Re: KVM: x86: Reconsider the current approach of vPMU

[Sean Christopherson]

On Thu, Feb 10, 2022, Like Xu wrote:
> On 10/2/2022 5:00 am, Sean Christopherson wrote:
> > On Wed, Feb 09, 2022, Peter Zijlstra wrote:
> > > Guests must not unilaterally steal the PMU.
> > 
> > The proposal is to add an option to allow userspace to gift the PMU to the guest,
> 
> Please define the verb "gift" in more details.

Add a knob that allows host userspace to control toggle between host perf having
sole ownership of the PMU, versus ownership of the PMU being "gifted" to KVM guests
upon VM-Entry and returned back to the host at VM-Exit.

IIUC, it's the same idea as PT's PT_MODE_HOST_GUEST mode, just applied to the PMU.

By default, the host would have sole ownership, and access to the knob would be
restricted appropriately.  KVM would disallow creation any VM that requires
joint ownership, e.g. launching a TDX guest would require the knob to be enabled.

> How do we balance the performance data collection needs of the
> 'hypervisor user space' and the 'system-wide profiler user space' ?

If host userspace enables the knob and transitions into a joint ownership mode,
then host userspace is explicitly acknowledging that it will no longer be able
to profile KVM guests.

Balancing between host and guest then gets factored into VM placement, e.g. VMs
that need or are paying for access to the PMU can only land on systems that are
configured for joint ownership.  If profiling the guest from the host is important,
then place those guests only on hosts with sole ownership.