[libvirt] Is seems necessary to pass "migratable=no/yes" to qemu.
The patch
http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=de0aeafe9ce3eb414c8b...
removes invtsc flag in the host-model CPU.
I'm wondering, will it be better to pass args "migratable=no/yes" to qemu,
and let qemu complete the
remaining work? As that qemu has checked whether it's necessary to use invtsc or not.
----------
"invtsc is available only if using: -cpu host,migratable=no,+invtsc."
http://git.qemu.org/?p=qemu.git;a=commit;h=120eee7d1fdb2eba15766cfff7b9bc...
----------
There's another problem, if we do not pass "migratable=no" to qemu.
Consider if we set host mode to pass-through
---------
<cpu mode='host-passthrough'>
</cpu>
---------
then the vm->def->cpu->features contains invtsc. however, qemu will automatically
remove this cpu flag
as that "migration=no" is not passed to it. thus, the guest will not start up.
This problem is in fact
caused by the patch:
http://libvirt.org/git/?p=libvirt.git;a=commit;h=fba6bc47cbcabbe08d422796...,
it forbids guest domain to start up if the host has INVTSC while the guest(qemu) does
not.
-------------
for (i = 0; def->cpu && i < def->cpu->nfeatures; i++) {
virCPUFeatureDefPtr feature = &def->cpu->features[i];
if (feature->policy != VIR_CPU_FEATURE_REQUIRE)
continue;
if (STREQ(feature->name, "invtsc") &&
!cpuHasFeature(guestcpu, feature->name)) {
virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
_("host doesn't support invariant TSC"));
goto cleanup;
}
}
break;
--------------
In conclusion:
1 Will it better to pass args "migratable=yes/no" to qemu rather than doing the
mask-invtsc job in libvirt?
2 If the guest has "pass-through" cpu mode, then it's unable to start up,
because qemu removes invtsc, and
vm->def->cpu->features has it. It seems a BUG.