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

[Daniel P. Berrange]

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/x86/kernel/cpu/powerflags.c?id=30321c7b#n18

Regards,
Daniel
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