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

From: Wanpeng Li
Date: Mon Jul 10 2017 - 22:44:36 EST

2017-07-11 8:13 GMT+08:00 Andy Lutomirski <luto@xxxxxxxxxx>:
> 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:

Yeah, I agreed.

> 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.

Sigh, I heard the same idea from a company called Huawei, they applied
the patent for the idea.

Wanpeng Li