Re: [PATCH 1/2] x86/mm: Do not zap PMD entry mapping unaccepted memory table during kdump.

[Kirill A. Shutemov]

On Tue, Feb 20, 2024 at 01:18:29AM +0000, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
> 
> During crashkernel boot only pre-allocated crash memory is presented as
> E820_TYPE_RAM. This can cause PMD entry mapping unaccepted memory table
> to be zapped during phys_pmd_init() as SNP/TDX guest use E820_TYPE_ACPI
> to store the unaccepted memory table and pass it between the kernels on
> kexec/kdump.
> 
> E820_TYPE_ACPI covers not only ACPI data, but also EFI tables and might
> be required by kernel to function properly.
> 
> The problem was discovered during debugging kdump for SNP guest. The
> unaccepted memory table stored with E820_TYPE_ACPI and passed between
> the kernels on kdump was getting zapped as the PMD entry mapping this
> is above the E820_TYPE_RAM range for the reserved crashkernel memory.
> 
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  arch/x86/mm/init_64.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
> index a0dffaca6d2b..207c6dddde0c 100644
> --- a/arch/x86/mm/init_64.c
> +++ b/arch/x86/mm/init_64.c
> @@ -524,7 +524,9 @@ phys_pmd_init(pmd_t *pmd_page, unsigned long paddr, unsigned long paddr_end,
>  			    !e820__mapped_any(paddr & PMD_MASK, paddr_next,
>  					     E820_TYPE_RAM) &&
>  			    !e820__mapped_any(paddr & PMD_MASK, paddr_next,
> -					     E820_TYPE_RESERVED_KERN))
> +					     E820_TYPE_RESERVED_KERN) &&
> +			    !e820__mapped_any(paddr & PMD_MASK, paddr_next,
> +					     E820_TYPE_ACPI))
>  				set_pmd_init(pmd, __pmd(0), init);
>  			continue;

Why do you single out phys_pmd_init()? I think it has to be addressed for
all page table levels as we do for E820_TYPE_RAM and E820_TYPE_RESERVED_KERN.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov