KVM: ppc: directly insert shadow mappings into the hardware TLB

Formerly, we used to maintain a per-vcpu shadow TLB and on every entry to the
guest would load this array into the hardware TLB. This consumed 1280 bytes of
memory (64 entries of 16 bytes plus a struct page pointer each), and also
required some assembly to loop over the array on every entry.

Instead of saving a copy in memory, we can just store shadow mappings directly
into the hardware TLB, accepting that the host kernel will clobber these as
part of the normal 440 TLB round robin. When we do that we need less than half
the memory, and we have decreased the exit handling time for all guest exits,
at the cost of increased number of TLB misses because the host overwrites some
guest entries.

These savings will be increased on processors with larger TLBs or which
implement intelligent flush instructions like tlbivax (which will avoid the
need to walk arrays in software).

In addition to that and to the code simplification, we have a greater chance of
leaving other host userspace mappings in the TLB, instead of forcing all
subsequent tasks to re-fault all their mappings.

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 15 insertions, 9 deletions

diff --git a/arch/powerpc/include/asm/kvm_44x.h b/arch/powerpc/include/asm/kvm_44x.h
index 72e593914adb..e770ea2bbb1c 100644
--- a/arch/powerpc/include/asm/kvm_44x.h
+++ b/arch/powerpc/include/asm/kvm_44x.h
@@ -22,19 +22,25 @@
 
 #include <linux/kvm_host.h>
 
-/* XXX Can't include mmu-44x.h because it redefines struct mm_context. */
 #define PPC44x_TLB_SIZE 64
 
+/* If the guest is expecting it, this can be as large as we like; we'd just
+ * need to find some way of advertising it. */
+#define KVM44x_GUEST_TLB_SIZE 64
+
+struct kvmppc_44x_shadow_ref {
+	struct page *page;
+	u16 gtlb_index;
+	u8 writeable;
+	u8 tid;
+};
+
 struct kvmppc_vcpu_44x {
 	/* Unmodified copy of the guest's TLB. */
-	struct kvmppc_44x_tlbe guest_tlb[PPC44x_TLB_SIZE];
-	/* TLB that's actually used when the guest is running. */
-	struct kvmppc_44x_tlbe shadow_tlb[PPC44x_TLB_SIZE];
-	/* Pages which are referenced in the shadow TLB. */
-	struct page *shadow_pages[PPC44x_TLB_SIZE];
-
-	/* Track which TLB entries we've modified in the current exit. */
-	u8 shadow_tlb_mod[PPC44x_TLB_SIZE];
+	struct kvmppc_44x_tlbe guest_tlb[KVM44x_GUEST_TLB_SIZE];
+
+	/* References to guest pages in the hardware TLB. */
+	struct kvmppc_44x_shadow_ref shadow_refs[PPC44x_TLB_SIZE];
 
 	struct kvm_vcpu vcpu;
 };