On Tue, Mar 27, 2012 at 8:34 PM, H.J. Lu <hjl.to...@gmail.com> wrote:

> %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.

Let me say this way: please propose the patch that implements your
suggestions. I will leave the approval of the patch to someone else.


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