[U-Boot] [PATCH 07/17] x86: Use fs for global data

[Graeme Russ <graeme.russ@gmail.com>]

Hi Simon,

On 04/01/12 16:36, Simon Glass wrote:
> Hi Graeme,
> 
> On Sun, Jan 1, 2012 at 8:09 PM, Graeme Russ <graeme.russ@gmail.com> wrote:
>> Use the base address of the 'F' segment as a pointer to the global data
>> structure. By adding the linear address (i.e. the 'D' segment address)
>> as the first word of the global data structure, the address of the
>> global data relative to the 'D' segment can be found simply, for
>> example, by:
>>
>>        fs movl 0, %eax
>>
>> This makes the gd 'pointer' writable prior to relocation (by reloading
>> the GDT) which brings x86 into line with all other arches
> 
> What is the GDT?

Global Descriptor Table - It's a kind of lookup table which the x86 CPU
uses to calculate physical addresses relative to 'segments' - By placing
the start of the 'F' segment to the physical address of the global data
structure, and adding a self-referencing physical address to the global
data structure (as the first member), reading the first word of the 'F'
segment provides the physical address of the global data

>>
>> NOTE: Writing to the gd 'pointer' is expensive (but we only do it
>> twice) but using it to access global data members (read and write) is
>> still fairly cheap
>>
>> Signed-off-by: Graeme Russ <graeme.russ@gmail.com>
>> ---
>>  arch/x86/cpu/cpu.c                 |   53 ++++++++++++++++++++--------------
>>  arch/x86/cpu/start.S               |    8 ++++-
>>  arch/x86/include/asm/global_data.h |   21 +++++++++----
>>  arch/x86/include/asm/processor.h   |    6 +++-
>>  arch/x86/include/asm/u-boot-x86.h  |    2 +
>>  arch/x86/lib/board.c               |   56 +++++++++++++++++++++++++-----------
>>  6 files changed, 98 insertions(+), 48 deletions(-)
>>
>> diff --git a/arch/x86/cpu/cpu.c b/arch/x86/cpu/cpu.c
>> index bf55c26..e7a5fc0 100644
>> --- a/arch/x86/cpu/cpu.c
>> +++ b/arch/x86/cpu/cpu.c
>> @@ -90,6 +90,37 @@ static void load_gdt(const u64 *boot_gdt, u16 num_entries)
>>        asm volatile("lgdtl %0\n" : : "m" (gdt));
>>  }
>>
>> +void init_gd(gd_t *id, u64 *gdt_addr)
>> +{
>> +       id->gd_addr = (ulong)id;
>> +       setup_gdt(id, gdt_addr);
>> +}
>> +
>> +void setup_gdt(gd_t *id, u64 *gdt_addr)
> 
> I will probably never understand this function but a comment might be nice.

Hmm, I guess I really don't know how to comment this to explain it clearly
- The GDT is a bit of a magical beast which you kind of have to learn by
osmosis ;)

> 
>> +{
>> +       /* CS: code, read/execute, 4 GB, base 0 */
>> +       gdt_addr[GDT_ENTRY_32BIT_CS] = GDT_ENTRY(0xc09b, 0, 0xfffff);
>> +
>> +       /* DS: data, read/write, 4 GB, base 0 */
>> +       gdt_addr[GDT_ENTRY_32BIT_DS] = GDT_ENTRY(0xc093, 0, 0xfffff);
>> +
>> +       /* FS: data, read/write, 4 GB, base (Global Data Pointer) */
>> +       gdt_addr[GDT_ENTRY_32BIT_FS] = GDT_ENTRY(0xc093, (ulong)id, 0xfffff);
>> +
>> +               /* 16-bit CS: code, read/execute, 64 kB, base 0 */
> 
> Extra indent?

Removed

[snip]

>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 203c63a..07897f9 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -24,13 +24,17 @@
>>  #ifndef __ASM_PROCESSOR_H_
>>  #define __ASM_PROCESSOR_H_ 1
>>
>> +#define GDT_ENTRY_SIZE         8
>> +
>>  #define GDT_ENTRY_NULL         0
>>  #define GDT_ENTRY_UNUSED       (GDT_ENTRY_NULL + 1)
>>  #define GDT_ENTRY_32BIT_CS     (GDT_ENTRY_UNUSED + 1)
>>  #define GDT_ENTRY_32BIT_DS     (GDT_ENTRY_32BIT_CS + 1)
>> -#define GDT_ENTRY_16BIT_CS     (GDT_ENTRY_32BIT_DS + 1)
>> +#define GDT_ENTRY_32BIT_FS     (GDT_ENTRY_32BIT_DS + 1)
>> +#define GDT_ENTRY_16BIT_CS     (GDT_ENTRY_32BIT_FS + 1)
>>  #define GDT_ENTRY_16BIT_DS     (GDT_ENTRY_16BIT_CS + 1)
>>
>>  #define GDT_NUM_ENTRIES                (GDT_ENTRY_16BIT_DS + 1)
>> +#define GDT_SIZE               (GDT_NUM_ENTRIES * GDT_ENTRY_SIZE)
> 
> Wasn't this already done in an earlier patch?

I've moved this change into patch #3

Regards,

Graeme