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Re: [libvirt] [PATCHv3 2/2] conf: allow fuzz in XML with cur balloon > max



Thank you!
ACK patch serials

On Sat, Mar 31, 2012 at 11:33 PM, Eric Blake <eblake redhat com> wrote:
> Commit 1b1402b introduced a regression.  Since older libvirt versions
> would silently round memory up (until the previous patch), but populated
> current memory based on querying the guest, it was possible to have
> dumpxml show cur > max by the amount of the rounding.  For example, if
> a user requested 1048570 KiB memory (just shy of 1GiB), the qemu
> driver would actually run with 1048576 KiB, and libvirt 0.9.10 would
> output a current that was 6KiB larger than the maximum.  Situations
> where this could have an impact include, but are not limited to,
> migration from old to new libvirt, managedsave in old libvirt and
> start in new libvirt, snapshot creation in old libvirt and revert in
> new libvirt - without this patch, the new libvirt would reject the
> VM because of the rounding discrepancy.
>
> Fix things by adding a fuzz factor, and silently clamp current down to
> maximum in that case, rather than failing to reparse XML for an existing
> VM.  From a practical standpoint, this has no user impact: 'virsh
> dumpxml' will continue to query the running guest rather than rely on
> the incoming xml, which will see the currect current value, and even if
> clamping down occurs during parsing, it will be by at most the fuzz
> factor of a megabyte alignment, and rounded back up when passed back to
> the hypervisor.
>
> Meanwhile, we continue to reject cur > max if the difference is beyond
> the fuzz factor of nearest megabyte.  But this is not a real change in
> behavior, since with 0.9.10, even though the parser allowed it, later
> in the processing stream we would reject it at the qemu layer; so
> rejecting it in the parser just moves error detection to a nicer place.
>
> * src/conf/domain_conf.c (virDomainDefParseXML): Don't reject
> existing XML.
> Based on a report by Zhou Peng.
> ---
>
> v3: improve the commit message (no change to the code)
>
>  src/conf/domain_conf.c |   19 +++++++++++++++----
>  1 files changed, 15 insertions(+), 4 deletions(-)
>
> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> index 7ea57b9..c800160 100644
> --- a/src/conf/domain_conf.c
> +++ b/src/conf/domain_conf.c
> @@ -7769,10 +7769,21 @@ static virDomainDefPtr virDomainDefParseXML(virCapsPtr caps,
>         goto error;
>
>     if (def->mem.cur_balloon > def->mem.max_balloon) {
> -        virDomainReportError(VIR_ERR_XML_ERROR,
> -                             _("current memory '%lluk' exceeds maximum '%lluk'"),
> -                             def->mem.cur_balloon, def->mem.max_balloon);
> -        goto error;
> +        /* Older libvirt could get into this situation due to
> +         * rounding; if the discrepancy is less than 1MiB, we silently
> +         * round down, otherwise we flag the issue.  */
> +        if (VIR_DIV_UP(def->mem.cur_balloon, 1024) >
> +            VIR_DIV_UP(def->mem.max_balloon, 1024)) {
> +            virDomainReportError(VIR_ERR_XML_ERROR,
> +                                 _("current memory '%lluk' exceeds "
> +                                   "maximum '%lluk'"),
> +                                 def->mem.cur_balloon, def->mem.max_balloon);
> +            goto error;
> +        } else {
> +            VIR_DEBUG("Truncating current %lluk to maximum %lluk",
> +                      def->mem.cur_balloon, def->mem.max_balloon);
> +            def->mem.cur_balloon = def->mem.max_balloon;
> +        }
>     } else if (def->mem.cur_balloon == 0) {
>         def->mem.cur_balloon = def->mem.max_balloon;
>     }
> --
> 1.7.7.6
>



-- 
Zhou Peng


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