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, 8 insertions, 11 deletions

diff --git a/arch/powerpc/kvm/44x.c b/arch/powerpc/kvm/44x.c
index 22054b164b5a..05d72fc8b478 100644
--- a/arch/powerpc/kvm/44x.c
+++ b/arch/powerpc/kvm/44x.c
@@ -96,21 +96,14 @@ void kvmppc_core_load_guest_debugstate(struct kvm_vcpu *vcpu)
 
 void kvmppc_core_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
-        int i;
-
-        /* Mark every guest entry in the shadow TLB entry modified, so that they
-         * will all be reloaded on the next vcpu run (instead of being
-         * demand-faulted). */
-        for (i = 0; i <= tlb_44x_hwater; i++)
-                kvmppc_tlbe_set_modified(vcpu, i);
 }
 
 void kvmppc_core_vcpu_put(struct kvm_vcpu *vcpu)
 {
-        /* Don't leave guest TLB entries resident when being de-scheduled. */
-        /* XXX It would be nice to differentiate between heavyweight exit and
-         * sched_out here, since we could avoid the TLB flush for heavyweight
-         * exits. */
+        /* XXX Since every guest uses TS=1 TID=0/1 mappings, we can't leave any TLB
+         * entries around when we're descheduled, so we must completely flush the
+         * TLB of all guest mappings. On the other hand, if there is only one
+         * guest, this flush is completely unnecessary. */
         _tlbia();
 }
 
@@ -130,6 +123,7 @@ int kvmppc_core_vcpu_setup(struct kvm_vcpu *vcpu)
 {
         struct kvmppc_vcpu_44x *vcpu_44x = to_44x(vcpu);
         struct kvmppc_44x_tlbe *tlbe = &vcpu_44x->guest_tlb[0];
+        int i;
 
         tlbe->tid = 0;
         tlbe->word0 = PPC44x_TLB_16M | PPC44x_TLB_VALID;
@@ -148,6 +142,9 @@ int kvmppc_core_vcpu_setup(struct kvm_vcpu *vcpu)
          * CCR1[TCS]. */
         vcpu->arch.ccr1 = mfspr(SPRN_CCR1);
 
+        for (i = 0; i < ARRAY_SIZE(vcpu_44x->shadow_refs); i++)
+                vcpu_44x->shadow_refs[i].gtlb_index = -1;
+
         return 0;
 }