[libvirt] [PATCH v2 1/3] qemu: Add capability for the ccf-assist pSeries feature

[Daniel Henrique Barboza]

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.

Signed-off-by: Daniel Henrique Barboza <danielhb413 at gmail.com>
---
 src/qemu/qemu_capabilities.c                    | 2 ++
 src/qemu/qemu_capabilities.h                    | 1 +
 tests/qemucapabilitiesdata/caps_4.0.0.ppc64.xml | 1 +
 3 files changed, 4 insertions(+)

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 46a056340b..b73534c0d1 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -545,6 +545,7 @@ VIR_ENUM_IMPL(virQEMUCaps,
               "incremental-backup",
               "query-cpu-model-baseline",
               "query-cpu-model-comparison",
+              "machine.pseries.cap-ccf-assist",
     );
 
 
@@ -1440,6 +1441,7 @@ 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[] = {
diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
index 72da3691f2..3da8e3d0f0 100644
--- a/src/qemu/qemu_capabilities.h
+++ b/src/qemu/qemu_capabilities.h
@@ -526,6 +526,7 @@ typedef enum { /* virQEMUCapsFlags grouping marker for syntax-check */
     QEMU_CAPS_INCREMENTAL_BACKUP, /* incremental backup is supported */
     QEMU_CAPS_QUERY_CPU_MODEL_BASELINE, /* qmp query-cpu-model-baseline */
     QEMU_CAPS_QUERY_CPU_MODEL_COMPARISON, /* qmp query-cpu-model-comparison */
+    QEMU_CAPS_MACHINE_PSERIES_CAP_CCF_ASSIST, /* -machine pseries.cap-ccf-assist */
 
     QEMU_CAPS_LAST /* this must always be the last item */
 } virQEMUCapsFlags;
diff --git a/tests/qemucapabilitiesdata/caps_4.0.0.ppc64.xml b/tests/qemucapabilitiesdata/caps_4.0.0.ppc64.xml
index 9ea6f4d046..5edd2c1c31 100644
--- a/tests/qemucapabilitiesdata/caps_4.0.0.ppc64.xml
+++ b/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>
-- 
2.21.0