Re: [patch v2 3/3] cpuidle-haltpoll: disable host side polling when kvm virtualized

From: Andrea Arcangeli <aarcange@redhat.com>
To: Joao Martins <joao.m.martins@oracle.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>,
	kvm-devel <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Radim Krcmar <rkrcmar@redhat.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Wanpeng Li <kernellwp@gmail.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Raslan KarimAllah <karahmed@amazon.de>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Ankur Arora <ankur.a.arora@oracle.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [patch v2 3/3] cpuidle-haltpoll: disable host side polling when kvm virtualized
Date: Thu, 6 Jun 2019 14:36:32 -0400	[thread overview]
Message-ID: <20190606183632.GA20928@redhat.com> (raw)
In-Reply-To: <cb11ef01-b579-1526-d585-0c815f2e1f6f@oracle.com>

Hello,

On Thu, Jun 06, 2019 at 07:25:28PM +0100, Joao Martins wrote:
> But I wonder whether we should fail to load cpuidle-haltpoll when host halt
> polling can't be disabled[*]? That is to avoid polling in both host and guest
> and *possibly* avoid chances for performance regressions when running on older
> hypervisors?

I don't think it's necessary: that would force an upgrade of the host
KVM version in order to use the guest haltpoll feature with an
upgraded guest kernel that can use the guest haltpoll.

The guest haltpoll is self contained in the guest, so there's no
reason to prevent that by design or to force upgrade of the KVM host
version. It'd be more than enough to reload kvm.ko in the host with
the host haltpoll set to zero with the module parameter already
available, to achieve the same runtime without requiring a forced host
upgrade.

The warning however sounds sensible.

Thanks,
Andrea