[libvirt] [PATCH 3/4] Cache result of QEMU capabilities extraction

[Eric Blake]

On 03/05/2014 10:53 AM, Daniel P. Berrange wrote:
> Extracting capabilities from QEMU takes a notable amount of time
> when all QEMU binaries are installed. Each system emulator
> needs about 200-300ms multiplied by 26 binaries == ~5-8 seconds.
> 
> This change causes the QEMU driver to save an XML file containing
> the content of the virQEMUCaps object instance in the cache
> dir eg /var/cache/libvirt/qemu/capabilities/$SHA256(binarypath).xml
> or $HOME/.cache/libvirt/qemu/cache/capabilities/$SHA256(binarypath).xml
> 
> We attempt to load this and only if it fails, do we fallback to
> probing the QEMU binary. The timestamp of the file is compared to
> the timestamp of the QEMU binary and discarded if the QEMU binary
> is newer.
> 
> Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
> ---
>  src/qemu/qemu_capabilities.c | 412 ++++++++++++++++++++++++++++++++++++++++++-
>  src/qemu/qemu_capabilities.h |   2 +
>  src/qemu/qemu_driver.c       |   1 +
>  3 files changed, 407 insertions(+), 8 deletions(-)
> 

> @@ -44,6 +45,7 @@
>  #include <unistd.h>
>  #include <sys/wait.h>
>  #include <stdarg.h>
> +#include <utime.h>

<utime.h> (and the utime() function) is deprecated by POSIX and
therefore no longer portable because it corrupts sub-second timestamps.
 Better is to use gnulib's utimensat (or futimens).  But what timestamps
do we actually have to munge?

> +
> +    ut.actime = qemuCaps->mtime;
> +    ut.modtime = qemuCaps->mtime;
> +    if (utime(filename, &ut) < 0) {
> +        virReportSystemError(errno,
> +                             _("Failed to set mtime on '%s' for '%s'"),
> +                             filename, qemuCaps->binary);

Why not just store the qemu binary timestamp in the XML, rather than
playing games with the mtime of the xml file?
> +
> +    /* Discard if cache is older that QEMU binary */
> +    /* XXX must also compare to libvirtd timestamp  */
> +    if (sb.st_mtime < qemuCaps->mtime) {

This ignores subsecond timestamps; you may want to use gnulib's
stat-time module to do a more accurate check.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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