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

On Tue, May 06, 2014 at 03:27:20PM +0200, Ján Tomko wrote: > > 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.