Re: [PATCH RFC 0/2] KVM: x86: Support using the VMX preemption timer for APIC Timer periodic/oneshot mode

From: Andy Lutomirski
Date: Mon Jul 10 2017 - 20:13:45 EST

On 10/11/2016 05:17 AM, Wanpeng Li wrote:
Most windows guests which I have on hand currently still utilize APIC Timer
periodic/oneshot mode instead of APIC Timer tsc-deadline mode:
- windows 2008 server r2
- windows 2012 server r2
- windows 7
- windows 10

This patchset adds the support using the VMX preemption timer for APIC Timer
periodic/oneshot mode.

I add a print in oneshot mode testcase of kvm-unit-tests/apic.flat and observed
that w/ patch the latency is ~2% of w/o patch. I think maybe something is still
not right in the patchset, in addition, tmcct in apic_get_tmcct() maybe is not
calculated correctly. Your comments to improve the patchset is a great appreciated.

Wanpeng Li (2):
KVM: lapic: Extract start_sw_period() to handle oneshot/periodic mode
KVM: x86: Support using the vmx preemption timer for APIC Timer periodic/one mode

arch/x86/kvm/lapic.c | 162 ++++++++++++++++++++++++++++++---------------------
1 file changed, 95 insertions(+), 67 deletions(-)

I think this is a step in the right direction, but I think there's a different approach that would be much, much faster: use the VMX preemption timer for *host* preemption. Specifically, do this:

1. Refactor the host TSC deadline timer a bit to allow the TSC deadline timer to be "borrow". It might look something like this:

u64 borrow_tsc_deadline(void (*timer_callback)());

The caller is now permitted to use the TSC deadline timer for its own nefarious purposes. The caller promises to call return_tsc_deadline() in a timely manner if the TSC exceeds the return value while the deadline timer is borrowed.

If the TSC deadline fires while it's borrowed, timer_callback() will be called.

void return_tsc_deadline(bool timer_fired);

The caller is done borrowing the TSC deadline timer. The caller need not reset the TSC deadline timer MSR to its previous value before calling this. It must be called with IRQs on and preemption off.

Getting this to work cleanly without races may be a bit tricky. So be it.

2. Teach KVM to use the VMX preemption timer as a substitute deadline timer while in guest mode. Specifically, KVM will borrow_tsc_deadline() (if TSC deadline is enabled) when entering guest mode and return_tsc_deadline() when returning out of guest mode.

3. Now KVM can change its MSR bitmaps to allow the guest to program the TSC deadline MSR directly. No exit at all needed to handle guest writes to the deadline timer.

Tglx, etc, would this be even remotely acceptable? I'm not personally touching this code with a ten-foot pole, but there seem to be plenty of people who really care about performance here.