Re: [PATCH v8 29/43] arm64: RME: Always use 4k pages for realms

[Gavin Shan <gshan@redhat.com>]

On 4/16/25 11:41 PM, 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.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v7:
>   * Rewritten commit message
> ---
>   arch/arm64/kvm/mmu.c | 4 ++++
>   1 file changed, 4 insertions(+)
> 

One nitpick below.

Reviewed-by: Gavin Shan <gshan@redhat.com>

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 02b66ee35426..29bab7a46033 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1653,6 +1653,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>   	if (logging_active || is_protected_kvm_enabled()) {
>   		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;

		/* Force PTE level mappings for realms */

>   	} else {
>   		vma_shift = get_vma_page_shift(vma, hva);
>   	}

Thanks,
Gavin