On Fri, 23 Jul 2021 16:55:39 +0100,
Alexandru Elisei [off-list ref] wrote:

Hi Marc,

On 7/17/21 10:55 AM, Marc Zyngier wrote:

quoted

Since we only support PMD-sized mappings for THP, getting
a permission fault on a level that results in a mapping
being larger than PAGE_SIZE is a sure indication that we have
already upgraded our mapping to a PMD.

In this case, there is no need to try and parse userspace page
tables, as the fault information already tells us everything.

Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/kvm/mmu.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index db6314b93e99..c036a480ca27 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -1088,9 +1088,14 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	 * If we are not forced to use page mapping, check if we are
 	 * backed by a THP and thus use block mapping if possible.
 	 */
-	if (vma_pagesize == PAGE_SIZE && !(force_pte || device))
-		vma_pagesize = transparent_hugepage_adjust(kvm, memslot, hva,
-							   &pfn, &fault_ipa);
+	if (vma_pagesize == PAGE_SIZE && !force_pte) {

Looks like now it's possible to call transparent_hugepage_adjust()
for devices (if fault_status != FSC_PERM). Commit 2aa53d68cee6
("KVM: arm64: Try stage2 block mapping for host device MMIO") makes
a good case for the !device check. Was the check removed
unintentionally?

That's what stupid bugs are made of. I must have resolved a rebase
conflict the wrong way, and lost this crucial bit. Thanks for spotting
this! Now fixed.

quoted

+		if (fault_status == FSC_PERM && fault_granule > PAGE_SIZE)
+			vma_pagesize = fault_granule;
+		else
+			vma_pagesize = transparent_hugepage_adjust(kvm, memslot,
+								   hva, &pfn,
+								   &fault_ipa);
+	}

This change makes sense to me - we can only get stage 2 permission
faults on a leaf entry since stage 2 tables don't have the
APTable/XNTable/PXNTable bits. The biggest block mapping size that
we support at stage 2 is PMD size (from
transparent_hugepage_adjust()), therefore if fault_granule is larger
than PAGE_SIZE, then it must be PMD_SIZE.

Yup, exactly.

Thanks again,

	M.

-- 
Without deviation from the norm, progress is not possible.