x86_64 cmpxchg behavior in qemu tcg does not match the real CPU

[Ilya Leoshkevich]

QEMU version:
1214d55d1c (HEAD, origin/master, origin/HEAD) Merge remote-tracking branch 'remotes/nvme/tags/nvme-next-pull-request' into staging

Consider the following little program:

$ cat 1.c
#include <stdio.h>
int main() {
  int mem = 0x12345678;
  register long rax asm("rax") = 0x1234567812345678;
  register int edi asm("edi") = 0x77777777;
  asm("cmpxchg %[edi],%[mem]"
      : [ mem ] "+m"(mem), [ rax ] "+r"(rax)
      : [ edi ] "r"(edi));
  long rax2 = rax;
  printf("rax2 = %lx\n", rax2);
}

According to the Intel Manual, cmpxchg should not touch the accumulator in case the values are equal, which is indeed the case on the real CPU:

$ gcc 1.c
$ ./a.out
rax2 = 1234567812345678

However, QEMU appears to zero extend EAX to RAX:

$ qemu-x86_64 ./a.out
rax2 = 12345678

This is also the case for lock cmpxchg.

Found in BPF development context: https://lore<email address hidden>

[Thomas Huth (th-huth)]

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on gitlab.com, please open a new ticket
for this problem in our new tracker here:

    https://gitlab.com/qemu-project/qemu/-/issues

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on gitlab.com and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes
anymore).

Thank you and sorry for the inconvenience.

[Launchpad Janitor (janitor)]

[Expired for QEMU because there has been no activity for 60 days.]