Re: [PATCH v6 29/43] arm64: RME: Always use 4k pages for realms

[Gavin Shan]

On 12/13/24 1:55 AM, Steven Price wrote:
> Always split up huge pages to avoid problems managing huge pages. There
> are two issues currently:
>
> 1. The uABI for the VMM allows populating memory on 4k boundaries even
>     if the underlying allocator (e.g. hugetlbfs) is using a larger page
>     size. Using a memfd for private allocations will push this issue onto
>     the VMM as it will need to respect the granularity of the allocator.
>
> 2. The guest is able to request arbitrary ranges to be remapped as
>     shared. Again with a memfd approach it will be up to the VMM to deal
>     with the complexity and either overmap (need the huge mapping and add
>     an additional 'overlapping' shared mapping) or reject the request as
>     invalid due to the use of a huge page allocator.
>
> For now just break everything down to 4k pages in the RMM controlled
> stage 2.
>
> Signed-off-by: Steven Price <steven.price@xxxxxxx>
> ---
>   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 e88714903ce5..9ede143ccef1 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1603,6 +1603,10 @@ 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 (kvm_is_realm(kvm)) {
> +		// Force PTE level mappings for realms
> +		force_pte = true;
> +		vma_shift = PAGE_SHIFT;
>   	} else {
>   		vma_shift = get_vma_page_shift(vma, hva);
>   	}

Since a memory abort is specific to a vCPU instead of a VM, so vcpu_is_rec()
instead of kvm_is_realm() is more accurate for the check. Besides, it looks
duplicate to the check added by "PATCH[20/43] arm64: RME: Runtime faulting
of memory", which is as below.

       /* FIXME: We shouldn't need to disable this for realms */
       if (vma_pagesize == PAGE_SIZE && !(force_pte || device || kvm_is_realm(kvm))) {
                                                                 ^^^^^^^^^^^^^^^^^
                                                                 Can be dropped now.

Thanks,
Gavin