[PATCH v4 12/14] x86: Add KHO support

[Wei Yang]

On Thu, Feb 06, 2025 at 03:27:52PM +0200, Mike Rapoport wrote:
>From: Alexander Graf <graf at amazon.com>
[...]
>diff --git a/arch/x86/kernel/e820.c b/arch/x86/kernel/e820.c
>index 82b96ed9890a..0b81cd70b02a 100644
>--- a/arch/x86/kernel/e820.c
>+++ b/arch/x86/kernel/e820.c
>@@ -1329,6 +1329,24 @@ 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
>+	 * 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);
>+

At the beginning of e820__memblock_setup() we call memblock_allow_resize(),
which means during adding memory region it could double the array. And the
memory used here is from some region just added.

But with KHO, I am afraid it would fail?

> 	/* Throw away partial pages: */
> 	memblock_trim_memory(PAGE_SIZE);
> 

-- 
Wei Yang
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