Re: [Xen-devel] [GIT PULL] x86/mm changes for v3.9-rc1

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Thu, Feb 21, 2013 at 4:34 PM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> wrote:
>
> This is a huge set of several partly interrelated (and concurrently
> developed) changes, which is why the branch history is messier than
> one would like.
>
> The *really* big items are two humonguous patchsets mostly developed
> by Yinghai Lu at my request, which completely revamps the way we
> create initial page tables.

Ugh. So I've tried to walk through this, and it's painful. If this
results in problems, we're going to be *so* screwed. Is it bisectable?

I also don't understand how "early_idt_handler" could *possibly* work.
In particular, it seems to rely on the trap number being set up in the
stack frame:

        cmpl $14,72(%rsp)       # Page fault?

but that's not even *true*. Why? Because we export both the
early_idt_handlers[] array (that sets up the trap number and makes the
stack frame be reliable) and the single early_idt_handler function
(that relies on the trap number and the reliable stack frame), AND
AFAIK WE USE THE LATTER!

See x86_64_start_kernel():

        for (i = 0; i < NUM_EXCEPTION_VECTORS; i++) {
#ifdef CONFIG_EARLY_PRINTK
                set_intr_gate(i, &early_idt_handlers[i]);
#else
                set_intr_gate(i, early_idt_handler);
#endif
        }

so unless you have CONFIG_EARLY_PRINTK, the interrupt gate will point
to that raw early_idt_handler function that doesn't *work* on its own,
afaik.

Btw, it's not just the page fault index testing that is wrong. The whole

        cmpl $__KERNEL_CS,96(%rsp)
        jne 11f

also relies on the stack frame being set up the same way for all
exceptions - which again is only true if we ran through the
early_idt_handlers[] prologue that added the extra stack entry.

How does this even work for me? I don't have EARLY_PRINTK enabled.

What am I missing?

                Linus

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