Re: [PATCHv2 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition_pgtable()

Kirill A. Shutemov
2024-08-15T09:18:39Z

On Thu, Aug 15, 2024 at 02:15:40PM +0800, Baoquan He wrote:
> Cc Eric and kexec mailing list.
>
> On 08/14/24 at 03:46pm, Kirill A. Shutemov wrote:
> > The init_transition_pgtable() function sets up transitional page tables.
> > It ensures that the relocate_kernel() function is present in the
> > identity mapping at the same location as in the kernel page tables.
> > relocate_kernel() switches to the identity mapping, and the function
> > must be present at the same location in the virtual address space before
> > and after switching page tables.
> >
> > init_transition_pgtable() maps a copy of relocate_kernel() in
> > image->control_code_page at the relocate_kernel() virtual address, but
> > the original physical address of relocate_kernel() would also work.
> >
> > It is safe to use original relocate_kernel() physical address cannot be
> > overwritten until swap_pages() is called, and the relocate_kernel()
> > virtual address will not be used by then.
>
> I haven't read these codes for long time, wondering if we still need
> copy relocate_kernel() to image->control_code_page + PAGE_SIZE as you
> said.

I think we can get away with only coping starting with identity_mapped().
But given that KEXEC_CONTROL_CODE_MAX_SIZE is 2K I don't see a reason to
change anything here.

--
Kiryl Shutsemau / Kirill A. Shutemov