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

From: David Woodhouse <dwmw2@infradead.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
	Eric Blake <eblake@redhat.com>
Cc: qemu-devel@nongnu.org, "Kevin Wolf" <kwolf@redhat.com>,
	"Hanna Reitz" <hreitz@redhat.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Anthony Perard" <anthony.perard@citrix.com>,
	"Paul Durrant" <paul@xen.org>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Jason Wang" <jasowang@redhat.com>,
	"Marcelo Tosatti" <mtosatti@redhat.com>,
	qemu-block@nongnu.org, xen-devel@lists.xenproject.org,
	kvm@vger.kernel.org, "Bernhard Beschow" <shentey@gmail.com>,
	"Joel Upham" <jupham125@gmail.com>
Subject: Re: [PATCH v3 28/28] docs: update Xen-on-KVM documentation
Date: Wed, 25 Oct 2023 20:02:31 +0100	[thread overview]
Message-ID: <0f7a176064f2b351576ab5d31a3a8e921cca4212.camel@infradead.org> (raw)
In-Reply-To: <21e8a265-bf5a-464c-86bc-f0fd7b5eb108@citrix.com>

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On Wed, 2023-10-25 at 19:56 +0100, Andrew Cooper wrote:
> 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 " -- ".
>  
>  Multiboot2 and PVH support a command line per module, which is the
> preferred way to pass the commandlines, given a choice.
>  

Thanks.

Indeed, I had *originally* thought I was going to need to implement
Multiboot2 in qemu in order to boot Shim + PV guest, but it turns out
we can make it work with just Multiboot1 support.

As long as the guest kernel doesn't want an *actual* initrd, that is ;)

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