KVM: PPC: Book3S HV: Allow nested guest creation when L0 hv_guest_state > L1

On powerpc, L1 hypervisor takes help of L0 using H_ENTER_NESTED
hcall to load L2 guest state in cpu. L1 hypervisor prepares the
L2 state in struct hv_guest_state and passes a pointer to it via
hcall. Using that pointer, L0 reads/writes that state directly
from/to L1 memory. Thus L0 must be aware of hv_guest_state layout
of L1. Currently it uses version field to achieve this. i.e. If
L0 hv_guest_state.version != L1 hv_guest_state.version, L0 won't
allow nested kvm guest.

This restriction can be loosened up a bit. L0 can be taught to
understand older layout of hv_guest_state, if we restrict the
new members to be added only at the end, i.e. we can allow
nested guest even when L0 hv_guest_state.version > L1
hv_guest_state.version. Though, the other way around is not
possible.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

1 files changed, 15 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/hvcall.h b/arch/powerpc/include/asm/hvcall.h
index c98f5141e3fc..cb4d07e2c0cc 100644
--- a/arch/powerpc/include/asm/hvcall.h
+++ b/arch/powerpc/include/asm/hvcall.h
@@ -535,9 +535,12 @@ struct h_cpu_char_result {
 	u64 behaviour;
 };
 
-/* Register state for entering a nested guest with H_ENTER_NESTED */
+/*
+ * Register state for entering a nested guest with H_ENTER_NESTED.
+ * New member must be added at the end.
+ */
 struct hv_guest_state {
-	u64 version;		/* version of this structure layout */
+	u64 version;		/* version of this structure layout, must be first */
 	u32 lpid;
 	u32 vcpu_token;
 	/* These registers are hypervisor privileged (at least for writing) */
@@ -571,6 +574,16 @@ struct hv_guest_state {
 /* Latest version of hv_guest_state structure */
 #define HV_GUEST_STATE_VERSION	1
 
+static inline int hv_guest_state_size(unsigned int version)
+{
+	switch (version) {
+	case 1:
+		return offsetofend(struct hv_guest_state, ppr);
+	default:
+		return -1;
+	}
+}
+
 /*
  * From the document "H_GetPerformanceCounterInfo Interface" v1.07
  *