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

[Oren Laadan]

Arnd Bergmann wrote:
> 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?

The idea was that x86-32 checkpoints can be restarted on a x86-64 node in
32 bit mode. Clearly it needed more thought...

> > 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.

I'm in favor about keeping the format identical between the variations of
each architecture. Note, however, that "struct pt_regs" won't do because it
may change with these variations.

So we'll take care of the padding and add r8..r15 in the next version.

Oren.


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