linux-kernel - Re: [PATCHv3 3/4] x86/64/kexec: Map original relocate_kernel() in init_transition

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Message-ID: <6464ab0f43f51f4526666f8924921262e8144027.camel@intel.com>
Date: Tue, 20 Aug 2024 11:06:34 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC: "ardb@...nel.org" <ardb@...nel.org>, "luto@...nel.org" <luto@...nel.org>,
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	<linux-acpi@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCHv3 3/4] x86/64/kexec: Map original relocate_kernel() in
 init_transition_pgtable()

> > 
> > So the (*) can actually access to the control page IIUC.
> > 
> > Now if we change to map VA of relocate_kernel() to the original one, then (*)
> > won't be able to access the control page.
> 
> No, it still will be able to access control page.
> 
> So we call relocate_kernel() in normal kernel text (within
> __START_KERNEL_map).
> 
> relocate_kernel() switches to identity mapping, VA is still the same.
> 
> relocate_kernel() jumps to identity_mapped() in the control page:
> 
> 
> 	/*
> 	 * get physical address of control page now
> 	 * this is impossible after page table switch
> 	 */
> 	movq	PTR(PA_CONTROL_PAGE)(%rsi), %r8
> 
> 	...
> 
> 	/* jump to identity mapped page */
> 	addq	$(identity_mapped - relocate_kernel), %r8
> 	pushq	%r8
> 	ANNOTATE_UNRET_SAFE
> 	ret
> 
> The ADDQ finds offset of identity_mapped() in the control page.
> 
> identity_mapping() finds start of the control page from *relative*
> position of relocate_page() to the current RIP in the control page:
> 
> 	leaq	relocate_kernel(%rip), %r8
> 
> It looks like this in my kernel binary:
> 
> 	lea    -0xfa(%rip),%r8

Ah I see.  I missed the *relative* addressing. :-)

> 
> What PA is mapped at the normal kernel text VA of relocate_kernel() makes
> zero affect to the calculation.

Yeah.

> 
> Does it make sense?
> 

Yes.  Thanks for explanation.

At later time:

        call    swap_pages                                                   
        movq    $virtual_mapped, %rax  	<---- (1)                            
        pushq   %rax
        ANNOTATE_UNRET_SAFE
        ret  				<---- (2)

(1) will load the VA which has __START_KERNEL_map to %rax, and after (2) the
kernel will run at VA of the original relocate_kernel() which maps to the PA
of the original relcoate_kernel().  But I think the memory page of the
original relocate_kernel() won't get corrupted after returning from the second
kernel, so should be safe to use?