[Feature]: Consider setting cache attributes on freed pages

[thomashinds]

Feature Overview

When memory is freed via free_pages, Patina sets certain attributes before freeing - preserving cache attributes and setting RP & XP.

ref:

patina/patina_dxe_core/src/gcd/spin_locked_gcd.rs

Lines 2652 to 2656 in 4e014bd

	if let Err(e) = self.set_memory_space_attributes_worker(
	current_range.start as usize,
	(current_range.end - current_range.start) as usize,
	MemoryProtectionPolicy::apply_free_memory_policy(desc.attributes),
	desc.attributes,

patina/patina_dxe_core/src/gcd.rs

Lines 228 to 238 in 4e014bd

	/// Rule: All free memory should be marked as EFI_MEMORY_RP, EFI_MEMORY_XP, and the preserved cache attributes.
	/// EFI_MEMORY_RP will cause the memory to be unmapped in the page table, but we still set EFI_MEMORY_XP to align
	/// with the originally added memory so that free memory can be coalesced into fewer blocks.
	///
	/// Arguments
	/// - * `attributes` - The existing attributes for the memory region
	///
	/// Use Case: This is called whenever memory is freed in the GCD
	pub(crate) const fn apply_free_memory_policy(attributes: u64) -> u64 {
	(attributes & efi::CACHE_ATTRIBUTE_MASK) | efi::MEMORY_RP | efi::MEMORY_XP
	}

Solution Overview

Consider enforcing opinionated cache attributes on freed pages, in order to have a stronger guarantee on the state of the pages' attributes if allocated in the future, and to allow freed regions to coalesce together if cache attributes were changed while in use. This should be reasonable to do because platforms should not impose requirements on the cache attributes of pooled free memory.

Alternatives Considered

• Make no changes - as I understand it this is closer to how EDK2's core behaves today but has the drawbacks identified above.
• Enforce that the caller should restore attributes before freeing.

Urgency

Low

Are you going to implement the feature request?

Someone else needs to implement the feature

Do you need maintainer feedback?

Maintainer feedback requested

Anything else?

No response