RE: [Xen-devel] [PATCH 0/4] HVM Virtual S3

["Yu, Ke" <ke.yu@xxxxxxxxx>]

Keir Fraser  wrote on 2007年5月17日 15:41:
> On 17/5/07 03:32, "Yu, Ke" <ke.yu@xxxxxxxxx> wrote:
> 
>> My concern here is that: save/restore is a heavy operation just like
>> S4 (hibernate), while the purpose of S3 is quick suspend and quick
>> resume comapred to S4. if we implement S3 like save/restore, I don't
>> see the value here, because HVM save/resotre or HVM S4 is just
>> enough. How do you think? 
> 
> I do not think that pure virtual S3 by itself makes sense. Unless the
> whole machine is going into S3, what really is the difference between
> a HVM guest in S3 and an HVM guest that simply is idle and so has all
> its VCPUs HLTed 
> 99.9% of the time? Both are tying up memory resource, neither is
> burning significant CPU resource or I/O resource.

Agree.Virtual S3 itself don't have practical value.

> 
> Virtual S3 *does* make sense with HVM save/restore because it makes
> even an HVM guest without PV drivers aware of the save/restore event
> so it can do things like resync its clock. That is why we are
> interested in virtual S3: only as an adjunct to save/restore.

I see your point. Virtual S3 can be used  to assist HVM save/resotor. It is 
fine. 

> 
> Another application would be as part of host S3. Given that those
> patches currently save/restore all domains (which actually I think is
> stupid, but that's for a separate email) you should automatically
> improve host S3 by integrating with save/restore. However, again, the
> benefit is not the effects of the state the guest finds itself in ---
> since S3 is not enormously meaningful in a virtualised environment
> --- but because of the side effects we enjoy when the guest awakens
> and resyncs its world. 
> 
>  -- Keir

Virtual S3 is also an important part of host S3, when HVM domain is assigned 
with physical device, e.g. with VT-d introduced. When host S3 begins, HVM 
domain should suspend its physical device,  otherwise the physical device may 
not work after resume. This is the reason why we develop the vritaul S3 
feature: help VT-d HVM domain to do power management. In this case, integrating 
save/resotre with virtual S3 is not necessary,  and it actually brings extra 
overhead. 

To summary, virtual S3 can have two functionality
- assist HVM to do save/restore
- assist HVM to do vt-d device power management
The first item need to call save/restore while the second item do not.

So is it possible that we have two code path, and use flags to tell virtual S3 
which path it should go?

BTW, I did not see current host S3 patches save/restore all domains. Could you 
please elabroate more?

Best Regards
Ke

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