Re: [PATCH v13 36/48] arm64: RMI: Always use 4k pages for realms

Joey Gouly
2026-03-19T10:24:59Z

Hi,

On Wed, Mar 18, 2026 at 03:54:00PM +0000, Steven Price wrote:
> 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.

The commit title/message mention 4K, but should probably say PAGE_SIZE or
something now that RMM isn't fixed to 4K.

Thanks,
Joey

>
> 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 | 7 +++++--
> 1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 73c18c2861a2..ad1300f366df 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1761,11 +1761,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> write_fault = kvm_is_write_fault(vcpu);
>
> /*
> - * Realms cannot map protected pages read-only
> + * Realms cannot map protected pages read-only, also force PTE mappings
> + * for Realms.
> * FIXME: It should be possible to map unprotected pages read-only
> */
> - if (vcpu_is_rec(vcpu))
> + if (vcpu_is_rec(vcpu)) {
> write_fault = true;
> + force_pte = true;
> + }
>
> exec_fault = kvm_vcpu_trap_is_exec_fault(vcpu);
> VM_WARN_ON_ONCE(write_fault && exec_fault);
> --
> 2.43.0
>
>