Re: [RFC PATCH 0/5] Utilizing VMX preemption for timer virtualization

[Jan Kiszka <jan.kiszka@web.de>]

On 2016-05-20 03:44, Yunhong Jiang wrote:
> The VMX-preemption timer is a feature on VMX, it counts down, from the
> value loaded by VM entry, in VMX nonroot operation. When the timer
> counts down to zero, it stops counting down and a VM exit occurs.
> 
> The VMX preemption timer for tsc deadline timer virtualization. The
> VMX preemption timer is armed when the vCPU is running, and a VMExit
> will happen if the virtual TSC deadline timer expires.
> 
> When the vCPU thread is scheduled out, the tsc deadline timer
> virtualization will be switched to use the current solution, i.e. use
> the timer for it. It's switched back to VMX preemption timer when the
> vCPU thread is scheduled int.
> 
> This solution replace the complex OS's hrtimer system, and also the
> host timer interrupt handling cost, with a preemption_timer VMexit. It
> fits well for some NFV usage scenario, when the vCPU is bound to a
> pCPU and the pCPU is isolated, or some similar scenarioes.
> 
> However, it possibly has impact if the vCPU thread is scheduled in/out
> very frequently, because it switches from/to the hrtimer emulation a
> lot. A module parameter is provided to turn it on or off.

IIRC, VMX preemption timer was broken on several older CPUs. That was
one reason we didn't use it to emulated nested preemption timer. Were
all those CPUs also not exposing the TCP deadline timer?

In any case, just checking for the feature bit like you do in patch 2
shouldn't be enough to catch those CPUs. Or were there microcode patches
distributed by now that translated the errata into bit removals?

Jan