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

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.

Map the original relocate_kernel() at the relocate_kernel() virtual
address in the identity mapping. It is preparation to replace the
init_transition_pgtable() implementation with a call to
kernel_ident_mapping_init().

Note that while relocate_kernel() switches to the identity mapping, it
does not flush global TLB entries (CR4.PGE is not cleared). This means
that in most cases, the kernel still runs relocate_kernel() from the
original physical address before the change.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/kernel/machine_kexec_64.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

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.

> 
> Map the original relocate_kernel() at the relocate_kernel() virtual
> address in the identity mapping. It is preparation to replace the
> init_transition_pgtable() implementation with a call to
> kernel_ident_mapping_init().
> 
> Note that while relocate_kernel() switches to the identity mapping, it
> does not flush global TLB entries (CR4.PGE is not cleared). This means
> that in most cases, the kernel still runs relocate_kernel() from the
> original physical address before the change.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/x86/kernel/machine_kexec_64.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 9c9ac606893e..645690e81c2d 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -157,7 +157,7 @@ static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
>  	pte_t *pte;
>  
>  	vaddr = (unsigned long)relocate_kernel;
> -	paddr = __pa(page_address(image->control_code_page)+PAGE_SIZE);
> +	paddr = __pa(relocate_kernel);
>  	pgd += pgd_index(vaddr);
>  	if (!pgd_present(*pgd)) {
>  		p4d = (p4d_t *)get_zeroed_page(GFP_KERNEL);
> -- 
> 2.43.0
>

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.