Re: [PATCH 1/2 RESEND v7] resource: add the new I/O resource descriptor 'IORES_DESC_RESERVED'

[Dave Hansen]

On 11/23/18 9:12 PM, Lianbo Jiang wrote:
> The upstream kernel can not accurately add the e820 reserved type to> kdump krenel e820 table.

	^ kernel

> Kdump uses walk_iomem_res_desc() to iterate io resources, then adds
> the matched resource ranges to the e820 table for kdump kernel. But,
> when convert the e820 type to the iores descriptor, several e820
> types are converted to 'IORES_DESC_NONE' in this function e820_type
> _to_iores_desc(). So the walk_iomem_res_desc() will get the redundant
> types(such as E820_TYPE_RAM/E820_TYPE_UNUSABLE/E820_TYPE_KERN) when
> walk through io resources with the descriptor 'IORES_DESC_NONE'.

Let me see if I understand.

When doing kexec, the old kernel makes a new e820 table for the new
kernel.  The old kernel constructs the new e820 table from
'iomem_resource'.  But, when creating the 'iomem_resource' tree,
reserved areas like E820_TYPE_RESERVED are not properly passed through.
 This causes problems like described in the next patch.

> This patch adds the new I/O resource descriptor 'IORES_DESC_RESERVED'
> for the iomem resources search interfaces. It is helpful to exactly
> match the reserved resource ranges when walking through iomem resources.

It's more than that, though.  You're specifically storing the reserved
area(s) when we see them come through the firmware.

> Furthermore, in order to make it still work after the new descriptor
> is added, these codes originally related to 'IORES_DESC_NONE' have
> been updated.

It would be nice to explain why they needed to be updated and what
breaks if they are not.

> diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
> index 5378d10f1d31..91b6112e7489 100644
> --- a/arch/x86/mm/ioremap.c
> +++ b/arch/x86/mm/ioremap.c
> @@ -83,7 +83,14 @@ static bool __ioremap_check_ram(struct resource *res)
>  
>  static int __ioremap_check_desc_other(struct resource *res)
>  {
> -	return (res->desc != IORES_DESC_NONE);
> +	/*
> +	 * The E820_TYPE_RESERVED was converted to the IORES_DESC_NONE
> +	 * before the new IORES_DESC_RESERVED is added, so it contained
> +	 * the e820 reserved type. In order to make it still work for
> +	 * SEV, here keep it the same as before.
> +	 */
> +	return ((res->desc != IORES_DESC_NONE) ||
> +		(res->desc != IORES_DESC_RESERVED));
>  }

After reading the changelog and the comment, I still have no idea why
this hunk is here.  Could you try to explain a bit more?