[RFC PATCH] arm/arm64: KVM: allow the use of THP on 2MB aligned memslots

[Marc Zyngier]

The THP code in KVM/ARM is a bit restrictive in not allowing a THP
to be used if the VMA is not 2MB aligned. Actually, it is not so much
the VMA that matters, but the associated memslot:

A process can perfectly mmap a region with no particular alignment
restriction, and then pass a 2MB aligned address to KVM. In this
case, KVM will only use this 2MB aligned region, and will ignore
the range between vma->vm_start and memslot->userspace_addr.

The fix is then to check the alignment of memslot->userspace_addr.

Cc: Christoffer Dall <christoffer.dall at linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
---
 arch/arm/kvm/mmu.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
index 5809069..cec641a 100644
--- a/arch/arm/kvm/mmu.c
+++ b/arch/arm/kvm/mmu.c
@@ -667,14 +667,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 		gfn = (fault_ipa & PMD_MASK) >> PAGE_SHIFT;
 	} else {
 		/*
-		 * Pages belonging to VMAs not aligned to the PMD mapping
+		 * Pages belonging to memslots not aligned to the PMD mapping
 		 * granularity cannot be mapped using block descriptors even
 		 * if the pages belong to a THP for the process, because the
 		 * stage-2 block descriptor will cover more than a single THP
 		 * and we loose atomicity for unmapping, updates, and splits
 		 * of the THP or other pages in the stage-2 block range.
 		 */
-		if (vma->vm_start & ~PMD_MASK)
+		if (memslot->userspace_addr & ~PMD_MASK)
 			force_pte = true;
 	}
 	up_read(&current->mm->mmap_sem);
-- 
1.8.2.3