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

Kevin Wolf
2023-10-26T08:27:52Z

Am 25.10.2023 um 20:56 hat Andrew Cooper geschrieben:
> On 25/10/2023 7:26 pm, David Woodhouse wrote:
> > On Wed, 2023-10-25 at 13:20 -0500, Eric Blake wrote:
> >> On Wed, Oct 25, 2023 at 03:50:42PM +0100, David Woodhouse wrote:
> >>> +
> >>> +Booting Xen PV guests
> >>> +---------------------
> >>> +
> >>> +Booting PV guest kernels is possible by using the Xen PV shim (a version of Xen
> >>> +itself, designed to run inside a Xen HVM guest and provide memory management
> >>> +services for one guest alone).
> >>> +
> >>> +The Xen binary is provided as the ``-kernel`` and the guest kernel itself (or
> >>> +PV Grub image) as the ``-initrd`` image, which actually just means the first
> >>> +multiboot "module". For example:
> >>> +
> >>> +.. 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.

> Multiboot2 and PVH support a command line per module, which is the
> preferred way to pass the commandlines, given a choice.

Multiboot 2 seems to integrate the string in a variable length module
structure instead of just having a pointer in a fixed length one, but
the model behind it is essentially the same as before.

Kevin