Re: [PATCH v11 5/5] kdump: update Documentation about crashkernel

[Catalin Marinas <catalin.marinas@arm.com>]

On Sat, Aug 01, 2020 at 09:08:56PM +0800, Chen Zhou wrote:
> diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
> index 2da65fef2a1c..4b58f97351d5 100644
> --- a/Documentation/admin-guide/kdump/kdump.rst
> +++ b/Documentation/admin-guide/kdump/kdump.rst
> @@ -299,7 +299,15 @@ Boot into System Kernel
>     "crashkernel=64M@16M" tells the system kernel to reserve 64 MB of memory
>     starting at physical address 0x01000000 (16MB) for the dump-capture kernel.
>  
> -   On x86 and x86_64, use "crashkernel=64M@16M".
> +   On x86 use "crashkernel=64M@16M".
> +
> +   On x86_64, use "crashkernel=X" to select a region under 4G first, and
> +   fall back to reserve region above 4G.
> +   We can also use "crashkernel=X,high" to select a region above 4G, which
> +   also tries to allocate at least 256M below 4G automatically and
> +   "crashkernel=Y,low" can be used to allocate specified size low memory.
> +   Use "crashkernel=Y@X" if you really have to reserve memory from specified
> +   start address X.
>  
>     On ppc64, use "crashkernel=128M@32M".
>  
> @@ -316,8 +324,15 @@ Boot into System Kernel
>     kernel will automatically locate the crash kernel image within the
>     first 512MB of RAM if X is not given.
>  
> -   On arm64, use "crashkernel=Y[@X]".  Note that the start address of
> -   the kernel, X if explicitly specified, must be aligned to 2MiB (0x200000).
> +   On arm64, use "crashkernel=X" to try low allocation in ZONE_DMA, and
> +   fall back to high allocation if it fails. And go for high allocation
> +   directly if the required size is too large. If crash_base is outside

I wouldn't mention crash_base in the admin guide. That's an
implementation detail really and admins are not supposed to read the
source code to make sense of the documentation. ZONE_DMA is also a
kernel internal, so you'd need to define what it is for arm64. At least
the DMA and DMA32 zones are printed during kernel boot.

> +   ZONE_DMA, try to allocate at least 256M in ZONE_DMA automatically.
> +   "crashkernel=Y,low" can be used to allocate specified size low memory.
> +   For non-RPi4 platforms, change ZONE_DMA memtioned above to ZONE_DMA32.
> +   Use "crashkernel=Y@X" if you really have to reserve memory from
> +   specified start address X. Note that the start address of the kernel,
> +   X if explicitly specified, must be aligned to 2MiB (0x200000).

-- 
Catalin