linux-kernel - Re: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST

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Message-ID: <4F26BC1A.8060002@redhat.com>
Date:	Mon, 30 Jan 2012 17:49:46 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Eric B Munson <emunson@...bm.net>
CC:	mingo@...hat.com, hpa@...or.com, ryanh@...ux.vnet.ibm.com,
	aliguori@...ibm.com, mtosatti@...hat.com,
	jeremy.fitzhardinge@...rix.com, kvm@...r.kernel.org,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Jan Kiszka <jan.kiszka@....de>
Subject: Re: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST_STOPPED

On 01/30/2012 05:32 PM, Eric B Munson wrote:
> > 
> > Can you point me to the discussion that moved this to be a vm ioctl?  In
> > general vm ioctls that do things for all vcpus are racy, like here. 
> > You're accessing variables that are protected by the vcpu mutex, and not
> > taking the mutex (nor can you, since it is held while the guest is
> > running, unlike most kernel mutexes).
> > 
>
> Jan Kiszka suggested that becuase there isn't a use case for notifying
> individual vcpus (can vcpu's be paused individually?

They can, though the guest will grind to a halt very soon.

> ) that it makes more sense
> to have a vm ioctl.
>
> http://thread.gmane.org/gmane.comp.emulators.qemu/131624
>
> If the per vcpu ioctl is the right choice I can resend those patches.

The races are solvable but I think it's easier in userspace.  It's also
more flexible, though I don't really see a use for this flexibility.

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

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