qemu: Add capability for the ccf-assist pSeries feature

Linux kernel 5.1 added a new PPC KVM capability named
KVM_PPC_CPU_CHAR_BCCTR_FLUSH_ASSIST, which is exposed to the QEMU guest
since QEMU commit 8ff43ee404d under a new sPAPR capability called
SPAPR_CAP_CCF_ASSIST. This cap indicates whether the processor supports
hardware acceleration for the count cache flush workaround, which
is a software workaround that flushes the count cache on context
switch. If the processor has this hardware acceleration, the software
flush can be shortened, resulting in performance gain.

This hardware acceleration is defaulted to 'off' in QEMU. The reason
is that earlier versions of the Power 9 processor didn't support
it (it is available on Power 9 DD2.3 and newer), and defaulting this
option to 'on' would break migration compatibility between the Power 9
processor class.

However, the user running a P9 DD2.3+ hypervisor might want to create
guests with ccf-assist=on, accepting the downside of only being able
to migrate them only between other P9 DD2.3+ hosts running upstream
kernel 5.1+, to get a performance boost.

This patch adds this new capability to Libvirt, with the name of
QEMU_CAPS_MACHINE_PSERIES_CAP_CCF_ASSIST.
Reviewed-by: NCole Robinson <crobinso@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_capabilities.c

@@ -546,6 +546,7 @@ VIR_ENUM_IMPL(virQEMUCaps,
               "query-cpu-model-baseline",
               "query-cpu-model-comparison",
               "ramfb",
+              "machine.pseries.cap-ccf-assist",
     );
 
 
@@ -1442,6 +1443,7 @@ static struct virQEMUCapsStringFlags virQEMUCapsMachinePropsPSeries[] = {
     { "cap-hpt-max-page-size", QEMU_CAPS_MACHINE_PSERIES_CAP_HPT_MAX_PAGE_SIZE },
     { "cap-htm", QEMU_CAPS_MACHINE_PSERIES_CAP_HTM },
     { "cap-nested-hv", QEMU_CAPS_MACHINE_PSERIES_CAP_NESTED_HV },
+    { "cap-ccf-assist", QEMU_CAPS_MACHINE_PSERIES_CAP_CCF_ASSIST },
 };
 
 static struct virQEMUCapsStringFlags virQEMUCapsMachinePropsVirt[] = {

src/qemu/qemu_capabilities.h

@@ -527,6 +527,7 @@ typedef enum { /* virQEMUCapsFlags grouping marker for syntax-check */
     QEMU_CAPS_QUERY_CPU_MODEL_BASELINE, /* qmp query-cpu-model-baseline */
     QEMU_CAPS_QUERY_CPU_MODEL_COMPARISON, /* qmp query-cpu-model-comparison */
     QEMU_CAPS_DEVICE_RAMFB, /* -device ramfb */
+    QEMU_CAPS_MACHINE_PSERIES_CAP_CCF_ASSIST, /* -machine pseries.cap-ccf-assist */
 
     QEMU_CAPS_LAST /* this must always be the last item */
 } virQEMUCapsFlags;

tests/qemucapabilitiesdata/caps_4.0.0.ppc64.xml

@@ -170,6 +170,7 @@
   <flag name='nbd-bitmap'/>
   <flag name='bochs-display'/>
   <flag name='migration-file-drop-cache'/>
+  <flag name='machine.pseries.cap-ccf-assist'/>
   <version>4000000</version>
   <kvmVersion>0</kvmVersion>
   <microcodeVersion>42900758</microcodeVersion>