On Wed, May 07, 2014 at 07:35:13PM -0300, Marcelo Tosatti wrote:
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.
Simply adding <feature name='invtsc'> <cpuid function='0x80000007' edx='0x00000100'/> </feature> To cpu_map.xml is sufficient to allow users to enable this feature on demand ? Haven't thought about all the corner cases, though.