Re: [Xen-devel] [PATCH 3/4] xen kconfig: add dom0 support help text

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 6 Jan 2012, Ian Campbell wrote:
> On Fri, 2012-01-06 at 09:26 +0000, Andrew Jones wrote:
> > 
> > ----- Original Message -----
> > > On Fri, 2012-01-06 at 08:57 +0000, Andrew Jones wrote:
> > > > Describe dom0 support in the config menu and supply help text for
> > > > it.
> > > 
> > > This turns a non-user visible symbol into a user visible one.
> > > Previously
> > > if Xen was enabled and the other prerequisites were met you would get
> > > dom0 support automatically -- do we really want to change that?
> > > According to 6b0661a5e6fbf it was a deliberate decision to have it
> > > this
> > > way.
> > 
> > I think it's a necessary evil in order to give users the ability to
> > compile kernels without the support. I know it doesn't make much sense
> > for most users, but...
> 
> Who actually wants to do this though and why? Do you have a bug report
> requesting this change?
> 
> Almost all of the things which dom0 needs (e.g. PCI device management
> etc) is also required by a domU with passthrough enabled so the savings
> are really very slight.
> 
> We are talking less than 1k of code AFAICT, 319 bytes for
> arch/x86/xen/vga.o and 573 for drivers/xen/xenfs/xenstored.o plus
> whatever xen_register_gsi (a couple of dozen lines of code) adds to
> arch/x86/pci/xen.o. grep doesn't show CONFIG_XEN_DOM0 being used
> anywhere else. What savings do you see in practice from disabling just
> this symbol?
> 
> We need to weigh up the size change against the complexity of asking the
> user yet another question, I'm not convinced the question is worth it on
> balance.

I agree, there is nothing to gain disabling dom0 support if the
conditions it depends upon are already met.
XEN_DOM0 was meant to be a silent option since the beginning to avoid
unnecessary complexity.

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