linux-kernel - Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

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Date:	Sat, 08 Sep 2012 14:36:00 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>, fes@...gle.com,
	aarcange@...hat.com, riel@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mikew@...gle.com, yinghan@...gle.com,
	virtualization@...ts.linux-foundation.org, yvugenfi@...hat.com,
	vrozenfe@...hat.com
Subject: Re: [PATCH] virtio-balloon spec: provide a version of the "silent deflate" feature that works

"Michael S. Tsirkin" <mst@...hat.com> writes:
> On Fri, Sep 07, 2012 at 04:09:50PM +0930, Rusty Russell wrote:
>> > So it looks like a bug: we should teach driver to tell host first on leak?
>> > Yan, Vadim, can you comment please?
>> >
>> > Also if true, looks like this bit will be useful to detect a fixed driver on
>> > the hypervisor side - to avoid unmapping such pages? Rusty what do you
>> > think?
>> 
>> So, feature is unimplemented in qemu, and broken in drivers.  I starting
>> to share Paolo's dislike of it.
>
> What is broken in drivers?

Because supporting the feature is *not* optional for a driver.

If the device said MUST_TELL_HOST, it meant that the driver *had* to
tell the host before it touched the page, otherwise Bad Things might
happen.  It was in the original spec precisely to allow devices to
actually *remove* pages.

Noone ever noticed the windows driver didn't support it, because qemu
never requires MUST_TELL_HOST.

So in practice, it's now an optional feature.  Since no device used it
anyway, we're better off discarding it than trying to fix it.

If someone wants an *optional* "tell me first" feature later, that's
easy to add, but I don't see why they'd want to.

> Do we really know there are no hypervisors implementing it?

As much as can be known.  Qemu doesn't, lkvm doesn't.

> As I said above drivers do have support.

Not the windows drivers.  So it's optional, thus removing it will likely
harm noone.

Cheers,
Rusty.
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