linux-kernel - Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw

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Message-ID: <4D5109CC.2030901@redhat.com>
Date:	Tue, 08 Feb 2011 11:15:56 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Zachary Amsden <zamsden@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: Convert tsc_write_lock to raw_spinlock

On 02/07/2011 07:23 PM, Jan Kiszka wrote:
> On 2011-02-07 18:10, Avi Kivity wrote:
> >  On 02/07/2011 06:59 PM, Jan Kiszka wrote:
> >>>
> >>>   (well, actually, cpufreq_notifier and kvm_arch_hardware_enable are
> >>>   already non preemptible, and the stats code should just go away?)
> >>
> >>  The stats code is trivial to convert, so it doesn't matter.
> >
> >  Removal is easier.
>
> Is that stat interface no longer used?

It's there for compatibility.  I'm itching to remove it.  See 
qemu-kvm.git/kvm/kvm_stat for the only known user, and for the 
replacement via tracepoints.

Tracepoints have marginally lower overhead when disabled, and somewhat 
higher overhead when enabled.  A disadvantage of tracepoints is that it 
is harder to associate an event with a vm when that event is triggered 
by a workqueue, but I don't think it matters in practice (kvm_stat 
doesn't even provide a per-vm breakdown).

> >
> >>  But what about mmu_shrink and its list_move_tail? How is this
> >>  synchronized against kvm_destroy_vm - already today?
> >
> >  kvm_destroy_vm() takes kvm_lock.  If a vm is destroyed before
> >  mmu_shrink(), mmu_shrink() will never see it.  If we reach mmu_shrink()
> >  before kvm_destroy_vm(), the latter will wait until mmu_shrink() is done.
> >
>
> Ah, I was confused. Would require some more logic if we wanted to make
> the loop lock-less, though.

Yes, the usual rcu_read_lock() / grab reference / rcu_read_unlock().

-- 
error compiling committee.c: too many arguments to function

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