On Tue, Mar 27, 2012 at 10:53 AM, Uros Bizjak <ubiz...@gmail.com> wrote:
> On Tue, Mar 27, 2012 at 7:28 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
>
>> GCC needs to move the value in the %fs segment
>> register into %r32 or %r64. This instruction
>>
>> "mov{l}\t{%%fs:0, %k0|%k0, DWORD PTR fs:0}"
>>
>> does exactly what GCC wants.
>
> Sorry, I really don't understand what you are trying to say.
>
> You are loading ptrmode (so, void *) pointer from %fs:0 to a DImode
> register. If you use movl, you can say that this instruction zero
> extends the value (void *, ptrmode, SImode) from a memory location
> pointed by %fs:0 to a DImode register. Please note the difference
> between:
>
> movl %fs:0, %eax
>
> and
>
> movl %fs, %eax.
>
> BTW: %fs is a 16bit register.
>
%fs and %gs are special in 64bit mode. For a memory operand
"%fs:address", its effective address is base address of %fs + address.
The base address of %fs and %fs are hidden. "mov %fs, %eax"
will only access the visible part of %fs, which is the 16bit segment
selector. In 64bit mode, UNSPEC_TP is the base address of %fs.
To access the base address of %fs, we can use system call:
int arch_prctl(int code, unsigned long addr);
int arch_prctl(int code, unsigned long *addr);
ARCH_SET_FS
Set the 64-bit base for the FS register to addr.
ARCH_GET_FS
Return the 64-bit base value for the FS register of the
current thread in the unsigned long pointed to by addr.
BTW, 4 new instructions are added to read/write base address of
%fs/%gs directly. For now, we have to use the system call to
update base address of %fs, To read the base address of %fs,
OS arranges that the base address of %fs points to a struct:
typedef struct
{
void *tcb; /* Pointer to the TCB. Not necessarily the
thread descriptor used by libpthread. */
...
}
and sets up tcb == the base address of %fs. Then we can use
"mov{l}\t{%%fs:0, %k0|%k0, DWORD PTR fs:0}"
to move the base address of %fs into %r32 and %r64 directly.
I hope this answers your questions.
Thanks.
--
H.J.
