[PATCH v2] KVM: arm64: Optimize protected mode and FWB

KVM opportunistically enables FWB if supported by the system for guest
VMs, which allows it to elude cache maintenance for data as they are
forced to be cacheable from stage-2.
In that case, __clean_dcache_guest_page() will immediately return.
However in protected mode, before calling __clean_dcache_guest_page()
it loops over the range and fix_map/unmap it, issuing TLB
invalidations, dsb() and isb() unnecessarily.

This can be optimized by returning early if FWB is supported,
kvm_pgtable_stage2_map() already issues dsb() and tlb invalidation
functions issue dsb() for the unmap path.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Reviewed-by: Fuad Tabba <redacted>
---
 arch/arm64/kvm/hyp/nvhe/mem_protect.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
index 4e329e39a695..049a09be2bbf 100644
--- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
+++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c

@@ -261,6 +261,13 @@ static void __apply_guest_page(void *va, size_t size,
 
 static void clean_dcache_guest_page(void *va, size_t size)
 {
+	/*
+	 * Guest stage-2 uses FWB if available, making it safe to elide CMOs.
+	 * In contrast, the host stage-2 never has FWB enabled.
+	 */
+	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
+		return;
+
 	__apply_guest_page(va, size, __clean_dcache_guest_page);
 }
 

-- 
2.55.0.229.g6434b31f56-goog