Re: [win-pv-devel] Problems with network using 8.2.0.x on Windows 2012R2 and Xen 4.8.1-pre

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Adam Goryachev [mailto:mailinglists@xxxxxxxxxxxxxxxxxxxxxx]
> Sent: 26 April 2017 15:21
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; win-pv-
> devel@xxxxxxxxxxxxxxxxxxxx
> Subject: Re: [win-pv-devel] Problems with network using 8.2.0.x on Windows
> 2012R2 and Xen 4.8.1-pre
> 
> 
> 
> On 26/4/17 18:29, Paul Durrant wrote:
> >> -----Original Message-----
> > [snip]
> >> Hi,
> >> I've been using xen for a number of years with a dozen windows 2012R2
> >> servers, and have been using one of the ejb drivers. I'm not upgrading
> >> to a new version of Xen from Debian testing packages, and having some
> >> problems to get the network interface working reliably. To get to this
> >> point, I've followed these steps:
> >> 1) snapshot the VM drive
> >> 2) boot the VM on the new xen platform
> >> 3) allow windows to fail to boot (can't find the HDD), boot into
> >> recovery menu
> >> 4) Boot into safe mode, uninstall the old drivers using an old uninstall
> >> script for GPLPV v10
> >> 5) Install the new 8.2.0.x drivers
> >> 6) Reboot after all drivers are installed
> >> 7) Everything works, except the network card which has the yellow ! mark
> >> in device manager
> >>
> >> After numerous attempts to fix this, I can see the following "minimal"
> >> steps will give a working device (until the next reboot):
> >> 1) Disable the non-working device
> >> 2) Enable the device
> >> Now, everything works as normal, but as noted, after a reboot it fails
> >> to start again.
> >>
> >> I've managed to enable debugging, and capture the bootup process, then
> >> below that is the log of what happens when I disable, and then enable
> >> the device. Apologies, but I've trimmed the logs to what I hope is the
> >> required information, please let me know if you need more.
> >>
> > Hi Adam,
> >
> >    Your problem is this:
> >
> > 3677@1493170121.472062:xen_platform_log xen platform:
> > XENBUS|GnttabExpand: fail1 (c000009a)
> >
> >    Something has swallowed most of your grant table. I suspect it is your
> storage interface... Unfortunately blkback has some rather bad defaults and
> you end up using multiple queues, with multiple pages per queue, and this
> takes a lot of grant entries. Try reducing to a single page per queue (IOW set
> blkback's 'max_ring_page_order' to 0) and see if that helps. An alternative
> would be boot Xen with a larger grant table size... E.g. I boot mine with
> 'gnttab_max_frames=128' (the default being 32).
> >    The fact that you can get the network interface up and running after boot
> is probably merely because there is less going on storage-wise and you're
> getting lucky.
> >
> 
> Thank you so much Paul, I added the gnttab_max_frames=128 to my grub
> config, and the network will now start automatically with bootup of the
> domU.
> 
> Three questions if you could please answer for me:
> 1) Is there any downside to increasing the value from the default 32 to
> 128? Is it just a little extra RAM usage or something? Or is there some
> other problem it might cause?

Hi Adam,

No, there shouldn't be any downside other than the extra pages used.

> 2) I was concerned that limiting the blkback might slow down the
> performance of that. I don't mind allocating additional RAM to solve
> problems, or to improve performance.

In practice I suspect that, unless you have a really really fast SSD or a 
ramdisk, then using a single page ring in blkback is not going to make a 
noticeable difference to your performance.

> 3) If I tried to start multiple domU's on this same dom0, would I end up
> running out, even though I've increased it? Would I run out when trying
> to start the 4th domU?

No, the grant table is per-domain so each domU will get 128 frames... there's 
no contention between domains in this respect.

Cheers,

  Paul

> 
> Regards,
> Adam
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