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Re: [Xen-devel] [PATCH] add privileged/unprivileged kernel feature indi

To: Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] add privileged/unprivileged kernel feature indication
From: Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Date: Tue, 5 Jul 2011 15:27:06 +0100
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On Tue, 2011-07-05 at 15:15 +0100, Jan Beulich wrote:
> >>> On 05.07.11 at 15:55, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Tue, 2011-07-05 at 13:48 +0100, Jan Beulich wrote:
> >> With our switching away from supporting 32-bit Dom0 operation, users
> >> complained that attempts (perhaps due to lack of knowledge of that
> >> change) to boot the no longer privileged kernel in Dom0 resulted in
> >> apparently silent failure. To make the mismatch explicit and visible,
> >> add feature flags that the kernel can set to indicate operation in
> >> what modes it supports. For backward compatibility, absence of both
> >> feature flags is taken to indicate a kernel that may be capable of
> >> operating in both modes.
> > 
> > While I agree that this is a useful change I think you should also try
> > and ensure that your bootloader configuration tool tries not to add
> > invalid combinations to the configuration. For example grub2's
> > "update-grub" command checks for CONFIG_XEN_PRIVILEGED_GUEST=y before
> > creating a dom0 style entry. IIRC grub1 did the same (although that may
> > have been Debian specific).
> That I would consider completely bogus - the boot loader should not
> have a need to know *anything* about the kind of kernel it boots.

I said bootloader configuration tool (i.e. the thing which helps users
generate /boot/grub/grub.cfg), not the bootloader itself. i.e. the tool
should not be adding xen+domU-only-kernel as an option in grub.cfg.

If this tool doesn't understand these things you end up presenting users
with booloader options which cannot ever work and then they get


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