[PATCH 6/6] arm64: KVM: Add support for VPIPT I-caches

Will Deacon will.deacon at arm.com
Fri Mar 10 12:32:25 PST 2017

A VPIPT I-cache has two main properties:

1. Lines allocated into the cache are tagged by VMID and a lookup can
   only hit lines that were allocated with the current VMID.

2. I-cache invalidation from EL1/0 only invalidates lines that match the
   current VMID of the CPU doing the invalidation.

This can cause issues with non-VHE configurations, where the host runs
at EL1 and wants to invalidate I-cache entries for a guest running with
a different VMID. VHE is not affected, because the host runs at EL2 and
I-cache invalidation applies as expected.

This patch solves the problem by invalidating the I-cache when unmapping
a page at stage 2 on a system with a VPIPT I-cache but not running with
VHE enabled. Hopefully this is an obscure enough configuration that the
overhead isn't anything to worry about, although it does mean that the
by-range I-cache invalidation currently performed when mapping at stage
2 can be elided on such systems, because the I-cache will be clean for
the guest VMID following a rollover event.

Signed-off-by: Will Deacon <will.deacon at arm.com>
 arch/arm64/include/asm/kvm_mmu.h |  9 +++++----
 arch/arm64/kvm/hyp/tlb.c         | 22 ++++++++++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
index dc3624d8b9db..d2293d49f555 100644
--- a/arch/arm64/include/asm/kvm_mmu.h
+++ b/arch/arm64/include/asm/kvm_mmu.h
@@ -242,12 +242,13 @@ static inline void __coherent_cache_guest_page(struct kvm_vcpu *vcpu,
 	kvm_flush_dcache_to_poc(va, size);
-	if (!icache_is_aliasing()) {		/* PIPT */
-		flush_icache_range((unsigned long)va,
-				   (unsigned long)va + size);
-	} else {
+	if (icache_is_aliasing()) {
 		/* any kind of VIPT cache */
+	} else if (is_kernel_in_hyp_mode() || !icache_is_vpipt()) {
+		/* PIPT or VPIPT at EL2 (see comment in __kvm_tlb_flush_vmid_ipa) */
+		flush_icache_range((unsigned long)va,
+				   (unsigned long)va + size);
diff --git a/arch/arm64/kvm/hyp/tlb.c b/arch/arm64/kvm/hyp/tlb.c
index e8e7ba2bc11f..f02c7e6a8db4 100644
--- a/arch/arm64/kvm/hyp/tlb.c
+++ b/arch/arm64/kvm/hyp/tlb.c
@@ -46,6 +46,28 @@ void __hyp_text __kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
+	/*
+	 * If the host is running at EL1 and we have a VPIPT I-cache,
+	 * then we must perform I-cache maintenance at EL2 in order for
+	 * it to have an effect on the guest. Since the guest cannot hit
+	 * I-cache lines allocated with a different VMID, we don't need
+	 * to worry about junk out of guest reset (we nuke the I-cache on
+	 * VMID rollover), but we do need to be careful when remapping
+	 * executable pages for the same guest. This can happen when KSM
+	 * takes a CoW fault on an executable page, copies the page into
+	 * a page that was previously mapped in the guest and then needs
+	 * to invalidate the guest view of the I-cache for that page
+	 * from EL1. To solve this, we invalidate the entire I-cache when
+	 * unmapping a page from a guest if we have a VPIPT I-cache but
+	 * the host is running at EL1. As above, we could do better if
+	 * we had the VA.
+	 *
+	 * The moral of this story is: if you have a VPIPT I-cache, then
+	 * you should be running with VHE enabled.
+	 */
+	if (!has_vhe() && icache_is_vpipt())
+		__flush_icache_all();
 	write_sysreg(0, vttbr_el2);

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