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

[Michal Privoznik]

On 10/29/2018 06:34 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) {
> +        virReportSystemError(errno,
> +                             _("Failed to stat %s"), kvm_device);
> +        return false;
> +    }
> +    kvm_ctime = sb.st_ctime;

This doesn't feel right. /dev/kvm ctime is changed every time qemu is
started or powered off (try running stat over it before and after a
domain is started/shut off). So effectively we will fork more often than
we would think. Should we cache inode number instead? Because for all
that we care is simply if the file is there.

> +
> +    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.

Michal