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[Qemu-devel] [PATCH qemu v2] spapr/target-ppc/kvm: Only add hcall-instru

From: Alexey Kardashevskiy
Subject: [Qemu-devel] [PATCH qemu v2] spapr/target-ppc/kvm: Only add hcall-instructions if KVM supports it
Date: Mon, 21 Mar 2016 13:14:02 +1100

ePAPR defines "hcall-instructions" device-tree property which contains
code to call hypercalls in ePAPR paravirtualized guests. In general
pseries guests should not be using this facility (as there is sPAPR
interface) and this property should not be present in the device tree
for pseries guests.

However a while ago this interface was chosen to implement a hypervisor
interface to speed up PR KVM guests and since then the property is always
present in the device tree. All KVM guests use it at least to read features
via the KVM_HC_FEATURES hypercall.

The property is populated by the code returned from the KVM's
KVM_PPC_GET_PVINFO ioctl; if not implemented in the KVM, QEMU writes there
"li r3, -1; nop; nop; nop". If QEMU does not create the property, and
the guest kernel is compiled with CONFIG_EPAPR_PARAVIRT (which is
normally the case), there is exactly the same stub at
@epapr_hypercall_start already.

Rather than maintaining the property (which used to be BE only; then was
fixed to be endian-agnostic) and confusing the guest (which might think
there is ePAPR host while there is none), it makes sense not to create
the property in the device tree in the first place if the host kernel
does not implement it.

This changes kvmppc_get_hypercall() to return 1 if the host kernel
does not implement KVM_CAP_PPC_GET_PVINFO. The caller can use it to decide
on whether to create the property or not.

This changes the pseries machine to not create the property if KVM does
not implement KVM_PPC_GET_PVINFO. In practice this means that from now
on the property will not be created if either HV KVM or TCG is used.

Signed-off-by: Alexey Kardashevskiy <address@hidden>
* changed the commit log
 hw/ppc/spapr.c   | 9 +++++----
 target-ppc/kvm.c | 2 +-
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index d0bb423..d43d6d9 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -497,10 +497,11 @@ static void *spapr_create_fdt_skel(hwaddr initrd_base,
              * Older KVM versions with older guest kernels were broken with the
              * magic page, don't allow the guest to map it.
-            kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
-                                 sizeof(hypercall));
-            _FDT((fdt_property(fdt, "hcall-instructions", hypercall,
-                              sizeof(hypercall))));
+            if (!kvmppc_get_hypercall(first_cpu->env_ptr, hypercall,
+                                      sizeof(hypercall))) {
+                _FDT((fdt_property(fdt, "hcall-instructions", hypercall,
+                                   sizeof(hypercall))));
+            }
diff --git a/target-ppc/kvm.c b/target-ppc/kvm.c
index 776336b..e5183db 100644
--- a/target-ppc/kvm.c
+++ b/target-ppc/kvm.c
@@ -2001,7 +2001,7 @@ int kvmppc_get_hypercall(CPUPPCState *env, uint8_t *buf, 
int buf_len)
     hc[2] = cpu_to_be32(0x48000008);
     hc[3] = cpu_to_be32(bswap32(0x3860ffff));
-    return 0;
+    return 1;
 static inline int kvmppc_enable_hcall(KVMState *s, target_ulong hcall)

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