Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO hypercall\

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Sep 29, 2017 at 10:18:25AM +0200, Paolo Bonzini wrote:
> On 28/09/2017 23:35, Marcelo Tosatti wrote:
> > On Thu, Sep 28, 2017 at 09:22:02AM +0200, Paolo Bonzini wrote:
> >> On 28/09/2017 02:44, Marcelo Tosatti wrote:
> >>>> Again: if you have many interruptions, it's not a flaw in KVM or QEMU's
> >>>> design, it's just that someone is doing something stupid.  It could be
> >>>> the guest (e.g. unnecessary devices or daemons as in the example above),
> >>>> QEMU (e.g. the RTC emulation used to trigger QEMU timers twice a second
> >>>> just to increment the clock), or the management (e.g. polling "is the VM
> >>>> running" 50 times per second).  But it can and must be fixed.
> >>>
> >>> No, i mean you can run anything in VCPU-0 (it is valid to do that).
> >>> And that "anything" can generate 1 interrupt per second, 1000 or 10.000
> >>> interrupts per second. Which are all valid things to be done.
> >>>
> >>> "I can't run a kernel compilation on VCPU-0 because that will impact
> >>> latency on the realtime VCPU-1" is not acceptable.
> >>
> >> That shouldn't happen.  Sources of frequent interruptions have all been
> >> fixed or moved outside the main thread.
> >>
> >> If there are more left, report the bug and we'll see how to fix it in
> >> userspace.
> > 
> > What should not happen? The generation of 10.000 interrupts per second
> > (say disk IO completion) on a given workload ?
> 
> If you know you have this kind disk workload, you must use virtio-blk or
> virtio-scsi with iothreads and place the iothreads on their own physical
> CPUs.
> 
> Among "run arbitrary workloads", "run real-time workloads", "pack stuff
> into as few physical CPUs as possible", you can only pick two.
> 
> Paolo

Thats not the state of things (userspace in vcpu-0 is not specially tailored
to not violate latencies in vcpu-1): that is not all user triggered
actions can be verified.

Think "updatedb", and so on...