Re: [libvirt] TSC scaling interface to management
On Tue, Sep 25, 2012 at 11:08:58AM +0100, Daniel P. Berrange wrote:
On Wed, Sep 12, 2012 at 12:39:39PM -0300, Marcelo Tosatti wrote:
>
>
> HW TSC scaling is a feature of AMD processors that allows a
> multiplier to be specified to the TSC frequency exposed to the guest.
>
> KVM also contains provision to trap TSC ("KVM: Infrastructure for
> software and hardware based TSC rate scaling" cc578287e3224d0da)
> or advance TSC frequency.
>
> This is useful when migrating to a host with different frequency and
> the guest is possibly using direct RDTSC instructions for purposes
> other than measuring cycles (that is, it previously calculated
> cycles-per-second, and uses that information which is stale after
> migration).
>
> "qemu-x86: Set tsc_khz in kvm when supported" (e7429073ed1a76518)
> added support for tsc_khz= option in QEMU.
>
> I am proposing the following changes so that management applications
> can work with this:
>
> 1) New option for tsc_khz, which is tsc_khz=host (QEMU command line
> option). Host means that QEMU is responsible for retrieving the
> TSC frequency of the host processor and use that.
> Management application does not have to deal with the burden.
FYI, libvirt already has support for expressing a number of different
TSC related config options, for support of Xen and VMWare's capabilities
in this area. What we currently allow for is
<timer name='tsc' frequency='NNN'
mode='auto|native|emulate|smpsafe'/>
In this context the frequency attribute provides the HZ value to
provide to the guest.
- auto == Emulate if TSC is unstable, else allow native TSC access
- native == Always allow native TSC access
- emulate = Always emulate TSC
- smpsafe == Always emulate TSC, and interlock SMP
These options can be mapped into KVM if necessary (they can map to
tsc_khz=XXX or to the module options (unfortunately not per-guest ATM)).
> Therefore it appears that this "tsc_khz=auto" option
can be specified
> only if the user specifies so (it can be a per-guest flag hidden
> in the management configuration/manual).
>
> Sending this email to gather suggestions (or objections)
> to this interface.
Daniel
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Karen had the suggestion to remove the burden of choice from the user,
which we can achieve by knowing whether or not the guest is using
a paravirtual clock.
The problem is that opens a can of races: Did migration happen before or
after guest boot process enabled the paravirtual clock etc.
I suppose leaving the option to the user is fine: if you run an obscure
operating system, which does not support paravirtual clock, then it
must be dealt with specialy (its in the manual, no big deal).