target/ppc: 'PVR != host PVR' in KVM_SET_SREGS workaround

Commit d5fc133eed ("ppc: Rework CPU compatibility testing
across migration") changed the way cpu_post_load behaves with
the PVR setting, causing an unexpected bug in KVM-HV migrations
between hosts that are compatible (POWER8 and POWER8E, for example).
Even with pvr_match() returning true, the guest freezes right after
cpu_post_load. The reason is that the guest kernel can't handle a
different PVR value other that the running host in KVM_SET_SREGS.

In [1] it was discussed the possibility of a new KVM capability
that would indicate that the guest kernel can handle a different
PVR in KVM_SET_SREGS. Even if such feature is implemented, there is
still the problem with older kernels that will not have this capability
and will fail to migrate.

This patch implements a workaround for that scenario. If running
with KVM, check if the guest kernel does not have the capability
(named here as 'cap_ppc_pvr_compat'). If it doesn't, calls
kvmppc_is_pr() to see if the guest is running in KVM-HV. If all this
happens, set env->spr[SPR_PVR] to the same value as the current
host PVR. This ensures that we allow migrations with 'close enough'
PVRs to still work in KVM-HV but also makes the code ready for
this new KVM capability when it is done.

A new function called 'kvmppc_pvr_workaround_required' was created
to encapsulate the conditions said above and to avoid calling too
many kvm.c internals inside cpu_post_load.

[1] https://lists.gnu.org/archive/html/qemu-ppc/2017-06/msg00503.html

Signed-off-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
[dwg: Fix for the case of using TCG on a PPC host]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 22 insertions, 0 deletions

diff --git a/target/ppc/machine.c b/target/ppc/machine.c
index abe0a1cdf0..e36b7100cb 100644
--- a/target/ppc/machine.c
+++ b/target/ppc/machine.c
@@ -9,6 +9,7 @@
 #include "mmu-hash64.h"
 #include "migration/cpu.h"
 #include "qapi/error.h"
+#include "kvm_ppc.h"
 
 static int cpu_load_old(QEMUFile *f, void *opaque, int version_id)
 {
@@ -249,6 +250,27 @@ static int cpu_post_load(void *opaque, int version_id)
         }
     }
 
+    /*
+     * If we're running with KVM HV, there is a chance that the guest
+     * is running with KVM HV and its kernel does not have the
+     * capability of dealing with a different PVR other than this
+     * exact host PVR in KVM_SET_SREGS. If that happens, the
+     * guest freezes after migration.
+     *
+     * The function kvmppc_pvr_workaround_required does this verification
+     * by first checking if the kernel has the cap, returning true immediately
+     * if that is the case. Otherwise, it checks if we're running in KVM PR.
+     * If the guest kernel does not have the cap and we're not running KVM-PR
+     * (so, it is running KVM-HV), we need to ensure that KVM_SET_SREGS will
+     * receive the PVR it expects as a workaround.
+     *
+     */
+#if defined(CONFIG_KVM)
+    if (kvmppc_pvr_workaround_required(cpu)) {
+        env->spr[SPR_PVR] = env->spr_cb[SPR_PVR].default_value;
+    }
+#endif
+
     env->lr = env->spr[SPR_LR];
     env->ctr = env->spr[SPR_CTR];
     cpu_write_xer(env, env->spr[SPR_XER]);