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.
What does this do at the hardware level ? This doesn't seem to be creating a second TSC timer source in the guest, rather it is setting a property related to the existing TSC timer. So I think it might make more sense as an attribute for <timer name='tsc'> element instead.
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.
Why is that ? The QEMU patch doesn't mention this restriction.
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.
Maybe I'm mis-interpreting the kernel source, but my reading of that was that this *does* show up in /proc/cpuinfo, but it is given the name 'constant_tsc' instead of 'invtsc'. On my host I see 'constant_tsc' and 'nonstop_tsc' in /proc/cpuinfo
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> or maybe add +invtsc to qemu args when the 'nonstop_tsc' flag is requested?
Yep, I could see that as a valid option. If it is visible in /proc/cpuinfo, then I think that's a compelling reason for libvirt to model it as a CPU flag too, rather than pretend it is a new type of timer when it is just an attribute of the base TSC timer.
[1]: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/arch/x8...
Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|