Re: [Qemu-devel] 3 new x86 instructions

[Torbjorn Granlund]

The new gdb (7.5.1) did not change anything.

The program behaves one way inside gdb and one way outside.
The behaviour inside gdb is surely incorrect.

I think the behaviour outside of gdb is also incorrect, but I haven't
spent much time thinking about it.

To repdoduce curious behaviour:

Compile the attached programs and link them.

foo.c
Description: Binary data

x.s
Description: Binary data

Execute.  I believe correct behaviour is to print:

    000000000000000d 000000000000000d 040000000000000b 000000000000000a

The program under your special qemu instead prints:

    000000000000000e 000000000000000d 040000000000000b 000000000000000a

Perhaps I am being silly and the program is behaving correctly.

But now fire up gdb.  Set a breakpoint at __gmpn_addmul_1.  Run.
Single-step to just before the first adox.  Then look at eflags:

    (gdb) info reg eflags
    eflags         0x246    [ PF ZF IF ]

We're about to read the 10 from the main program's a[4]:

    (gdb) p *(unsigned long *)($rdi+8*$rcx)
    $3 = 10

Single-step over the adox.  Again look at eflags:

    (gdb) info reg eflags
    eflags         0x3a0ede [ PF #3 AF ZF SF IF DF OF VM VIF VIP ID ]

Oops.  Lots of undesirable things happened to the flags.  Now continuing
will lock up in the jo .  insn, unsurprisingly.


Guest uname -a:
FreeBSD hwl.gmplib.org 9.1-RELEASE FreeBSD 9.1-RELEASE #0 r243825: Tue Dec 4 
09:23:10 UTC 2012
address@hidden:/usr/obj/usr/src/sys/GENERIC amd64

-- 
Torbjörn