Re: [PATCH kvm-unit-tests] realmode: load above stack

[Thomas Huth <thuth@redhat.com>]

On 04/06/2024 16.35, Paolo Bonzini wrote:
> The bottom 32K of memory are generally reserved for use by the BIOS;
> for example, traditionally the boot loader is placed at 0x7C00 and
> the stack grows below that address.
> 
> It turns out that with some versions of clang, realmode.flat has
> become big enough that it overlaps the stack used by the multiboot
> option ROM loader.  The result is that a couple instructions are
> overwritten.  Typically one or two tests fail and that's it...
> 
> Move the code above the forbidden region, in real 90s style.
> 
> Reported-by: Thomas Huth <thuth@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>   x86/realmode.lds | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/x86/realmode.lds b/x86/realmode.lds
> index 0ed3063b..e4782a98 100644
> --- a/x86/realmode.lds
> +++ b/x86/realmode.lds
> @@ -1,6 +1,6 @@
>   SECTIONS
>   {
> -    . = 16K;
> +    . = 32K;
>       stext = .;
>       .text : { *(.init) *(.text) }
>       . = ALIGN(4K);

  Hi Paolo!

FYI, the realmode kvm-unit-test now also fails with the recent version of 
GCC 16 for the i386 target:

  https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/14195195727

It was working fine some weeks ago with GCC 15.1:

  https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/jobs/13977260961

When I apply your patch, the problem seems to be gone again in this case, 
but since there were some other issues with this (with older versions of 
GCC, I think):

https://lore.kernel.org/kvm/49f8aadf-6e3f-4d2b-a32a-8ba941a3a2a1@redhat.com/

... there must be a better way to fix it?

Could you please have a look?

  Thanks,
   Thomas