linux-kernel - Re: [PATCH 1/4 -mm] kexec based hibernation -v7 : kexec jump

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Message-ID: <20071210223117.GC27124@redhat.com>
Date:	Mon, 10 Dec 2007 17:31:17 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"Huang, Ying" <ying.huang@...el.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Machek <pavel@....cz>, nigel@...el.suspend2.net,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	linux-pm@...ts.linux-foundation.org,
	Kexec Mailing List <kexec@...ts.infradead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4 -mm] kexec based hibernation -v7 : kexec jump

On Fri, Dec 07, 2007 at 03:53:30PM +0000, Huang, Ying wrote:
> This patch implements the functionality of jumping between the kexeced
> kernel and the original kernel.
> 
> To support jumping between two kernels, before jumping to (executing)
> the new kernel and jumping back to the original kernel, the devices
> are put into quiescent state, and the state of devices and CPU is
> saved. After jumping back from kexeced kernel and jumping to the new
> kernel, the state of devices and CPU are restored accordingly. The
> devices/CPU state save/restore code of software suspend is called to
> implement corresponding function.
> 
> To support jumping without reserving memory. One shadow backup page
> (source page) is allocated for each page used by new (kexeced) kernel
> (destination page). When do kexec_load, the image of new kernel is
> loaded into source pages, and before executing, the destination pages
> and the source pages are swapped, so the contents of destination pages
> are backupped. Before jumping to the new (kexeced) kernel and after
> jumping back to the original kernel, the destination pages and the
> source pages are swapped too.
> 
> A jump back protocol for kexec is defined and documented. It is an
> extension to ordinary function calling protocol. So, the facility
> provided by this patch can be used to call ordinary C function in real
> mode.
> 
> A set of flags for sys_kexec_load are added to control which state are
> saved/restored before/after real mode code executing. For example, you
> can specify the device state and FPU state are saved/restored
> before/after real mode code executing.
> 
> The states (exclude CPU state) save/restore code can be overridden
> based on the "command" parameter of kexec jump. Because more states
> need to be saved/restored by hibernating/resuming.
> 


[..]
>  
> -#define KEXEC_ON_CRASH  0x00000001
> -#define KEXEC_ARCH_MASK 0xffff0000
> +#define KEXEC_ON_CRASH		0x00000001
> +#define KEXEC_PRESERVE_CPU	0x00000002
> +#define KEXEC_PRESERVE_CPU_EXT	0x00000004
> +#define KEXEC_SINGLE_CPU	0x00000008
> +#define KEXEC_PRESERVE_DEVICE	0x00000010
> +#define KEXEC_PRESERVE_CONSOLE	0x00000020

Hi,

Why do we need so many different flags for preserving different types
of state (CPU, CPU_EXT, Device, console) ? To keep things simple,
can't we can create just one flag KEXEC_PRESERVE_CONTEXT, which will
indicate any special action required for preserving the previous kernel's
context so that one can swith back to old kernel?

Thanks
Vivek
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