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

[Michal Privoznik]

On 10/29/18 6:34 PM, 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 at redhat.com>
> Signed-off-by: Marc Hartmayer <mhartmay at 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 */
> +    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) {

I'd report error here iff 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)
> @@ -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)
> 

Otherwise looking good. I agree with Martin that this might not be the
best solution but it's the best we have (also, the problem is not on our
side).

ACK (will wait a day or two for others to chime in before pushing)

Michal