KVM: ppc: Write only modified shadow entries into the TLB on exit

Track which TLB entries need to be written, instead of overwriting everything
below the high water mark. Typically only a single guest TLB entry will be
modified in a single exit.

Guest boot time performance improvement: about 15%.

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

arch/powerpc/include/asm/kvm_host.h

@@ -82,6 +82,9 @@ struct kvm_vcpu_arch {
 	/* 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];
+
 	u32 host_stack;
 	u32 host_pid;
 	u32 host_dbcr0;