Il 28/05/2013 17:09, Michael S. Tsirkin ha scritto:
> On Tue, May 28, 2013 at 04:32:44PM +0200, Paolo Bonzini wrote:
>> Il 28/05/2013 16:29, Michael S. Tsirkin ha scritto:
>>> On Tue, May 28, 2013 at 04:06:02PM +0200, Paolo Bonzini wrote:
>>>> Il 28/05/2013 15:32, Michael S. Tsirkin ha scritto:
>>>>> At this point I am confused. I think there are two changes in your patch:
>>>>>
>>>>> 1. Handling of VIRTIO_F_GUEST_MUST_TELL_HOST
>>>>> Is this functionally identical to what I proposed?
>>>>> If yes, I am fine with either change being applied.
>>>>
>>>> Yes.
>>>>
>>>>> 2. New SILENT_DEFLATE feature
>>>>> Since guest can get same functionality by not acking
>>>>> TELL_HOST, I still don't see what good it does:
>>>>> Historically a host with no features supports silent
>>>>> deflate and guest with no features can do silent deflate.
>>>>> I conclude silent deflate is the default behaviour for
>>>>> both host and guest, and we can't change default without
>>>>> breaking compatibility.
>>>>
>>>> You're right that for correctness the existing feature is enough:
>>>> if it is not negotiated by the guest, the host ensures correctness by
>>>> only giving the guest a fake balloon.
>>>>
>>>> However, the new feature is about optimization, not correctness.
>>>> In fact, VIRTIO_BALLOON_F_SILENT_DEFLATE is the optimization
>>>> feature that VIRTIO_BALLOON_F_MUST_TELL_HOST was meant to be.
>>>>
>>>> What I'm interested in, is drivers that can _optionally_ use silent
>>>> deflation (as an optimization). These should not get a fake balloon!
>>>>
>>>> With the new feature bit, these drivers should propose both
>>>> VIRTIO_BALLOON_F_GUEST_TELLS_HOST and VIRTIO_BALLOON_F_SILENT_DEFLATE.
>>>> The driver can then use silent deflation if and only if the host
>>>> has negotiated VIRTIO_BALLOON_F_SILENT_DEFLATE too. Like this:
>>>>
>>>> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
>>>> index bd3ae32..05fe948 100644
>>>> --- a/drivers/virtio/virtio_balloon.c
>>>> +++ b/drivers/virtio/virtio_balloon.c
>>>> @@ -186,12 +186,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
>>>> vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
>>>> }
>>>>
>>>> - /*
>>>> - * Note that if
>>>> - * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
>>>> - * is true, we *have* to do it in this order
>>>> - */
>>>> - tell_host(vb, vb->deflate_vq);
>>>> + if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_SILENT_DEFLATE)
>>>> + tell_host(vb, vb->deflate_vq);
>>>> mutex_unlock(&vb->balloon_lock);
>>>> release_pages_by_pfn(vb->pfns, vb->num_pfns);
>>>> }
>>>> @@ -543,6 +539,7 @@ static int virtballoon_restore(struct virtio_device *vdev)
>>>> static unsigned int features[] = {
>>>> VIRTIO_BALLOON_F_MUST_TELL_HOST,
>>>> VIRTIO_BALLOON_F_STATS_VQ,
>>>> + VIRTIO_BALLOON_F_SILENT_DEFLATE,
>>>> };
>>>>
>>>> static struct virtio_driver virtio_balloon_driver = {
>>>>
>>>>
>>>> Of course with the current implementation of the balloon it does not
>>>> matter much. But for example, with Luiz's work, releasing pages as soon
>>>> as the shrinker is called will increase effectiveness of the shrinker.
>>>> At the same time, not all is lost if the guest prefers not to allow
>>>> silent deflation (e.g. because there is an assigned device).
>>>>
>>>> On old hosts, a guest that can optionally use silent deflation will
>>>> not use it. That's the same as for any other feature bit.
>>>>
>>>>> How about splitting the patches so we can discuss them separately?
>>>>
>>>> I can do that, but I hope the above clarifies it.
>>>
>>> Maybe I'm just dense.
>>> Let's see the split spec patchset?
>>
>> What's unclear exactly? I'm not sure the spec patchset improves things
>> that much, I can split it in two or three (change old feature, add new
>> feature, add explanation) but it's not like changing logic in a program.
>>
>> Paolo
>
> Both your code and what you say here about the new bit seem to break
> compatibility with old hosts and guests.
What is the exact scenario that you have in mind? Here are all the
possibilities:
- host that requires tell-first for a real balloon (doesn't
propose new bit), any guest (doesn't matter if they are old
or new since the new bit is never negotiated).
Here, the old text suggested that the host need not do anything special,
but it wouldn't have worked with Windows guests. The host has to
provide a fake balloon, or prevent the driver from working (e.g. hide
the virtqueues). So this is a change indeed, but the same change is
present with your 1-word change too. We have:
negotiated old bit host operation guest operation
F fake balloon need not tell host
T real balloon tells host
Note that the guest ignores the result of negotiating the old bit,
only the host cares and only if it requires tell-first.
The other cases have no such change:
- old host, doesn't require tell-first (doesn't propose new bit):
negotiated old bit host operation guest operation
F real balloon need not tell host
T real balloon tells host
- new host, doesn't require tell-first (proposes new bit), old guest
(doesn't propose new bit, hence it is never negotiated):
negotiated old bit host operation guest operation
F real balloon need not tell host
T real balloon tells host
Same as the previous case, since in both cases the new bit is not
negotiated.
Note that the host ignores the result of negotiating the new bit,
only the guest cares.
- new host, doesn't require tell-first (proposes new bit), new guest
(proposes new bit, thus it is negotiated):
negotiated old bit negotiated new bit host operation guest operation
F T real balloon need not tell host
T T real balloon need not tell host
- host that requires tell-first for a real balloon (doesn't matter
if old or new since it doesn't propose the new bit, hence it is
never negotiated), new guest:
negotiated old bit negotiated new bit host operation guest operation
T F real balloon need not tell host
The very last case is the interesting one. Without the new bit, the
guest has to promise it will tell the host first when deflating.
With the new bit, the guest is just telling that it *can* tell
the host first; the host can still say "don't worry about that".
So the guest sees the new bit and thinks "I told the host I *could*
tell it about deflated pages, but the host told me I need not, so
I won't do it". This is the optimized case I was talking about.
> If it's in spec, I think it would be clearer what are we trying to
> achieve, and how.
Having one or three patches doesn't change the final text...
Paolo
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