Re: [PATCH 3/8] KVM: arm64: Propagate and use kvm_s2_fault_result on S2 fault

When stage 2 page tables fault the net result may either be that a page is
mapped, an error occurred or the fault should be retried (-EAGAIN).

When a fault succeeds it may be upgraded to a PMD size via
transparent_hugepage_adjust().

In order to support KVM pre-faulting the outcome of the fault and the
mapping size must be recorded.

Track this in the new kvm_s2_fault_result struct, which is threaded through
gmem_abort(), user_mem_abort() and kvm_s2_fault_map().

PKVM and SEA aren't relevant to synthetic pre-faulting so neither
kvm_inject_sea() nor pkvm_mem_abort() are altered.

Actual hardware faulting doesn't require this information, so
kvm_handle_guest_abort() simply passes NULL kvm_s2_fault_result to
gmem_abort() and user_mem_abort().

Faults are necessarily ephemeral and pre-faulting can't guarantee what may
happen in parallel, so do not store the GFN or PFN in
kvm_s2_fault_result. Pre-faulting only needs to know what was mapped in at
the point of the fault.

This struct could be replaced with a pointer to an unsigned long, however
it's clearer to separate out the mapped flag and having a struct allows us
to easily add additional fields in future as needed.

No functional change intended.

Suggested-by: Vincent Donnefort <redacted>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
---
 arch/arm64/kvm/mmu.c | 33 +++++++++++++++++++++++++++------
 1 file changed, 27 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 80cb520e25b9..da15da4e40e6 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -1607,6 +1607,11 @@ struct kvm_s2_fault_desc {
 	struct kvm_s2_mmu	*mmu;
 };
 
+struct kvm_s2_fault_result {
+	unsigned long mapping_size;
+	bool mapped;
+};
+
 static bool kvm_s2_fault_is_perm(const struct kvm_s2_fault_desc *s2fd)
 {
 	return esr_fsc_is_permission_fault(s2fd->esr);

@@ -1632,7 +1637,17 @@ static u64 kvm_s2_perm_fault_granule(const struct kvm_s2_fault_desc *s2fd)
 	return BIT(ARM64_HW_PGTABLE_LEVEL_SHIFT(level));
 }
 
-static int gmem_abort(const struct kvm_s2_fault_desc *s2fd)
+static void populate_fault_result(struct kvm_s2_fault_result *result,
+				  unsigned long mapping_size)
+{
+	/* A THP upgrade may have altered mapping size. */
+	result->mapping_size = mapping_size;
+	/* -EAGAIN is swallowed so be explicit when we actually map. */
+	result->mapped = true;