Re: [PATCH -mm] kexec jump -v9

["Huang, Ying" <ying.huang@intel.com>]

On Wed, 2008-05-14 at 16:52 -0400, Vivek Goyal wrote:
[...]
> Ok, I have done some testing on this patch. Currently I have just
> tested switching back and forth between two kernels and it is working for
> me.

Thanks.

[...]
> > +/*
> > + * Entry point for jumping back from kexeced kernel, the paging is
> > + * turned off.
> > + */
> > +kexec_jump_back_entry:
> > +	call	1f
> > +1:
> > +	popl	%ebx
> > +	subl	$(1b - kexec_relocate_page), %ebx
> > +	movl	%edi, KJUMP_ENTRY_OFF(%ebx)
> > +	movl	CP_VA_CONTROL_PAGE(%ebx), %edi
> > +	lea	STACK_TOP(%ebx), %esp
> > +	movl	CP_PA_SWAP_PAGE(%ebx), %eax
> > +	movl	CP_PA_BACKUP_PAGES_MAP(%ebx), %edx
> > +	pushl	%eax
> > +	pushl	%edx
> > +	call	swap_pages
> > +	addl	$8, %esp
> > +	movl	CP_PA_PGD(%ebx), %eax
> > +	movl	%eax, %cr3
> > +	movl	%cr0, %eax
> > +	orl	$(1<<31), %eax
> > +	movl	%eax, %cr0
> > +	lea	STACK_TOP(%edi), %esp
> > +	movl	%edi, %eax
> > +	addl	$(virtual_mapped - kexec_relocate_page), %eax
> > +	pushl	%eax
> > +	ret
> 
> Upon re-entering the kernel, what happens to GDT table? So gdtr will be
> pointing to GDT of other kernel (which is not there as pages have been
> swapped)? Do we need to reload the gdtr upon re-entering the kernel.

After re-entering the kernel and returning from machine_kexec,
restore_processor_state() is called, where the GDTR and some other CPU
state such as FPU, IDT, etc are restored.

> [..]
> > @@ -197,8 +282,54 @@ identity_mapped:
> >  	xorl	%eax, %eax
> >  	movl	%eax, %cr3
> >  
> > +	movl	CP_PA_SWAP_PAGE(%edi), %eax
> > +	pushl	%eax
> > +	pushl	%ebx
> > +	call	swap_pages
> > +	addl	$8, %esp
> > +
> > +	/* To be certain of avoiding problems with self-modifying code
> > +	 * I need to execute a serializing instruction here.
> > +	 * So I flush the TLB, it's handy, and not processor dependent.
> > +	 */
> > +	xorl	%eax, %eax
> > +	movl	%eax, %cr3
> > +
> > +	/* set all of the registers to known values */
> > +	/* leave %esp alone */
> > +
> > +	movl	KJUMP_MAGIC_OFF(%edi), %eax
> > +	cmpl	$KJUMP_MAGIC_NUMBER, %eax
> > +	jz 1f
> > +	xorl	%edi, %edi
> > +	xorl	%eax, %eax
> > +	xorl	%ebx, %ebx
> > +	xorl    %ecx, %ecx
> > +	xorl    %edx, %edx
> > +	xorl    %esi, %esi
> > +	xorl    %ebp, %ebp
> > +	ret
> > +1:
> > +	popl	%edx
> > +	movl	CP_PA_SWAP_PAGE(%edi), %esp
> > +	addl	$PAGE_SIZE_asm, %esp
> > +	pushl	%edx
> > +2:
> > +	call	*%edx
> 
> > +	movl	%edi, %edx
> > +	popl	%edi
> > +	pushl	%edx
> > +	jmp	2b
> > +
> 
> What does above piece of code do? Looks like redundant for switching
> between the kernels? After call *%edx, we never return here. Instead
> we come back to "kexec_jump_back_entry"?

For switching between the kernels, this is redundant. Originally another
feature of kexec jump is to call some code in physical mode. This is
used to provide a C ABI to called code.

Now, Eric suggests to use a C ABI compatible mode to pass the jump back
entry point too, that is, use the return address on stack instead of %
edi. I think that is reasonable. Maybe we can revise this code to be
compatible with C ABI and provide a convenient interface for both kernel
and other physical mode code.

> [..]
> > --- /dev/null
> > +++ b/Documentation/i386/jump_back_protocol.txt
> > @@ -0,0 +1,66 @@
> > +		THE LINUX/I386 JUMP BACK PROTOCOL
> > +		---------------------------------
> > +
> > +		Huang Ying <ying.huang@intel.com>
> > +		    Last update 2007-12-19
> > +
> > +Currently, the following versions of the jump back protocol exist.
> > +
> > +Protocol 1.00:	Jumping between original kernel and kexeced kernel
> > +		support. Calling ordinary C function support.
> > +
> > +
> > +*** JUMP BACK ENTRY
> > +
> > +At jump back entry of callee, the CPU must be in 32-bit protected mode
> > +with paging disabled; the CS, DS, ES and SS must be 4G flat segments;
> > +CS must have execute/read permission, and DS, ES and SS must have
> > +read/write permission; interrupt must be disabled; the contents of
> > +registers and corresponding memory must be as follow:
> > +
> > +Offset/Size	Meaning
> > +
> > +%edi		Real jump back entry of caller if supported,
> > +		otherwise 0.
> > +%esp		Stack top pointer, the size of stack is about 4k bytes.
> > +(%esp)/4	Helper jump back entry of caller if %edi != 0,
> > +		otherwise undefined.
> > +
> 
> I am not sure what is helper jump back entry? I understand that you 
> are using %edi to pass around entry point between two kernels. Can
> you please shed some more light on this?

Helper jump back entry is used to provide a C ABI to some physical mode
code other than kernel. It is the above redundant code.

Best Regards,
Huang Ying


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