On 2/14/25 2:14 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(+)
The change log looks confusing to me. Currently, there are 3 classes of stage2 faults,
handled by their corresponding handlers like below.
stage2 fault in the private space: private_memslot_fault()
stage2 fault in the MMIO space: io_mem_abort()
stage2 fault in the shared space: user_mem_abort()
Only the stage2 fault in the private space needs to allocate a 4KB page from guest-memfd.
This patch is changing user_mem_abort(), which is all about the stage2 fault in the shared
space, where a guest-memfd isn't involved. The only intersection between the private/shared
space is the stage2 page table. I'm guessing we want to have enforced 4KB page is due to
the shared stage2 page table by the private/shared space, or I'm wrong.
What I'm understanding from the change log: it's something to be improved in future due to
only 4KB pages can be supported by guest-memfd. Please correct me if I'm wrong.
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 994e71cfb358..8c656a0ef4e9 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -1641,6 +1641,10 @@ 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;
} else {
vma_shift = get_vma_page_shift(vma, hva);
}
Thanks,
Gavin
