Re: vPT rework (and timer mode)

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.07.2020 11:02, Roger Pau Monné wrote:
> It's my understanding that the purpose of pt_update_irq and
> pt_intr_post is to attempt to implement the "delay for missed ticks"
> mode, where Xen will accumulate timer interrupts if they cannot be
> injected. As shown by the patch above, this is all broken when the
> timer is added to a vCPU (pt->vcpu) different than the actual target
> vCPU where the interrupt gets delivered (note this can also be a list
> of vCPUs if routed from the IO-APIC using Fixed mode).
> 
> I'm at lost at how to fix this so that virtual timers work properly
> and we also keep the "delay for missed ticks" mode without doing a
> massive rework and somehow keeping track of where injected interrupts
> originated, which seems an overly complicated solution.
> 
> My proposal hence would be to completely remove the timer_mode, and
> just treat virtual timer interrupts as other interrupts, ie: they will
> be injected from the callback (pt_timer_fn) and the vCPU(s) would be
> kicked. Whether interrupts would get lost (ie: injected when a
> previous one is still pending) depends on the contention on the
> system. I'm not aware of any current OS that uses timer interrupts as
> a way to track time. I think current OSes know the differences between
> a timer counter and an event timer, and will use them appropriately.

Fundamentally - why not, the more that this promises to be a
simplification. The question we need to answer up front is whether
we're happy to possibly break old OSes (presumably ones no-one
ought to be using anymore these days, due to their support life
cycles long having ended).

Jan