Re: [libvirt] [RFC PATCH] Add support for invtsc timer

On Tue, May 06, 2014 at 03:27:20PM +0200, Ján Tomko wrote: > Not yet merged in upstream QEMU: > https://lists.gnu.org/archive/html/qemu-devel/2014-04/msg05024.html > > Add support for invariant TSC timer running at constant rate in > all ACPI P-, C- and T-states. > > It can be enabled by specifying: > <clock> > <timer name='invtsc' present='yes'/> > </clock> > in the domain XML. > > Migration and saving the domain does not work with this timer. > > The support for this timer is indicated by bit 8 of EDX after calling > CPUID with 0x80000007. It does not show up in /proc/cpuinfo [1] > and since we're calling qemu without 'enforce', it doesn't error > out if the host doesn't support this. It is not a timer, really, but a CPU flag. > Alternatively, we could expose it in libvirt as a cpu flag: > <cpu mode='custom' match='exact'> > <model fallback='forbid'>qemu64</model> > <feature policy='require' name='invtsc'/> > </cpu> Would prefer that option. Can't one modify QEMU's "-cpu" parameters, via libvirt XML? > or maybe add +invtsc to qemu args when the 'nonstop_tsc' flag is > requested? No, since it blocks migration better not condition "+invtsc" on presence of 'nonstop_tsc' flag.