Re: [PATCH 0/3] x86/pvh: Support relocating dom0 kernel

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Jason Andryuk <jason.andryuk@amd.com>,
	xen-devel@lists.xenproject.org,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <george.dunlap@citrix.com>,
	Julien Grall <julien@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH 0/3] x86/pvh: Support relocating dom0 kernel
Date: Thu, 7 Mar 2024 11:20:53 +0100	[thread overview]
Message-ID: <ZemVBa1rxMsNI1pp@macbook> (raw)
In-Reply-To: <f6cbaf67-f5c7-4947-a07a-0f1d8f693462@suse.com>

On Thu, Mar 07, 2024 at 11:08:37AM +0100, Jan Beulich wrote:
> On 07.03.2024 11:00, Roger Pau Monné wrote:
> > On Wed, Mar 06, 2024 at 01:50:29PM -0500, Jason Andryuk wrote:
> >> Xen tries to load a PVH dom0 kernel at the fixed guest physical address
> >> from the elf headers.  For Linux, this defaults to 0x1000000 (16MB), but
> >> it can be configured.
> >>
> >> Unfortunately there exist firmwares that have reserved regions at this
> >> address, so Xen fails to load the dom0 kernel since it's not RAM.
> >>
> >> The other issue is that the Linux PVH entry point is not
> >> position-independent.  It expects to run at the compiled
> >> CONFIG_PHYSICAL_ADDRESS.
> >>
> >> This patch set expands the PVH dom0 builder to try to relocate the
> >> kernel if needed and possible.  XENFEAT_pvh_relocatable is added for
> >> kernels to indicate they are relocatable.  However, we may want to
> >> switch to an additional ELF note with the kernel alignment.  Linux
> >> specifies a kernel alignment in the bzImage boot_params.setup_header,
> >> but that is not present the ELF vmlinux file.
> > 
> > I wonder whether we need a pair of notes, to signal the min/max
> > addresses the kernel supports being relocated to.
> 
> Plus, as per your other reply, a 3rd one to specify alignment needs.

Alignment we could in theory get from the ELF program header, if OSes
fill those reliably.  FreeBSD seems to do so, haven't checked Linux.

> Then again - a single note can have multiple values. So perhaps it
> doesn't need to be more than one new notes (except if dealing with
> multi-value ones is deemed too complicated).

I've never dealt with a multi-value note, if that's not overly
complicated I would be fine with it, otherwise multiple notes are OK
IMO.

Thanks, Roger.