From: Launchpad Bug Tracker <1915327@bugs.launchpad.net>
To: qemu-devel@nongnu.org
Subject: [Bug 1915327] Re: x86_64 cmpxchg behavior in qemu tcg does not match the real CPU
Date: Tue, 13 Jul 2021 04:17:23 -0000 [thread overview]
Message-ID: <162614984350.15811.8705644630794054093.malone@loganberry.canonical.com> (raw)
In-Reply-To: 161298968608.29108.9925609875505640314.malonedeb@soybean.canonical.com
[Expired for QEMU because there has been no activity for 60 days.]
** Changed in: qemu
Status: Incomplete => Expired
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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:
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:
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prev parent reply other threads:[~2021-07-13 4:33 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
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 [this message]
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