Re: [PATCH v3 5/5] x86/boot: Clarify comment

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/10/2024 4:06 pm, Alejandro Vallejo wrote:
> On Fri Oct 11, 2024 at 2:58 PM BST, Frediano Ziglio wrote:
>> On Fri, Oct 11, 2024 at 2:38 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
>> wrote:
>>> On 11/10/2024 2:28 pm, Alejandro Vallejo wrote:
>>>> On Fri, Oct 11, 2024 at 02:08:37PM +0100, Frediano Ziglio wrote:
>>>>> On Fri, Oct 11, 2024 at 1:56 PM Alejandro Vallejo
>>>>> <alejandro.vallejo@xxxxxxxxx> wrote:
>>>>>> On Fri, Oct 11, 2024 at 09:52:44AM +0100, Frediano Ziglio wrote:
>>>>>>> Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
>>>>>>> ---
>>>>>>>  xen/arch/x86/boot/reloc.c | 2 +-
>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/xen/arch/x86/boot/reloc.c b/xen/arch/x86/boot/reloc.c
>>>>>>> index e50e161b27..e725cfb6eb 100644
>>>>>>> --- a/xen/arch/x86/boot/reloc.c
>>>>>>> +++ b/xen/arch/x86/boot/reloc.c
>>>>>>> @@ -65,7 +65,7 @@ typedef struct memctx {
>>>>>>>      /*
>>>>>>>       * Simple bump allocator.
>>>>>>>       *
>>>>>>> -     * It starts from the base of the trampoline and allocates 
>>>>>>> downwards.
>>>>>>> +     * It starts on top of space reserved for the trampoline and 
>>>>>>> allocates downwards.
>>>>>> nit: Not sure this is much clearer. The trampoline is not a stack (and 
>>>>>> even if
>>>>>> it was, I personally find "top" and "bottom" quite ambiguous when it 
>>>>>> grows
>>>>>> backwards), so calling top to its lowest address seems more confusing 
>>>>>> than not.
>>>>>>
>>>>>> If anything clarification ought to go in the which direction it takes. 
>>>>>> Leaving
>>>>>> "base" instead of "top" and replacing "downwards" by "backwards" to make 
>>>>>> it
>>>>>> crystal clear that it's a pointer that starts where the trampoline 
>>>>>> starts, but
>>>>>> moves in the opposite direction.
>>>>>>
>>>>> Base looks confusing to me, but surely that comment could be confusing.
>>>>> For the trampoline 64 KB are reserved. Last 4 KB are used as a normal
>>>>> stack (push/pop/call/whatever), first part gets a copy of the
>>>>> trampoline code/data (about 6 Kb) the rest (so 64 - 4 - ~6 = ~54 kb)
>>>>> is used for the copy of MBI information. That "rest" is what we are
>>>>> talking about here.
>>>> Last? From what I looked at it seems to be the first 12K.
>>>>
>>>>    #define TRAMPOLINE_STACK_SPACE  PAGE_SIZE
>>>>    #define TRAMPOLINE_SPACE        (KB(64) - TRAMPOLINE_STACK_SPACE)
>>>>
>>>> To put it another way, with left=lo-addr and right=hi-addr. The code seems 
>>>> to
>>>> do this...
>>>>
>>>>  |<--------------64K-------------->|
>>>>  |<-----12K--->|                   |
> s/12K/4K/
>
> My brain merged the 12bits in the wrong place. Too much bit twiddling.
>
>>>>  +-------------+-----+-------------+
>>>>  | stack-space | mbi | trampoline  |
>>>>  +-------------+-----+-------------+
>>>>                ^  ^
>>>>                |  |
>>>>                |  +-- copied Multiboot info + modules
>>>>                +----- initial memctx.ptr
>>>>
>>>> ... with the stack growing backwards to avoid overflowing onto mbi.
>>>>
>>>> Or am I missing something?
>>> So I was hoping for some kind of diagram like this, to live in
>>> arch/x86/include/asm/trampoline.h with the other notes about the trampoline.
>>>
>>> But, is that diagram accurate?  Looking at
>>        /* Switch to low-memory stack which lives at the end of
>> trampoline region. */
>>        mov     sym_esi(trampoline_phys), %edi
>>        lea     TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE(%edi),%esp
>>        lea     trampoline_boot_cpu_entry-trampoline_start(%edi),%eax
>>        pushl   $BOOT_CS32
>>        push    %eax
>>
>>        /* Copy bootstrap trampoline to low memory, below 1MB. */
>>        lea     sym_esi(trampoline_start), %esi
>>        mov     $((trampoline_end - trampoline_start) / 4),%ecx
>>        rep movsl
>>
>> So, from low to high
>> - trampoline code/data (%edi at beginning of copy is trampoline_phys,
>> %esi is trampoline_start)
>> - space (used for MBI copy)
>> - stack (%esp is set to trampoline_phys + TRAMPOLINE_SPACE +
>> TRAMPOLINE_STACK_SPACE)
>>
>> Frediano
> So it's reversed from what I thought
>
>  |<--------------64K-------------->|
>  |                   |<-----4K---->|
>  +-------------+-----+-------------+
>  |  text-(ish) | mbi | stack-space |
>  +-------------+-----+-------------+
>                   ^                ^
>                   |                |
>                   |                +-- initial memctx.ptr
>                   +------------------- copied Multiboot info + modules
>
>
> Your version of the comment is a definite improvement over the nonsense that
> was there before. Sorry for the noise :)

Today, the pointer that becomes memctx.ptr is phys+SPACE, which does not
include the stack.

So initial memctx.ptr starts immediately below the stack, and the bump
allocator goes backwards (leftwards).

~Andrew