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

From: Baoquan He <bhe@redhat.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Ard Biesheuvel <ardb@kernel.org>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	Sean Christopherson <seanjc@google.com>,
	linux-kernel@vger.kernel.org, linux-acpi@vger.kernel.org,
	kexec@lists.infradead.org
Subject: Re: [PATCHv2 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition_pgtable()
Date: Thu, 15 Aug 2024 14:15:40 +0800	[thread overview]
Message-ID: <Zr2dDKP4JuRUSOL3@MiWiFi-R3L-srv> (raw)
In-Reply-To: <20240814124613.2632226-4-kirill.shutemov@linux.intel.com>

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
> 

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