Re: Emulation failure

[Duy Nguyen TN <duy.nguyen@dektech.com.au>]

Vào T2, ngày 19, 08 năm 2013 lúc 11:27 +0200, Paolo Bonzini viết:
> > The disassembled code is
> > 
> >    0x1dd10:    push   %rbx
> >    0x1dd11:    mov    $0x6e,%eax
> >    0x1dd16:    mov    %rdi,%rbx
> >    0x1dd19:    sub    $0x20,%rsp
> >    0x1dd1d:    test   %rdi,%rdi
> >    0x1dd20:    je     0xb1dd92
> >    0x1dd22:    mov    0x4bf1e0(%rip),%eax
> >    0x1dd28:    cmp    $0xffffffff,%eax
> >    0x1dd2b:    je     0xb1ddd0
> >    0x1dd31:    test   %eax,%eax
> >    0x1dd33:    jne    0xb1dd92
> >    0x1dd35:    mov    0xe1f55c(%rip),%rax
> >    0x1dd3c:    cmpq   $0x0,0xf0(%rax)
> >    0x1dd44:    fildll 0xf0(%rax)
> >    0x1dd4a:    js     0xb1ddf0
> >    0x1dd50:    mov    0xe1f54a(%rip),%eax
> >    0x1dd56:    mov    %rax,-0x80(%rsp)
> >    0x1dd5b:    fildll -0x80(%rsp)
> >    0x1dd5f:    fmulp  %st,%st(1)
> > 
> > Not sure if it helps but rax after 0xb1dd35 contains the pointer to
> > mmap'd memory of /dev/hpet
> 
> I think this wouldn't work even with the latest kernel.  Emulation of
> x87 instructions is not supported yet.

I'm confused. How could this program work? It produces similar assembly
listing

-- 8< --
#include <stdio.h>
#include <stdint.h>

uint64_t s_rtcClockPeriod = 10;
uint64_t mc = 3000000000;
int main(int ac, char **av)
{
        uint64_t value = (uint64_t)((long double)mc * 
                         (long double)s_rtcClockPeriod /
                        1000000000.0L);
        printf("%lu\n", value);
        return 0;
}
-- 8< --

and the assembly I got is

-- 8< --
sub    $0x18,%rsp
cmpq   $0x0,0x200adc(%rip)
fildll 0x200ad6(%rip)
js     0x4005f8 <main+184>
cmpq   $0x0,0x200ac0(%rip)
fildll 0x200aba(%rip)
js     0x400612 <main+210>
fmulp  %st,%st(1)
fdivs  0x1ac(%rip)
flds   0x1aa(%rip)
fxch   %st(1)
fucomi %st(1),%st
jae    0x4005c0 <main+128>
fstp   %st(1)
fnstcw 0x16(%rsp)
...
-- 8< --