Re: [PATCH 3/3] x86/PVH: Support relocatable dom0 kernels

[Juergen Gross <jgross@xxxxxxxx>]

On 07.03.24 18:01, Jason Andryuk wrote:
> On 2024-03-07 04:30, Roger Pau Monné wrote:
> > On Wed, Mar 06, 2024 at 01:50:32PM -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 PVH entry code is not relocatable - it loads from absolute
> > > addresses, which fail when the kernel is loaded at a different address.
> > > With a suitably modified kernel, a reloctable entry point is possible.
> > >
> > > Add the XENFEAT_pvh_relocatable flag to let a kernel indicate that it
> > > supports a relocatable entry path.
> > >
> > > Change the loading to check for an acceptable load address.  If the
> > > kernel is relocatable, support finding an alternate load address.
> > >
> > > Linux cares about its physical alignment.  This can be pulled out of the
> > > bzImage header, but not from the vmlinux ELF file.  If an alignment
> > > can't be found, use 2MB.
> > While I'm fine with having a Linux specific way, there needs to be a
> > generic way of passing the alignment for non-bzImage kernels.
> >
> > ELF program headers have an align field, would that be suitable to
> > use?
> Unfortunately, it doesn't seem correct.  Linux has CONFIG_PHYSICAL_ALIGN, and it 
> doesn't seem to be used in the elf headers.  As a quick test, I set 
> CONFIG_PHYSICAL_ALIGN=0x800000, but the elf align values are still 0x200000.
An excerpt from the kernel's arch/x86/Makefile:

#
# The 64-bit kernel must be aligned to 2MB.  Pass -z max-page-size=0x200000 to
# the linker to force 2MB page size regardless of the default page size used
# by the linker.
#
ifdef CONFIG_X86_64
LDFLAGS_vmlinux += -z max-page-size=0x200000
endif


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature.asc
Description: OpenPGP digital signature