graphic: 0.594
hypervisor: 0.505
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PID: 0.487
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permissions: 0.476
arm: 0.461
ppc: 0.460
peripherals: 0.460
user-level: 0.456
risc-v: 0.454
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debug: 0.439
vnc: 0.436
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virtual: 0.423
kernel: 0.418
files: 0.415
architecture: 0.401
mistranslation: 0.384
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TCG: 0.377
register: 0.377
socket: 0.340
KVM: 0.332
x86: 0.328
assembly: 0.324
i386: 0.309

linux-user: Qemu doesn't support `set_robust_list` used by glibc robust mutex implementation
Description of problem:
It seems that syscall set_robust_list is not implemented on Qemu for any Linux platform: [link]( https://github.com/qemu/qemu/blob/master/linux-user/syscall.c#L12811)
Steps to reproduce:
1.  Use below toy program `set_robust_list.c` and compile it without optimizations like:
```
    gcc -Wall -W -Wextra -std=gnu17 -pedantic set_robust_list.c -o set_robust_list
```

```
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
#include <linux/futex.h>
#include <syscall.h>

int main(void)
{
#ifdef __NR_set_robust_list
    struct robust_list_head head;
    size_t len = sizeof(struct robust_list_head);

    // This call to set_robust_list function should fail
    int err = syscall(__NR_set_robust_list, &head, -1);
    if (err < 0)
        perror("1st set_robust_list error");
    else
        puts("1st set_robust_list OK");

    // This call to set_robust_list function should be sucessful
    err = syscall(__NR_set_robust_list, &head, len);
    if (err < 0)
        perror("2nd set_robust_list error");
    else
        puts("2nd set_robust_list OK");
#else
    puts("No set_robust_list support");
#endif
    exit(0);
}
```

2. Run program on Qemu and compare output with output from x64 build. In my case it looks like:
```
root@AMDC4705:/runtime/set_robust_list# ./set_robust_list
1st set_robust_list error: Invalid argument
2nd set_robust_list OK
root@AMDC4705:/runtime/set_robust_list# ./set_robust_list-riscv
1st set_robust_list error: Function not implemented
2nd set_robust_list error: Function not implemented
```
Additional information:
Working `set_robust_list` on Linux is quite important in context of named robust mutexes. In NPTL `set_robust_list` is used internally at ld.so initialization time to perform following check: [link](https://github.com/bminor/glibc/blob/master/sysdeps/nptl/dl-tls_init_tp.c#L96)

When syscall fails, later `pthread_mutex_init` (with `PTHREAD_MUTEX_ROBUST` + `PTHREAD_PROCESS_SHARED` attributes) end up with `ENOTSUP` error [link](https://github.com/bminor/glibc/blob/master/nptl/pthread_mutex_init.c#L99).

In dotnet we use robust mutexes for process synchronization purpose. Although there are other available techniques like named semaphores or file locks, robust mutexes are better locking option in case of unexpected process death.