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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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--->|                   |
>  +-------------+-----+-------------+
>  | 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