[Crash-utility] [PATCH] x86_64: Make the conversion between 4level and 5level paging automatically

[Dave Anderson]

----- Original Message -----
> Currently, Crash only enable support for kernel-only 5-level page tables by
> entering the command line option "--machdep vm=5level". Since Linux 4.17,
> the Linux kernel can be both 4level and 5level page tables. This command line
> can't work well for this.
> 
> Using the "pgtable_l5_enabled" got from vmcore to detect whether the kernel
> proper for 5 level page tables automatically.

Hello Dou,

Presumably by the time arch_crash_save_vmcoreinfo calls pgtable_l5_enabled(),
things have been initialized up appropriately, and so this should work OK for
kdump-generated vmcores.  But have you looked into how this should be accomplished
for for live systems?  Since kernel commit 51be1335 reverts __pgtable_l5_enabled 
from being __initdata to __ro_after_init, would it be as simple as just reading
__pgtable_l5_enabled at POST_RELOC time?

Thanks,
  Dave

> Signed-off-by: Dou Liyang <douly.fnst at cn.fujitsu.com>
> ---
>  x86_64.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/x86_64.c b/x86_64.c
> index 6d1ae2f..be6164b 100644
> --- a/x86_64.c
> +++ b/x86_64.c
> @@ -203,6 +203,10 @@ x86_64_init(int when)
>  			machdep->machspec->kernel_image_size = dtol(string, QUIET, NULL);
>  			free(string);
>  		}
> +		if ((string = pc->read_vmcoreinfo("NUMBER(pgtable_l5_enabled)"))) {
> +			machdep->flags |= VM_5LEVEL;
> +			free(string);
> +		}
>  		if (SADUMP_DUMPFILE() || QEMU_MEM_DUMP_NO_VMCOREINFO() ||
>  		    VMSS_DUMPFILE())
>  			/* Need for calculation of kaslr_offset and phys_base */
> --
> 2.14.3
> 
> 
> 
>