syscall: 0.535
instruction: 0.287
runtime: 0.178



QEMU-user ignores MADV_DONTNEED

There is comment int the code "This is a hint, so ignoring and returning success is ok"
https://github.com/qemu/qemu/blob/b1cffefa1b163bce9aebc3416f562c1d3886eeaa/linux-user/syscall.c#L11941

But it seems incorrect with the current state of Linux

"man madvise" or https://man7.org/linux/man-pages/man2/madvise.2.html
says the following:
>>  These advice values do not influence the semantics
>>       of the application (except in the case of MADV_DONTNEED)

>> After a successful MADV_DONTNEED operation, the semantics
>> of memory access in the specified region are changed:
>> subsequent accesses of pages in the range will succeed,
>> but will result in either repopulating the memory contents
>> from the up-to-date contents of the underlying mapped file
>> (for shared file mappings, shared anonymous mappings, and
>> shmem-based techniques such as System V shared memory
>> segments) or zero-fill-on-demand pages for anonymous
>> private mappings.

Some applications use this behavior clear memory and it
would be nice to be able to run them on QEMU without
workarounds.

Reproducer on "Debian 5.10.24 x86_64 GNU/Linux" as a host.


```
#include "assert.h"
#include "stdio.h"
#include <sys/mman.h>
#include <errno.h>

int main() {
  char *P = (char *)mmap(0, 4096, PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  assert(P);
  *P = 'A';
  while (madvise(P, 4096, MADV_DONTNEED) == -1 && errno == EAGAIN) {
  }
  assert(*P == 0);

  printf("OK\n");
}

/*
gcc /tmp/madvice.c -o /tmp/madvice

qemu-x86_64 /tmp/madvice
madvice: /tmp/madvice.c:13: main: Assertion `*P == 0' failed.
qemu: uncaught target signal 6 (Aborted) - core dumped
Aborted

/tmp/madvice
OK


*/

```