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Re: [libvirt] [PATCH v2] qemu: Introduce caching whether /dev/kvm is accessible



On Mon, Oct 29, 2018 at 06:34:58PM +0100, Marc Hartmayer wrote:
Introduce caching whether /dev/kvm is usable as the QEMU user:QEMU
group. This reduces the overhead of the QEMU capabilities cache
lookup. Before this patch there were many fork() calls used for
checking whether /dev/kvm is accessible. Now we store the result
whether /dev/kvm is accessible or not and we only need to re-run the
virFileAccessibleAs check if the ctime of /dev/kvm has changed.

Suggested-by: Daniel P. Berrangé <berrange redhat com>
Signed-off-by: Marc Hartmayer <mhartmay linux ibm com>
---
src/qemu/qemu_capabilities.c | 54 ++++++++++++++++++++++++++++++++++--
1 file changed, 52 insertions(+), 2 deletions(-)

diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index e228f52ec0bb..85516954149b 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -3238,6 +3238,10 @@ struct _virQEMUCapsCachePriv {
    virArch hostArch;
    unsigned int microcodeVersion;
    char *kernelVersion;
+
+    /* cache whether /dev/kvm is usable as runUid:runGuid */

Even though this doesn't solve all of it (the ctime change with udev etc.) it
makes it better.  However this needs to be checked (or cached) per seclabel of
the domain as it can run under non-default user.

+    virTristateBool kvmUsable;
+    time_t kvmCtime;
};
typedef struct _virQEMUCapsCachePriv virQEMUCapsCachePriv;
typedef virQEMUCapsCachePriv *virQEMUCapsCachePrivPtr;
@@ -3824,6 +3828,52 @@ virQEMUCapsSaveFile(void *data,
}


+/* Determine whether '/dev/kvm' is usable as QEMU user:QEMU group. */
+static bool
+virQEMUCapsKVMUsable(virQEMUCapsCachePrivPtr priv)
+{
+    struct stat sb;
+    static const char *kvm_device = "/dev/kvm";
+    virTristateBool value;
+    virTristateBool cached_value = priv->kvmUsable;
+    time_t kvm_ctime;
+    time_t cached_kvm_ctime = priv->kvmCtime;
+
+    if (stat(kvm_device, &sb) < 0) {
+        virReportSystemError(errno,
+                             _("Failed to stat %s"), kvm_device);
+        return false;
+    }
+    kvm_ctime = sb.st_ctime;
+
+    if (kvm_ctime != cached_kvm_ctime) {
+        VIR_DEBUG("%s has changed (%lld vs %lld)", kvm_device,
+                  (long long)kvm_ctime, (long long)cached_kvm_ctime);
+        cached_value = VIR_TRISTATE_BOOL_ABSENT;
+    }
+
+    if (cached_value != VIR_TRISTATE_BOOL_ABSENT)
+        return cached_value == VIR_TRISTATE_BOOL_YES;
+
+    if (virFileAccessibleAs(kvm_device, R_OK | W_OK,
+                            priv->runUid, priv->runGid) == 0) {
+        value = VIR_TRISTATE_BOOL_YES;
+    } else {
+        value = VIR_TRISTATE_BOOL_NO;
+    }
+
+    /* There is a race window between 'stat' and
+     * 'virFileAccessibleAs'. However, since we're only interested in
+     * detecting changes *after* the virFileAccessibleAs check, we can
+     * neglect this here.
+     */
+    priv->kvmCtime = kvm_ctime;
+    priv->kvmUsable = value;
+
+    return value == VIR_TRISTATE_BOOL_YES;
+}
+
+
static bool
virQEMUCapsIsValid(void *data,
                   void *privData)
@@ -3872,8 +3922,7 @@ virQEMUCapsIsValid(void *data,
        return true;
    }

-    kvmUsable = virFileAccessibleAs("/dev/kvm", R_OK | W_OK,
-                                    priv->runUid, priv->runGid) == 0;
+    kvmUsable = virQEMUCapsKVMUsable(priv);

    if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_KVM) &&
        kvmUsable) {
@@ -4684,6 +4733,7 @@ virQEMUCapsCacheNew(const char *libDir,
    priv->runUid = runUid;
    priv->runGid = runGid;
    priv->microcodeVersion = microcodeVersion;
+    priv->kvmUsable = VIR_TRISTATE_BOOL_ABSENT;

    if (uname(&uts) == 0 &&
        virAsprintf(&priv->kernelVersion, "%s %s", uts.release, uts.version) < 0)
--
2.17.0

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