Re: [RFC PATCH 1/1] kvm: Note an RCU quiescent state on guest exit

[Sean Christopherson]

On Fri, May 10, 2024, Leonardo Bras wrote:
> As of today, KVM notes a quiescent state only in guest entry, which is good
> as it avoids the guest being interrupted for current RCU operations.
> 
> While the guest vcpu runs, it can be interrupted by a timer IRQ that will
> check for any RCU operations waiting for this CPU. In case there are any of
> such, it invokes rcu_core() in order to sched-out the current thread and
> note a quiescent state.
> 
> This occasional schedule work will introduce tens of microsseconds of
> latency, which is really bad for vcpus running latency-sensitive
> applications, such as real-time workloads.
> 
> So, note a quiescent state in guest exit, so the interrupted guests is able
> to deal with any pending RCU operations before being required to invoke
> rcu_core(), and thus avoid the overhead of related scheduler work.

Are there any downsides to this?  E.g. extra latency or anything?  KVM will note
a context switch on the next VM-Enter, so even if there is extra latency or
something, KVM will eventually take the hit in the common case no matter what.
But I know some setups are sensitive to handling select VM-Exits as soon as possible.

I ask mainly because it seems like a no brainer to me to have both VM-Entry and
VM-Exit note the context switch, which begs the question of why KVM isn't already
doing that.  I assume it was just oversight when commit 126a6a542446 ("kvm,rcu,nohz:
use RCU extended quiescent state when running KVM guest") handled the VM-Entry
case?