linux-kernel - Re: [RFC][PATCH 2/4] checkpoint/restart: x86 support

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Date:	Sat, 9 Aug 2008 00:29:27 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	containers@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, Theodore Tso <tytso@....edu>,
	"Serge E. Hallyn" <serue@...ibm.com>
Subject: Re: [RFC][PATCH 2/4] checkpoint/restart: x86 support

On Friday 08 August 2008, Oren Laadan wrote:

> > It seems weird that you use __u64 members for the registers, but don't
> > include r8..r15 in the list. As a consequence, this structure does not
> > seem well suited for either x86-32 or x86-64.
> 
> In the context of CR, x86-32 and x86-64 are distinct architectures because
> you cannot always migrate from one to the other (though 32->64 is sometimes
> possible). Therefore, each architecture can have a separate checkpoint file
> format (eg r8..r15 only for x86-64).

So why do you use __u64 members for your 32 bit registers?

> Except for this special case (32 bit running 64 bit), simple conversion can
> be done in the kernel if needed, but most conversion between kernel the
> format for different kernel versions (should it change) can be done in
> user space (eg. with a filter).

The 32bit on 64bit case is quite common on non-x86 architectures, e.g.
powerpc or sparc, where 64 bit kernels typically run 32 bit user space.

A particularly interesting case is mixing 32 and 64 bit tasks in a container
that you are checkpointing. This is a very realistic scenario, so there
may be good arguments for keeping the format identical between the variations
of each architecture.

> > I would suggest either using struct pt_regs by reference, or defining
> > it so that you can use the same structure for both 32 and 64 bit x86.
> 
> We prefer not to use the kernel structure directly, but an intermediate
> structure that can help mitigate subtle incompatibilities issues (between
> kernel configurations, versions, and even compiler versions).
> 
> Anyway, either a single structure for both 32 and 64 bit x86, or separate
> "struct cr_hdr_cpu{_32,_64}", one for each architecture.

struct pt_regs is part of the kernel ABI, it will not change.

	Arnd <><


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