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

[Daniel P. Berrangé]

On Mon, Oct 29, 2018 at 05:53:58PM +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 at redhat.com>
> Signed-off-by: Marc Hartmayer <mhartmay at linux.ibm.com>
> ---
>  src/qemu/qemu_capabilities.c | 56 ++++++++++++++++++++++++++++++++++--
>  1 file changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> index e228f52ec0bb..ea95915f0f71 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 +3828,54 @@ 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 (cached_value != VIR_TRISTATE_BOOL_ABSENT) {
> +        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);
> +            goto update;
> +        }
> +
> +        return cached_value == VIR_TRISTATE_BOOL_YES;
> +    }
> +
> + update:

I don't like this use of goto's. It could be simplified thus to give
a linear flow:

     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;
> +}

Regards,
Daniel
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