linux-kernel - Re: [PATCHv2 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition

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Message-ID: <6a5jgjdwnjseqxsxfzfcyflqyrpmzr7n5ki5haxc3e6llu5cbe@3lmi4qbfol7q>
Date: Thu, 15 Aug 2024 12:18:32 +0300
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Baoquan He <bhe@...hat.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, 
	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>, 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, kexec@...ts.infradead.org
Subject: Re: [PATCHv2 3/4] x86/64/kexec: Map original relocate_kernel() in
 init_transition_pgtable()

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