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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 11/10/2024 2:38 pm, Andrew Cooper 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--->|                   |
>>  +-------------+-----+-------------+
>>  | 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

Sorry, sent too early.

GDB says that trampoline_end-trampoline_start is 6512, so one and a half
pages.

TRAMPOLINE_STACK_SPACE is 4k, and subtracted from 64k to make
TRAMPOLINE_SPACE

So this is why code uses TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE (==
64k) for it's size calculation.

Within that, we've got MBI_SPACE_MIN (8k) used in the linker assertion,
for SPACE (60k) - (end - start)(~7k).


What we're really saying is that the total size is 64k, with the top 4k
being stack, the bottom 7k being .text(ish), and the middle 53k being
the MBI relocation area.

And memctx is a backwards bump allocator within the middle 53k.

Maybe we should defer this until after the Hyperlaunch BootInfo series
has come in.  Later parts have a major change on how we handle the MBI
block, and I can see some substantial pruning opportunities.

~Andrew