint64_t pc = (uint64_t)udata;
qemu_logf("PC 0x%" PRIx64 " accessed memory at 0x%" PRIx64 "\n", pc, va);
}
static void vcpu_tb_trans(qemu_plugin_id_t id, struct qemu_plugin_tb *tb)
{
size_t n = qemu_plugin_tb_n_insns(tb);
for (size_t i = 0; i < n; i++) {
struct qemu_plugin_insn *insn = qemu_plugin_tb_get_insn(tb, i);
uint64_t pc = qemu_plugin_insn_vaddr(insn);
qemu_plugin_register_vcpu_insn_exec_cb(insn, before_insn_cb,
QEMU_PLUGIN_CB_R_REGS, (void *)pc);
qemu_plugin_register_vcpu_mem_cb(insn, mem_cb, QEMU_PLUGIN_CB_NO_REGS,
QEMU_PLUGIN_MEM_RW, (void*)pc);
}
}
QEMU_PLUGIN_EXPORT
int qemu_plugin_install(qemu_plugin_id_t id, const qemu_info_t *info,
int argc, char **argv)
{
qemu_plugin_register_vcpu_tb_trans_cb(id, vcpu_tb_trans);
return 0;
}
}