Re: [PATCH v6 11/14] x86: add KHO support

[Dave Hansen <dave.hansen@intel.com>]

On 4/29/25 01:06, Mike Rapoport wrote:
> On Mon, Apr 28, 2025 at 03:05:55PM -0700, Dave Hansen wrote:
>> On 4/10/25 22:37, Changyuan Lyu wrote:
>>> From: Alexander Graf <graf@amazon.com>
>>>
>>> +/*
>>> + * If KHO is active, only process its scratch areas to ensure we are not
>>> + * stepping onto preserved memory.
>>> + */
>>
>> Same thing on the imperative voice here.
>>
>> I'm also not fully understanding the comment. Do these "scratch" regions
>> basically represent all the memory that's not being handed over? It's
>> not obvious.
> 
> Scratch memory represents areas created at the boot of the first kernel and
> it's known that scratch areas won't contain any memory that's being handed
> over.
>  
>>> diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
>>> index 57120f0749cc3..c314212a5ecd5 100644
>>> --- a/arch/x86/kernel/e820.c
>>> +++ b/arch/x86/kernel/e820.c
>>> @@ -1300,6 +1300,24 @@ void __init e820__memblock_setup(void)
>>>  		memblock_add(entry->addr, entry->size);
>>>  	}
>>>  
>>> +	/*
>>> +	 * At this point with KHO we only allocate from scratch memory.
>>> +	 * At the same time, we configure memblock to only allow
>>> +	 * allocations from memory below ISA_END_ADDRESS which is not
>>> +	 * a natural scratch region, because Linux ignores memory below
>>> +	 * ISA_END_ADDRESS at runtime. Beside very few (if any) early
>>> +	 * allocations, we must allocate real-mode trapoline below
>>
>> 						trampoline ^
>>
>>> +	 * ISA_END_ADDRESS.
>>> +	 *
>>> +	 * To make sure that we can actually perform allocations during
>>> +	 * this phase, let's mark memory below ISA_END_ADDRESS as scratch
>>> +	 * so we can allocate from there in a scratch-only world.
>>> +	 *
>>> +	 * After real mode trampoline is allocated, we clear scratch
>>> +	 * marking from the memory below ISA_END_ADDRESS
>>> +	 */
>>> +	memblock_mark_kho_scratch(0, ISA_END_ADDRESS);
>>
>> This isn't making a whole ton of sense to me.
>>
>> Is this *only* to facilitate possible users that need <ISA_END_ADDRESS
>> allocations? If so, please say that.
>>
>> I _think_ this is trying to say that KHO kernels are special and are
>> trying to only allocate from scratch areas. But <ISA_END_ADDRESS
>> allocations are both necessary and not marked by KHO _as_ a scratch area
>> which causes a problem.
> 
> Yes :)

So, on both of these, could the submitters please add or revise the
comments to make it more clear?