Re: [PATCH v4 00/14] kexec: introduce Kexec HandOver (KHO)

[Mike Rapoport]

On Thu, Feb 20, 2025 at 05:43:48PM +0100, Alexander Gordeev wrote:
> On Thu, Feb 20, 2025 at 09:49:52AM +0800, Dave Young wrote:
> > On Wed, 19 Feb 2025 at 21:55, Alexander Graf <graf@xxxxxxxxxx> wrote:
> > > >>> What architecture exactly does this KHO work fine?   Device Tree
> > > >>> should be ok on arm*, x86 and power*, but how about s390?
> > > >> KHO does not use device tree as the boot protocol, it uses FDT as a data
> > > >> structure and adds architecture specific bits to the boot structures to
> > > >> point to that data, very similar to how IMA_KEXEC works.
> > > >>
> > > >> Currently KHO is implemented on arm64 and x86, but there is no fundamental
> > > >> reason why it wouldn't work on any architecture that supports kexec.
> > > > Well,  the problem is whether there is a way to  add dtb in the early
> > > > boot path,  for X86 it is added via setup_data,  if there is no such
> > > > way I'm not sure if it is doable especially for passing some info for
> > > > early boot use.  Then the KHO will be only for limited use cases.
> > >
> > >
> > > Every architecture has a platform specific way of passing data into the
> > > kernel so it can find its command line and initrd. S390x for example has
> > > struct parmarea. To enable s390x, you would remove some of its padding
> > > and replace it with a KHO base addr + size, so that the new kernel can
> > > find the KHO state tree.
> > 
> > Ok, thanks for the info,  I cced s390 people maybe they can provide inputs.
> 
> If I understand correctly, the parmarea would be used for passing the
> FDT address - which appears to be fine. However, s390 does not implement
> early_memremap()/early_memunmap(), which KHO needs.

KHO uses early_memremap()/early_memunmap() because it parses FDT before
phys_to_virt() is available on arm64 and x86. AFAIU on s390 phys_to_virt()
can be used at setup_arch() time, so it shouldn't be a problem to add
appropriate wrappers.
 
-- 
Sincerely yours,
Mike.