Re: [Fastboot] [PATCH 03/03] kexec: Avoid overwriting the current pgd (V2, x86_64)

["Magnus Damm" <magnus.damm@gmail.com>]

On 5/27/06, Eric W. Biederman <ebiederm@xmission.com> wrote:
> "Magnus Damm" <magnus.damm@gmail.com> writes:
>
> > The code is not broken. The code does exactly what you are talking
> > about. Maybe I was a bit unclear.
>
> Ack.  I got a little confused.  I was still thinking of what
> the identity mapping was trying to achieve, and you don't achieve
> the same thing.

No problem. The remapping code is all but simple.

> > "page_table_a" contains two mappings. One which is the same as where
> > the page is mapped in the kernel (the virtual address where
> > relocate_new_kernel is located), and one identity mapped. So when the
> > switch occurs the cpu will continue to run on the same virtual
> > address.
>
> In the general case which you want to cover page_table_a is still
> slightly wrong.  In particular you can theoretically get a conflict
> between the virtual address of your page table switching function and
> the physical address of the control code page.

Are you sure?

The mappings in page_table_a are created by C code in the
kernel/hypervisor, right? One of the mappings is an identity mapping,
and the other one is mapping the control page at the same address as a
"NX-safe" page in the kernel/hypervisor.

When we are about to kexec into the new kernel, the kernel/hypervisor
is accessing the regular page tables as usual. machine_kexec() then
jumps to the assembly code where we disable interrupts and switch page
tables. Could you please clarify what you mean by "page table
switching function"?

The only conflict that I see is when the two mappings in page_table_a
happen to be on the same address. But that is not a problem,
everything should work as expected in that case too. The page_table_a
code should in that case just use one mapping instead of two.

I could of course be wrong and I'd really like to hear all doubts
people have regarding my "page_table_a" solution. So if anyonel thinks
that there's a problem somewhere please give me a detailed
description.

> However while I agree that you need to do this in assembly for
> control I disagree that this code should be part of the
> relocate_new_kernel function.
>
> Please move the code that uses page_table_a to a separate function,
> that when it is done jumps to the control_code page.  Then you can
> map this page both virtually and physically with a statically
> allocated page table built a compile time.

This function, you write "uses page_table_a". Do you mean that the
function allocates it? Or fills it in? Or maybe switches to it? Please
clarify!

> This is a little simpler as you don't need to build this first
> page table dynamically and a little clearer as you aren't trying to
> get the control code page to serve two different functions.

But doesn't a static set of pages used for page_table_a just mean that
you are wasting valuable unswappable kernel memory? Also, how can you
be sure that the static pages are in a DMA-safe address range?

> If this function was more than 3 lines of assembly it could even
> be written in C for clarity with a special section so that
> the linker would not map it twice.  Of course that would still need
> to address the stack usage problem.

I do not really see the benefit with this static solution actually.
The special section also feels a bit complicated. Maybe it fits your
big picture better and that is of course good.

It does however make the Xen porting more difficult. =(

> >> So you need at least one additional entry in your page table.
> >>
> >> The fact that this bug did not jump out is a clear sign you were
> >> changing too many things at once, and did not have an adequate
> >> explanation in your change log.
> >
> > I will ignore that last part for now. I've tested the code on i386,
> > i386/pae, x86_64 (both opteron and athlon64) and all the combinations
> > with and without xen, and all configurations except x86_64 with both
> > kexec and kdump.
>
> Testing is important here.  But given that 99% of the bug hunting must
> be done via code review and thinking about the problem testing is not
> sufficient.

Agreed.

Thanks!

/ magnus
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