powerpc: Disable relocation on exceptions whenever PR KVM is active

For PR KVM we allow userspace to map 0xc000000000000000. Because
transitioning from userspace to the guest kernel may use the relocated
exception vectors we have to disable relocation on exceptions whenever
PR KVM is active as we cannot trust that address.

This issue does not apply to HV KVM, since changing from a guest to the
hypervisor will never use the relocated exception vectors.

Currently the hypervisor interface only allows us to toggle relocation
on exceptions on a partition wide scope, so we need to globally disable
relocation on exceptions when the first PR KVM instance is started and
only re-enable them when all PR KVM instances have been destroyed.

It's a bit heavy handed, but until the hypervisor gives us a lightweight
way to toggle relocation on exceptions on a single thread it's only real
option.

Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/arch/powerpc/kvm/book3s_pr.c b/arch/powerpc/kvm/book3s_pr.c
index 28d38adeca73..67e4708388a0 100644
--- a/arch/powerpc/kvm/book3s_pr.c
+++ b/arch/powerpc/kvm/book3s_pr.c
@@ -34,6 +34,7 @@
 #include <asm/kvm_book3s.h>
 #include <asm/mmu_context.h>
 #include <asm/switch_to.h>
+#include <asm/firmware.h>
 #include <linux/gfp.h>
 #include <linux/sched.h>
 #include <linux/vmalloc.h>
@@ -1284,12 +1285,21 @@ void kvmppc_core_flush_memslot(struct kvm *kvm, struct kvm_memory_slot *memslot)
 {
 }
 
+static unsigned int kvm_global_user_count = 0;
+static DEFINE_SPINLOCK(kvm_global_user_count_lock);
+
 int kvmppc_core_init_vm(struct kvm *kvm)
 {
 #ifdef CONFIG_PPC64
 	INIT_LIST_HEAD(&kvm->arch.spapr_tce_tables);
 #endif
 
+	if (firmware_has_feature(FW_FEATURE_SET_MODE)) {
+		spin_lock(&kvm_global_user_count_lock);
+		if (++kvm_global_user_count == 1)
+			pSeries_disable_reloc_on_exc();
+		spin_unlock(&kvm_global_user_count_lock);
+	}
 	return 0;
 }
 
@@ -1298,6 +1308,14 @@ void kvmppc_core_destroy_vm(struct kvm *kvm)
 #ifdef CONFIG_PPC64
 	WARN_ON(!list_empty(&kvm->arch.spapr_tce_tables));
 #endif
+
+	if (firmware_has_feature(FW_FEATURE_SET_MODE)) {
+		spin_lock(&kvm_global_user_count_lock);
+		BUG_ON(kvm_global_user_count == 0);
+		if (--kvm_global_user_count == 0)
+			pSeries_enable_reloc_on_exc();
+		spin_unlock(&kvm_global_user_count_lock);
+	}
 }
 
 static int kvmppc_book3s_init(void)