linux-kernel - Re: [RFC][PATCH] virtio balloon: kill tell-host-first logic

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Message-ID: <20110412164341.GQ13631@amit-x200.redhat.com>
Date:	Tue, 12 Apr 2011 22:13:42 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [RFC][PATCH] virtio balloon: kill tell-host-first logic

On (Tue) 12 Apr 2011 [09:22:32], Dave Hansen wrote:
> 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. :)

I should've been clearer: care to do the qemu patch? :-)

> 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

Actually this is just the bit number; it doesn't get set.

		Amit
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