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

[Mostafa Saleh]

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 at google.com>
Reviewed-by: Fuad Tabba <fuad.tabba at linux.dev>
---
 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