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qemu: Introduce caching whether /dev/kvm is accessible

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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>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
This commit is contained in:
Marc Hartmayer 2018-10-29 18:34:58 +01:00 committed by Michal Privoznik
parent e05d8e570b
commit 91684829be
1 changed files with 54 additions and 2 deletions
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56
src/qemu/qemu_capabilities.c
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@ -3264,6 +3264,10 @@ struct _virQEMUCapsCachePriv {
virArch hostArch;
unsigned int microcodeVersion;
char *kernelVersion;
/* cache whether /dev/kvm is usable as runUid:runGuid */
virTristateBool kvmUsable;
time_t kvmCtime;
};
typedef struct _virQEMUCapsCachePriv virQEMUCapsCachePriv;
typedef virQEMUCapsCachePriv *virQEMUCapsCachePrivPtr;
@ -3891,6 +3895,54 @@ virQEMUCapsKVMSupportsNesting(void)
}
/* 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) {
if (errno != ENOENT) {
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)
@ -3940,8 +3992,7 @@ 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) {
@ -4763,6 +4814,7 @@ 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)