linux-kernel - [PATCHv4 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition

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Message-ID: <20240822124000.1171321-4-kirill.shutemov@linux.intel.com>
Date: Thu, 22 Aug 2024 15:39:59 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Baoquan He <bhe@...hat.com>
Cc: Ard Biesheuvel <ardb@...nel.org>,
	Tom Lendacky <thomas.lendacky@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Sean Christopherson <seanjc@...gle.com>,
	linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>
Subject: [PATCHv4 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@...ux.intel.com>
Reviewed-by: Kai Huang <kai.huang@...el.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