On Tue, Apr 25, 2017 at 12:22:30PM +0200, Christoffer Dall wrote: > On Tue, Apr 25, 2017 at 11:49:27AM +0200, Daniel Lezcano wrote: > > [...] > > > > > > > The idle code is very much *not* aware of anything concerning that guest > > > timer. > > > > Just for my own curiosity, if there are two VM (VM1 and VM2). VM1 sets a timer1 > > at <time> and exits, VM2 runs and sets a timer2 at <time+delta>. > > > > The timer1 for VM1 is supposed to expire while VM2 is running. IIUC the virtual > > timer is under control of VM2 and will expire at <time+delta>. > > > > Is the host wake up with the SW timer and switch in VM1 which in turn restores > > the timer and jump in the virtual timer irq handler? > > > The thing that may be missing here is that a VCPU thread (more of which > in a collection is a VM) is just a thread from the point of view of > Linux, and whether or not a guest schedules a timer, should not effect > the scheduler's decision to run a given thread, if the thread is > runnable. > > Whenever we run a VCPU thread, we look at its timer state (in software) > and calculate if the guest should see a timer interrupt and inject such > a one (the hardware arch timer is not involved in this process at all). > > We use timers in exactly two scenarios: > > 1. The hardware arch timers are used to force an exit to the host when > the guest programmed the timer, so we can do the calculation in > software I mentioned above and inject a virtual software-generated > interrupt when the guest expects to see one. > > 2. The guest goes to sleep (WFI) but has programmed a timer to be woken > up at some point. KVM handles a WFI by blocking the VCPU thread, > which basically means making the thread interruptible and putting it > on a waitqueue. In this case we schedule a software timer to make > the thread runnable again when the software timer fires (and the > scheduler runs that thread when it wants to after that). > > If you have a VCPU thread from VM1 blocked, and you run a VCPU thread > from VM2, then the VCPU thread from VM2 will program the hardware arch > timer with the context of the VM2 VCPU thread while running, and this > has nothing to do with the VCPU thread from VM1 at this point, because > it relies on the host Linux time keeping infrastructure to become > runnable some time in the future, and running a guest naturally doesn't > mess with the host's time keeping. > > Hope this helps, Yes, definitively. Thanks for the detailed description. -- Daniel -- <http://www.linaro.org/> Linaro.org ? Open source software for ARM SoCs Follow Linaro: <http://www.facebook.com/pages/Linaro> Facebook | <http://twitter.com/#!/linaroorg> Twitter | <http://www.linaro.org/linaro-blog/> Blog
