[PATCH] x86/kexec: set MIN_KERNEL_LOAD_ADDR to 0x01000000

[Baoquan He]

Hi John,

On 10/23/23 at 02:54pm, John Sperbeck wrote:
> On Sun, Oct 22, 2023 at 7:42 PM H. Peter Anvin <hpa at zytor.com> wrote:
......
> > >---
> > > arch/x86/kernel/kexec-bzimage64.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > >diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> > >index a61c12c01270..d6bf6c13dab1 100644
> > >--- a/arch/x86/kernel/kexec-bzimage64.c
> > >+++ b/arch/x86/kernel/kexec-bzimage64.c
> > >@@ -36,7 +36,7 @@
> > >  */
> > > #define MIN_PURGATORY_ADDR    0x3000
> > > #define MIN_BOOTPARAM_ADDR    0x3000
> > >-#define MIN_KERNEL_LOAD_ADDR  0x100000
> > >+#define MIN_KERNEL_LOAD_ADDR  0x1000000
> > > #define MIN_INITRD_LOAD_ADDR  0x1000000
> > >
> > > /*
> >
> > This doesn't make any sense to me. There is already a high water mark for his much memory the kernel needs until an initrd or setup_data item can appear. This is just a hack, please fix it properly.
> 
> The startup_64() code in head_64.S changes behavior based on whether
> it's running below or above LOAD_PHYSICAL_ADDR:
> 
> #ifdef CONFIG_RELOCATABLE
>         leaq    startup_32(%rip) /* - $startup_32 */, %rbp
>         movl    BP_kernel_alignment(%rsi), %eax
>         decl    %eax
>         addq    %rax, %rbp
>         notq    %rax
>         andq    %rax, %rbp
>         cmpq    $LOAD_PHYSICAL_ADDR, %rbp
>         jae     1f
> #endif
>         movq    $LOAD_PHYSICAL_ADDR, %rbp
> 1:
> 
> In my example, we were running from address 0x00400000.  The %rbp
> register will start with 0x00400000, but will be changed to 0x01000000
> after the check against LOAD_PHYSICAL_ADDR fails.
> 
> The 0x01000000 value in %rbp is passed to extract_kernel as the
> 'output' argument.  Unless choose_random_location() decides
> differently, this will be where the kernel is decompressed to.  The
> size of the kernel is large enough in my example that the
> decompression overruns the initrd.
> 
> If the startup_64() code didn't have the LOAD_PHYSICAL_ADDR check and
> used %rpb as is, then there would be no issue.  The decompression
> would have been to 0x00400000 and would have completed before reaching
> the initrd memory.
> 
> That is, the kexec code is being careful to ensure that the kernel and
> initrd memory doesn't overlap, but isn't paying attention to what
> happens if the kernel memory is below LOAD_PHYSICAL_ADDR (the kernel
> address is effectively changed to a different location).  My proposed
> change makes it aware, and avoids such addresses.

Wondering why kexec-ed kernel is located under 0x1000000. The loading
code will search physical memory regions bottom up for an available one.
Usually, kexec kernel will be loaded above 16M.

I have posted a patchset to load kernel at top of system RAM for kexec_file
load just as kexec_load has been doing. Do you think it's helpful?

[PATCH 0/2] kexec_file: Load kernel at top of system RAM if required
https://lore.kernel.org/all/20231114091658.228030-1-bhe@redhat.com/T/#u

Thanks
Baoquan