[PATCH v10 29/43] arm64: RME: Always use 4k pages for realms

Steven Price
2025-08-20T14:59:35Z

Guest_memfd doesn't yet natively support huge pages, and there are
currently difficulties for a VMM to manage huge pages efficiently so for
now always split up mappings to PTE (4k).

The two issues that need progressing before supporting huge pages for
realms are:

1. guest_memfd needs to be able to allocate from an appropriate
allocator which can provide huge pages.

2. The VMM needs to be able to repurpose private memory for a shared
mapping when the guest VM requests memory is transitioned. Because
this can happen at a 4k granularity it isn't possible to
free/reallocate while huge pages are in use. Allowing the VMM to
mmap() the shared portion of a huge page would allow the huge page
to be recreated when the memory is unshared and made protected again.

These two issues are not specific to realms and don't affect the realm
API, so for now just break everything down to 4k pages in the RMM
controlled stage 2. Future work can add huge page support without
changing the uAPI.

Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
---
Changes since v7:
* Rewritten commit message
---
arch/arm64/kvm/mmu.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a5bd78bfc5ec..676c01d80875 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1674,6 +1674,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
if (logging_active) {
force_pte = true;
vma_shift = PAGE_SHIFT;
+ } else if (vcpu_is_rec(vcpu)) {
+ /* Force PTE level mappings for realms */
+ force_pte = true;
+ vma_shift = PAGE_SHIFT;
} else {
vma_shift = get_vma_page_shift(vma, hva);
}
--
2.43.0