id = 2495
title = "A bug in x86-64 MMX instructions"
state = "closed"
created_at = "2024-08-11T14:42:09.034Z"
closed_at = "2024-08-14T02:52:58.775Z"
labels = ["Closed::Fixed", "accel: TCG", "target: i386"]
url = "https://gitlab.com/qemu-project/qemu/-/issues/2495"
host-os = "Ubuntu 22.04"
host-arch = "x86-64"
qemu-version = "qemu-x86_64 version 9.0.91 (v9.1.0-rc1-6-g0f397dcfec)"
guest-os = "N/A (qemu-user)"
guest-arch = "x86_64"
description = """It seems QEMU emits invalid TCG when lifting MMX instructions with redundant REX prefixes. For example, when lifting `490f7ec0 (movq r8, mm0)`, QEMU generates the following valid TCG.

```
 ---- 00000000004011f2 0000000000000000
 call enter_mmx,$0x0,$0,env
 ld_i64 loc0,env,$0x270
 mov_i64 r8,loc0
 mov_i64 rip,$0x4011f6
 exit_tb $0x0
 set_label $L0
 exit_tb $0x7f84f82ec143
```

However, after changing the value of the rex prefix to `4f` , so the instruction becomes `4f0f7ec0 (rex.WRXB movq r8, mm0)`, the lifted TCG is changed to:

```
 ---- 00000000004011f2 0000000000000000
 call enter_mmx,$0x0,$0,env
 ld_i64 loc0,env,$0x2f0 // The offset to MM0 is changed
 mov_i64 r8,loc0
 mov_i64 rip,$0x4011f6
 exit_tb $0x0
 set_label $L0
 exit_tb $0x7f98e82ec143
```

I have observed this bug in numerous MMX instructions. For example, `410fdaff (rex.B pminub mm7, mm7)` is lifted to the wrong TCGs.

It seems this bug looks similar to #2474."""
reproduce = """1. Write `test.c` 
```
#include <stdint.h>
#include <stdio.h>
#include <string.h>

uint8_t i_R8[8] = { 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0 };
uint8_t i_MM0[8] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
uint8_t o_R8[8];

void __attribute__ ((noinline)) show_state() {
    printf("R8: ");
    for (int i = 0; i < 8; i++) {
        printf("%02x ", o_R8[i]);
    }
    printf("\\n");
}

void __attribute__ ((noinline)) run() {
    __asm__ (
        ".intel_syntax noprefix\\n"
        "mov r8, qword ptr [rip + i_R8]\\n"
        "movq mm0, qword ptr [rip + i_MM0]\\n"
        ".byte 0x4f, 0x0f, 0x7e, 0xc0\\n"
        "mov qword ptr [rip + o_R8], r8\\n"
        ".att_syntax\\n"
    );
}

int main(int argc, char **argv) {
    run();
    show_state();
    return 0;
}
```    
2. Compile `test.bin` using this command: `gcc-12 -O2 -no-pie ./test.c -o ./test.bin`
3. Run QEMU using this command: `qemu-x86_64 ./test.bin` 
4. The program, runs on top of the buggy QEMU, prints the value of R8 as `00 00 00 00 00 00 00 00`. It should print `ff ff ff ff ff ff ff ff` after the bug is fixed."""
additional = """"""