Re: [win-pv-devel] How to diagnose windows domU hang on boot?

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: firemeteor.guo@xxxxxxxxx [mailto:firemeteor.guo@xxxxxxxxx] On
> Behalf Of G.R.
> Sent: 08 February 2017 13:56
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> Cc: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [win-pv-devel] How to diagnose windows domU hang on boot?
> 
> >> > So, I suggest you stick with dom0 backends but limit your storage to a
> single
> >> page ring. I can't remember the exact blkback module param you need to
> set
> >> to do that, but it shouldn't be hard to find.
> >> >
> >> I'm not sure I understand what you mean by 'mutipage rings'. Must
> >> refer to something internal which is not mentioned in the doc:
> >> http://xenbits.xen.org/docs/4.8-testing/misc/xl-disk-configuration.txt
> >
> > No. I'm referring to the blkback feature. See
> https://lkml.org/lkml/2015/6/3/25
> >
> > I suggest you blkback's max_ring_page_order parameter to 0, because I
> think it is currently defaulting to 4... which is stupidly large for most use-
> cases.
> 
> Just would like to report back that this trick really fixed the issue.

Glad to hear it.

> Thank you so much for your help!!!
> (I'm currently using 2 instead of 0, may do some further tweaking).
> 

No problem. You may find that 2 is still a little high if you want to use 
multiple VBDs. You may want to consider setting Xen's gnttab_max_frames 
parameter on the boot command line. It defaults to 32 pages so you may want to 
give it more e.g. I typically bump it up to 128.

> One thing I'm curious is that why all other PV drivers (those in linux
> / freebsd, and old windows gpl pv) are working just fine, while this
> new windows driver suffers?

Probably because they either ignore or limit the order of multi-page rings.

> It's really not user friendly for the default config value to behave badly.

Agreed.

> Is there anything we can do to make it better? Either to propose a
> more reasonable default or make it more tolerant in the new windows pv
> driver?

Well it would be nice if blkback didn't ask for such ludicrously large shared 
ring sizes in the first place but, given that it does, the obvious thing to do 
would be to introduce a registry override in XENVBD for the max ring page order 
and then give it a default value of 0. I'll have a look at doing this.

  Cheers,

    Paul

> 
> Thanks,
> Rui
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