[libvirt] [PATCH 4/5] qemu: Add -mem-path even with numa

[Martin Kletzander]

On Thu, Oct 01, 2015 at 07:01:17PM -0400, John Ferlan wrote:
>
>
>On 10/01/2015 08:10 AM, Martin Kletzander wrote:
>> Of course this will be used only in case we don't need the
>> memory-backend-file backend, so it should not fire until later.
>>
>
>So if I'm reading things right... when building the numa string, if it's
>determine that "-object" and ",memdev=ram-node%zu" will be required,
>then it's also required to have "-mem-alloc -mem-path ..."
>
>(furrowing through the qemu docs was more difficult than I thought)
>

This was harder for me to grasp as well, I'll try to explain.  If you
use -object memory-backend-file, that object itself has a path where
to use hugepages from.  If you instead use '-numa ...,mem=SIZE', then
there is no way to say where from can this particular node take the
hugepages, but you can use '-mem-prealloc -mem-path
/path/to/hugetlbfs/mount' to specify it for the whole domain.  So you
need '-mem-path' if hugepages backing is requested and there is no
need for -object memory-backend-file'.

I hope that's understandable, let me know if I should try explaining
it more clearly.

>> Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
>> ---
>>  src/qemu/qemu_command.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>
>Perhaps "outside" the domain of these changes; however, I note that the
>call to qemuBuildNumaArgStr is from qemuBuildCommandLine as follows:
>
>    if (virDomainNumaGetNodeCount(def->numa)) {
>        if (qemuBuildNumaArgStr(cfg, def, cmd, qemuCaps, nodeset) < 0)
>
>one of the first things done in qemuBuildNumaArgStr:
>
>    size_t ncells = virDomainNumaGetNodeCount(def->numa);
>
>oh and of course the pesky
>
>    const long system_page_size = virGetSystemPageSizeKB();
>
>I mentioned previously
>
>BTW: Not noted by Coverity, but interestingly later in the code there's:
>
>        if (ncells) {
>            /* Fortunately, we allow only guest NUMA nodes to be continuous
>             * starting from zero. */
>            pos = ncells - 1;
>        }
>
>Which if I'm reading things correct - cannot happen.
>

I haven't noticed this, but the way I'm reading it is that
qemuBuildNumaArgStr() is called only if there are some numa nodes, so
this will happen *every* time qemuBuildNumaArgStr().

>Just a note - you don't have to change/fix this, but well if you want to...
>
>ACK to what's here as it seems reasonable - perhaps a better reason in
>the commit message would help
>

I'll update the commit message, reading it after myself now I don't
understand it either :)

>John
>
>> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
>> index 17e5cfd71702..321a5e931350 100644
>> --- a/src/qemu/qemu_command.c
>> +++ b/src/qemu/qemu_command.c
>> @@ -8132,6 +8132,10 @@ qemuBuildNumaArgStr(virQEMUDriverConfigPtr cfg,
>>          }
>>      }
>>
>> +    if (!needBackend &&
>> +        qemuBuildMemPathStr(cfg, def, qemuCaps, cmd) < 0)
>> +        goto cleanup;
>> +
>>      for (i = 0; i < ncells; i++) {
>>          VIR_FREE(cpumask);
>>          if (!(cpumask = virBitmapFormat(virDomainNumaGetNodeCpumask(def->numa, i))))
>>
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