Re: [Xen-devel] [PATCH] netback: allow arbitrary mtu size until frontend connects

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Mon, 2011-02-07 at 09:49 +0000, Jan Beulich wrote:
> >>> On 07.02.11 at 10:11, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> wrote:
> > On Mon, 2011-02-07 at 09:03 +0000, Jan Beulich wrote:
> >> >>> On 06.02.11 at 14:42, Olaf Hering <olaf@xxxxxxxxx> wrote:
> >> > Allow arbitrary mtu size until the frontend is connected.  Once the
> >> > connection is established, adjust mtu by checking if the backend
> >> > supports the 'feature-sg'.  If the backend does support it, keep the
> >> > current mtu. Otherwise set it to the default value, which is 1500.
> >> > 
> >> > This helps the vif-bridge hotplug script to set the mtu size to 9000
> >> > while bringing up the guest.
> >> 
> >> Isn't this functionally the same as
> >> 
> > http://xenbits.xen.org/XCP/linux-2.6.32.pq.hg?file/043b76e4943c/xen-netback-A
> >  
> > llow-setting-of-large-MTU-before-rings.patch
> >> or its pv-ops parent 
> > https://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=commitdiff;h=bee2
> >  
> > eec2355c4bf4e149a426d5e30527162de566
> >> (in which case it would seem preferable to pull in that change)?
> >> 
> >> Ian, looking at that patch of yours again, the adjustment to
> >> netbk_set_sg() seems a little odd: Why is it necessary to
> >> reduce dev->mtu unconditionally (i.e. independent of the
> >> value of "data") here?
> > 
> > Hmm. I don't recall. Seems like a bug looking at it now...
> > 
> > Luckily for me it appears to have been correct by Paul in
> > d532fa93d4eeabbfc0176a6a9a93b0d6ade3f6c4
> 
> Indeed, to some part: How would dev->mtu grow again if sg
> got re-enabled after having been disabled?

Er, manual intervention?

For other features (e.g. gso, csum) I think we track the requested
configuration separately from the currently in force configuration.
Presumably we could do the same here if that's what we want.

Ian.


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