Re: [PATCH v3 28/28] docs: update Xen-on-KVM documentation

[David Woodhouse <dwmw2@xxxxxxxxxxxxx>]

On Thu, 2023-10-26 at 10:26 +0200, Kevin Wolf wrote:
> 
> > > > > +.. parsed-literal::
> > > > > +
> > > > > +  |qemu_system| --accel kvm,xen-version=0x40011,kernel-irqchip=split 
> > > > > \\
> > > > > +       -chardev stdio,id=char0 -device xen-console,chardev=char0 \\
> > > > > +       -display none  -m 1G  -kernel xen -initrd bzImage \\
> > > > > +       -append "pv-shim console=xen,pv -- console=hvc0 
> > > > > root=/dev/xvda1" \\
> > > > > +       -drive file=${GUEST_IMAGE},if=xen
> > > > Is the space between -- and console= intentionsl?
> > > Yes, that one is correct. The -- is how you separate Xen's command line
> > > (on the left) from the guest kernel command line (on the right).
> > 
> > To expand on this a bit.
> > 
> > Multiboot1 supports multiple modules but only a single command line.  As
> > one of the modules passed to Xen is the dom0 kernel, we need some way to
> > pass it's command line, hence the " -- ".
> 
> That's not right, even Multiboot 1 contains a 'string' field in the
> module structure that is defined to typically hold a command line. The
> exact meaning is OS dependent, so Xen could use it however it wants.
> 
> In QEMU (and I believe this is the same behaviour as in GRUB),
> everything before the space in an -initrd argument is treated as a
> filename to load, everything after it is just passed as the command
> line.
> 
> So it would have been entirely possible to use -initrd 'bzImage
> console=hvc0 root=/dev/xvda1' if Xen worked like that.

Xen does allow that too. I didn't realise our multiboot loader did though.

So yes, you *can* use  -initrd 'bzImage root=/dev/xvda1'. 

And you can even load more than one module, it seems. Separate them by
commas, so -initrd 'bzImage,initrd.img' should work.

You can even do both at the same time. If you have commas on the kernel
command line, *double* them:

 -initrd 'bzImage root=/dev/xvda earlyprintk=xen,,keep,initrd.img'

I'll update the documentation accordingly.

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