linux-kernel - Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT

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Message-ID: <10635834-459a-9ec1-624d-febd6b5af243@redhat.com>
Date:   Thu, 28 Sep 2017 09:22:02 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        mingo@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 3/3] x86: kvm guest side support for KVM_HC_RT_PRIO
 hypercall\

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.

Paolo