Re: plugins: Missing Store Exclusive Memory Accesses

[Aaron Lindsay via <qemu-devel@nongnu.org>]

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;
}

}