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    SubjectRe: [Devel] Re: [PATCH 08/10] Introduce functions to restart a process
    On Wed, Oct 22, 2008 at 12:06:19PM +0200, Greg Kurz wrote:
    > On Wed, 2008-10-22 at 11:25 +0200, Louis Rilling wrote:
    > > Do you checkpoint uninterruptible syscalls as well? If only interruptible
    > > syscalls are checkpointed, I'd say that either this syscall uses ERESTARTSYS or
    > > ERESTART_RESTARTBLOCK, and then signal handling code already does the trick, or
    > > this syscall does not restart itself when interrupted, and well, this is life,
    > > userspace just sees -EINTR, which is allowed by the syscall spec.
    > > Actually this is how we checkpoint/migrate tasks in interruptible syscalls in
    > > Kerrighed and this works.
    > >
    > > Louis
    > >
    > I don't know Kerrighed internals but I understand you perform checkpoint
    > with a signal handler. Right ?

    Right. This is an kernel-internal-only signal, so all signals remain available
    for userspace.

    > This approach has a huge benefit: the
    > signal handling code do all the arch dependant stuff to save registers
    > in user memory.

    Hm, I'm not sure to understand what you mean here. We just rely on arch code
    that jumps to signal handling to correctly setup struct pt_regs, which is then
    passed to the checkpoint code. So yes, userspace registers are mostly saved by
    existing arch code. But in x86-64 for instance, segment registers still need to
    be saved by the checkpoint code (a bit like copy_thread() does), and I don't
    know arch-independent functions doing this.


    Dr Louis Rilling Kerlabs
    Skype: louis.rilling Batiment Germanium
    Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes 35700 Rennes
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