linux-kernel - Re: [PATCH v3 0/4] implement vcpu preempted check

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Date:   Fri, 30 Sep 2016 02:58:58 -0400 (EDT)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Pan Xinhui <xinhui@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Pan Xinhui <xinhui.pan@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        virtualization@...ts.linux-foundation.org,
        linux-s390@...r.kernel.org, xen-devel-request@...ts.xenproject.org,
        kvm@...r.kernel.org, benh@...nel.crashing.org, paulus@...ba.org,
        mpe@...erman.id.au, mingo@...hat.com, paulmck@...ux.vnet.ibm.com,
        will deacon <will.deacon@....com>, kernellwp@...il.com,
        jgross@...e.com, bsingharora@...il.com,
        Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH v3 0/4] implement vcpu preempted check

> > > > Please consider s390 and (x86/arm) KVM. Once we have a few, more can
> > > > follow later, but I think its important to not only have PPC support for
> > > > this.
> > >
> > > Actually the s390 preemted check via sigp sense running  is available for
> > > all hypervisors (z/VM, LPAR and KVM) which implies everywhere as you can
> > > no longer buy s390 systems without LPAR.
> > >
> > > As Heiko already pointed out we could simply use a small inline function
> > > that calls cpu_is_preempted from arch/s390/lib/spinlock (or
> > > smp_vcpu_scheduled from smp.c)
> >
> > Sure, and I had vague memories of Heiko's email. This patch set however
> > completely fails to do that trivial hooking up.
> 
> sorry for that.
> I will try to work it out on x86.

x86 has no hypervisor support, and I'd like to understand the desired
semantics first, so I don't think it should block this series.  In
particular, there are at least the following choices:

1) exit to userspace (5-10.000 clock cycles best case) counts as
lock holder preemption

2) any time the vCPU thread not running counts as lock holder
preemption

To implement the latter you'd need a hypercall or MSR (at least as
a slow path), because the KVM preempt notifier is only active
during the KVM_RUN ioctl.

Paolo