[libvirt] [PATCH v2] qemu: Add check for whether KVM nesting is enabled

[John Ferlan]

Support for nested KVM is handled via a kernel module configuration
adjustment which if done after libvirtd is started and/or the last
QEMU capabilities adjustment can result in the inability to start a
guest and use nested KVM until the capabilities cache is invalidated.
This is because without knowing, the CPU settings for a guest may not
add the vmx=on to/for the guest config.

Thus, let's fetch and save the setting during initialization and then
when the capabilities are checked for various host related adjustments
that could affect whether the capabilities cache is updated add a check
whether the nested value was set for Intel, AMD, or s390 to force a
refetch of the capabilities.

Signed-off-by: John Ferlan <jferlan at redhat.com>
---
 v1 was part of an RFC:

 https://www.redhat.com/archives/libvir-list/2018-November/msg00494.html

 This patch alters that code slightly to add the check Marc Hartmayer
 requested for S390 and to use "kvm" in the API names and variables to
 make it clearer that it's not CapsIsNested but CapsKVMIsNested.

 If it's felt the new check should slide down further in virQEMUCapsIsValid
 then that's fine - just let me know what it should follow.

 src/qemu/qemu_capabilities.c | 45 ++++++++++++++++++++++++++++++++++++
 src/qemu/qemu_capspriv.h     |  2 ++
 tests/qemucapabilitiestest.c |  3 +++
 3 files changed, 50 insertions(+)

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index fde27010e4..c377733fe6 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -40,6 +40,7 @@
 #include "virnodesuspend.h"
 #include "virnuma.h"
 #include "virhostcpu.h"
+#include "virkmod.h"
 #include "qemu_monitor.h"
 #include "virstring.h"
 #include "qemu_hostdev.h"
@@ -557,6 +558,7 @@ struct _virQEMUCaps {
     virObject parent;
 
     bool usedQMP;
+    bool kvmIsNested;
 
     char *binary;
     time_t ctime;
@@ -1528,6 +1530,7 @@ virQEMUCapsPtr virQEMUCapsNewCopy(virQEMUCapsPtr qemuCaps)
         return NULL;
 
     ret->usedQMP = qemuCaps->usedQMP;
+    ret->kvmIsNested = qemuCaps->kvmIsNested;
 
     if (VIR_STRDUP(ret->binary, qemuCaps->binary) < 0)
         goto error;
@@ -3587,6 +3590,9 @@ virQEMUCapsLoadCache(virArch hostArch,
     virQEMUCapsInitHostCPUModel(qemuCaps, hostArch, VIR_DOMAIN_VIRT_KVM);
     virQEMUCapsInitHostCPUModel(qemuCaps, hostArch, VIR_DOMAIN_VIRT_QEMU);
 
+    qemuCaps->kvmIsNested = virXPathBoolean("count(./kvmIsNested) > 0",
+                                            ctxt) > 0;
+
     ret = 0;
  cleanup:
     VIR_FREE(str);
@@ -3806,6 +3812,9 @@ virQEMUCapsFormatCache(virQEMUCapsPtr qemuCaps)
     if (qemuCaps->sevCapabilities)
         virQEMUCapsFormatSEVInfo(qemuCaps, &buf);
 
+    if (qemuCaps->kvmIsNested)
+        virBufferAddLit(&buf, "<kvmIsNested/>\n");
+
     virBufferAdjustIndent(&buf, -2);
     virBufferAddLit(&buf, "</qemuCaps>\n");
 
@@ -3846,6 +3855,30 @@ virQEMUCapsSaveFile(void *data,
 }
 
 
+static bool
+virQEMUCapsKVMIsNested(void)
+{
+    VIR_AUTOFREE(char *) kConfig = NULL;
+
+    /* Intel, AMD, and s390 related checks */
+    if ((kConfig = virKModConfig()) &&
+        (strstr(kConfig, "kvm_intel nested=1") ||
+         strstr(kConfig, "kvm_amd nested=1") ||
+         strstr(kConfig, "kvm nested=1")))
+        return true;
+    return false;
+}
+
+
+void
+virQEMUCapsClearKVMIsNested(virQEMUCapsPtr qemuCaps)
+{
+    /* For qemucapabilitiestest to avoid printing the </kvmIsNested> on
+     * hosts with nested set in the kernel */
+    qemuCaps->kvmIsNested = false;
+}
+
+
 static bool
 virQEMUCapsIsValid(void *data,
                    void *privData)
@@ -3854,6 +3887,7 @@ virQEMUCapsIsValid(void *data,
     virQEMUCapsCachePrivPtr priv = privData;
     bool kvmUsable;
     struct stat sb;
+    bool kvmIsNested;
 
     if (!qemuCaps->binary)
         return true;
@@ -3886,6 +3920,15 @@ virQEMUCapsIsValid(void *data,
         return false;
     }
 
+    /* Check if someone changed the nested={0|1} value for the kernel from
+     * the previous time we checked. If so, then refresh the capabilities. */
+    kvmIsNested = virQEMUCapsKVMIsNested();
+    if (kvmIsNested != qemuCaps->kvmIsNested) {
+        VIR_WARN("changed kernel nested kvm value was %d", qemuCaps->kvmIsNested);
+        qemuCaps->kvmIsNested = kvmIsNested;
+        return false;
+    }
+
     if (!virQEMUCapsGuestIsNative(priv->hostArch, qemuCaps->arch)) {
         VIR_DEBUG("Guest arch (%s) is not native to host arch (%s), "
                   "skipping KVM-related checks",
@@ -4472,6 +4515,8 @@ virQEMUCapsInitQMP(virQEMUCapsPtr qemuCaps,
     if (virQEMUCapsInitQMPMonitor(qemuCaps, cmd->mon) < 0)
         goto cleanup;
 
+    qemuCaps->kvmIsNested = virQEMUCapsKVMIsNested();
+
     if (virQEMUCapsGet(qemuCaps, QEMU_CAPS_KVM)) {
         virQEMUCapsInitQMPCommandAbort(cmd);
         if ((rc = virQEMUCapsInitQMPCommandRun(cmd, true)) != 0) {
diff --git a/src/qemu/qemu_capspriv.h b/src/qemu/qemu_capspriv.h
index 8d1a40fe74..edfe2cd6f6 100644
--- a/src/qemu/qemu_capspriv.h
+++ b/src/qemu/qemu_capspriv.h
@@ -48,6 +48,8 @@ int
 virQEMUCapsInitQMPMonitor(virQEMUCapsPtr qemuCaps,
                           qemuMonitorPtr mon);
 
+void virQEMUCapsClearKVMIsNested(virQEMUCapsPtr qemuCaps);
+
 int
 virQEMUCapsInitQMPMonitorTCG(virQEMUCapsPtr qemuCaps,
                              qemuMonitorPtr mon);
diff --git a/tests/qemucapabilitiestest.c b/tests/qemucapabilitiestest.c
index 8fe5a55e1d..90942c6fce 100644
--- a/tests/qemucapabilitiestest.c
+++ b/tests/qemucapabilitiestest.c
@@ -63,6 +63,9 @@ testQemuCaps(const void *opaque)
                                   qemuMonitorTestGetMonitor(mon)) < 0)
         goto cleanup;
 
+    /* Don't apply what the host has... force clear for testing purposes */
+    virQEMUCapsClearKVMIsNested(capsActual);
+
     if (virQEMUCapsGet(capsActual, QEMU_CAPS_KVM)) {
         qemuMonitorResetCommandID(qemuMonitorTestGetMonitor(mon));
         if (virQEMUCapsInitQMPMonitorTCG(capsActual,
-- 
2.17.2