Subject: Re: [RFC][PATCH] virtio balloon: kill tell-host-first logic
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Amit Shah <amit.shah@redhat.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
In-Reply-To: <20110412054330.GH26678@amit-x200.redhat.com>
References: <20110407174325.ED21C82B@kernel>
	 <20110411110131.GA23123@redhat.com> <1302559871.7286.16749.camel@nimitz>
	 <20110412054330.GH26678@amit-x200.redhat.com>
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Date: Tue, 12 Apr 2011 09:22:32 -0700
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On Tue, 2011-04-12 at 11:13 +0530, Amit Shah wrote:
> 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? 

If Rusty hasn't pushed the commit out anywhere, we can still amend the
commit.  Otherwise, we're in a _bit_ of a pickle since you can't patch
git logs. :)

Whatever is easiest for Rusty works for me.  

How about this for a replacement log?

--

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
feature, 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; we're under no obligation to do _anything_.
That's the state of affairs in current qemu:

	#define VIRTIO_BALLOON_F_MUST_TELL_HOST 0

This patch makes the "tell host first" logic the only case.  This
should make everybody happy, and reduce the amount of untested or
untestable code in the kernel.

This _also_ means that we don't have to preserve a pfn list
after the pages are freed, which should let us get rid of some
temporary storage (vb->pfns) eventually.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>

-- Dave

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