Date: Tue, 12 Apr 2011 11:13:30 +0530
From: Amit Shah <amit.shah@redhat.com>
To: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
        Anthony Liguori <aliguori@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org
Subject: Re: [RFC][PATCH] virtio balloon: kill tell-host-first logic
Message-ID: <20110412054330.GH26678@amit-x200.redhat.com>
References: <20110407174325.ED21C82B@kernel>
 <20110411110131.GA23123@redhat.com>
 <1302559871.7286.16749.camel@nimitz>
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On (Mon) 11 Apr 2011 [15:11:11], Dave Hansen wrote:
> On Mon, 2011-04-11 at 14:01 +0300, Michael S. Tsirkin wrote:
> > On Thu, Apr 07, 2011 at 10:43:25AM -0700, Dave Hansen wrote:
> > > The virtio balloon driver has a VIRTIO_BALLOON_F_MUST_TELL_HOST
> > > feature bit.  Whenever the bit is set, we must always tell the
> > > host before we free pages back to the allocator.  Without this
> > > we might free a page (and have another user touch it) while the
> > > hypervisor is unprepared for it.
> > > 
> > > But, if the bit is _not_ set, we are under no obligation to
> > > reverse the order.  Furthermore, all modern qemus set this bit.
> > 
> > Which qemus do this, specifically?  Amit Shah just pointed out to me
> > that upstream qemu.git and qemu-kvm.git don't seem to do this.
> 
> I had a conversation with Anthony about it, and I think I managed to
> confuse myself somewhere.  Just to be clear, all that I see in the
> qemu-kvm git tree right now (df85c051d780bca0ee2462cfeb8ef6d9552a19b0)
> is this:
> 
> hw/virtio-balloon.h:#define VIRTIO_BALLOON_F_MUST_TELL_HOST 0 /* Tell before reclaiming pages */
> 
> > Which qemu did you test this with? 
> 
> Probably a week or two old qemu-kvm.
> 
> My changelog could probably use some work, but the patch still stands.
> The only requirement we have is that when
> VIRTIO_BALLOON_F_MUST_TELL_HOST is set we *MUST* tell the host, first.
> But, when it's not set, we can do whatever we want.
> 
> So, we might as well always have the ...F_MUST_TELL_HOST behavior all
> the time.  

Sure, the only contention was on the commit message, where you stated
modern qemus set this... qemu doesn't, and it should.  Care to do a
patch for that?

Thanks,

		Amit
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