* [Bug 1915327] [NEW] x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
@ 2021-02-10 20:41 Ilya Leoshkevich
2021-05-13 12:04 ` [Bug 1915327] " Thomas Huth
2021-07-13 4:17 ` Launchpad Bug Tracker
0 siblings, 2 replies; 3+ messages in thread
From: Ilya Leoshkevich @ 2021-02-10 20:41 UTC (permalink / raw)
To: qemu-devel
Public bug reported:
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.kernel.org/bpf/b1792bb3c51eb3e94b9d27e67665d3f2209bba7e.camel@linux.ibm.com
** Affects: qemu
Importance: Undecided
Status: New
--
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https://bugs.launchpad.net/bugs/1915327
Title:
x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
Status in QEMU:
New
Bug description:
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.kernel.org/bpf/b1792bb3c51eb3e94b9d27e67665d3f2209bba7e.camel@linux.ibm.com
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1915327/+subscriptions
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug 1915327] Re: x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
2021-02-10 20:41 [Bug 1915327] [NEW] x86_64 cmpxchg behavior in qemu tcg does not match the real CPU Ilya Leoshkevich
@ 2021-05-13 12:04 ` Thomas Huth
2021-07-13 4:17 ` Launchpad Bug Tracker
1 sibling, 0 replies; 3+ messages in thread
From: Thomas Huth @ 2021-05-13 12:04 UTC (permalink / raw)
To: qemu-devel
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.
** Changed in: qemu
Status: New => Incomplete
** Tags added: i386 tcg
--
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1915327
Title:
x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
Status in QEMU:
Incomplete
Bug description:
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.kernel.org/bpf/b1792bb3c51eb3e94b9d27e67665d3f2209bba7e.camel@linux.ibm.com
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1915327/+subscriptions
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug 1915327] Re: x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
2021-02-10 20:41 [Bug 1915327] [NEW] x86_64 cmpxchg behavior in qemu tcg does not match the real CPU Ilya Leoshkevich
2021-05-13 12:04 ` [Bug 1915327] " Thomas Huth
@ 2021-07-13 4:17 ` Launchpad Bug Tracker
1 sibling, 0 replies; 3+ messages in thread
From: Launchpad Bug Tracker @ 2021-07-13 4:17 UTC (permalink / raw)
To: qemu-devel
[Expired for QEMU because there has been no activity for 60 days.]
** Changed in: qemu
Status: Incomplete => Expired
--
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1915327
Title:
x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
Status in QEMU:
Expired
Bug description:
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.kernel.org/bpf/b1792bb3c51eb3e94b9d27e67665d3f2209bba7e.camel@linux.ibm.com
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1915327/+subscriptions
^ permalink raw reply [flat|nested] 3+ messages in thread
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2021-02-10 20:41 [Bug 1915327] [NEW] x86_64 cmpxchg behavior in qemu tcg does not match the real CPU Ilya Leoshkevich
2021-05-13 12:04 ` [Bug 1915327] " Thomas Huth
2021-07-13 4:17 ` Launchpad Bug Tracker
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