linux-kernel - Re: [PATCH v4 1/9] dax/hmem, e820, resource: Defer Soft Reserved insertion until hmem is ready

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Message-ID: <692f65ecb5603_261c110090@dwillia2-mobl4.notmuch>
Date: Tue, 2 Dec 2025 14:19:24 -0800
From: <dan.j.williams@...el.com>
To: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>,
	<linux-cxl@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<nvdimm@...ts.linux.dev>, <linux-fsdevel@...r.kernel.org>,
	<linux-pm@...r.kernel.org>
CC: Alison Schofield <alison.schofield@...el.com>, Vishal Verma
	<vishal.l.verma@...el.com>, Ira Weiny <ira.weiny@...el.com>, Dan Williams
	<dan.j.williams@...el.com>, Jonathan Cameron <jonathan.cameron@...wei.com>,
	Yazen Ghannam <yazen.ghannam@....com>, Dave Jiang <dave.jiang@...el.com>,
	Davidlohr Bueso <dave@...olabs.net>, Matthew Wilcox <willy@...radead.org>,
	Jan Kara <jack@...e.cz>, "Rafael J . Wysocki" <rafael@...nel.org>, Len Brown
	<len.brown@...el.com>, Pavel Machek <pavel@...nel.org>, Li Ming
	<ming.li@...omail.com>, Jeff Johnson <jeff.johnson@....qualcomm.com>, "Ying
 Huang" <huang.ying.caritas@...il.com>, Yao Xingtao <yaoxt.fnst@...itsu.com>,
	Peter Zijlstra <peterz@...radead.org>, Greg KH <gregkh@...uxfoundation.org>,
	Nathan Fontenot <nathan.fontenot@....com>, Terry Bowman
	<terry.bowman@....com>, Robert Richter <rrichter@....com>, Benjamin Cheatham
	<benjamin.cheatham@....com>, Zhijian Li <lizhijian@...itsu.com>, "Borislav
 Petkov" <bp@...en8.de>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v4 1/9] dax/hmem, e820, resource: Defer Soft Reserved
 insertion until hmem is ready

Smita Koralahalli wrote:
> From: Dan Williams <dan.j.williams@...el.com>
> 
> Insert Soft Reserved memory into a dedicated soft_reserve_resource tree
> instead of the iomem_resource tree at boot. Delay publishing these ranges
> into the iomem hierarchy until ownership is resolved and the HMEM path
> is ready to consume them.
> 
> Publishing Soft Reserved ranges into iomem too early conflicts with CXL
> hotplug and prevents region assembly when those ranges overlap CXL
> windows.
> 
> Follow up patches will reinsert Soft Reserved ranges into iomem after CXL
> window publication is complete and HMEM is ready to claim the memory. This
> provides a cleaner handoff between EFI-defined memory ranges and CXL
> resource management without trimming or deleting resources later.

Please, when you modify a patch from an original, add your
Co-developed-by: and clarify what you changed.

> 
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> Signed-off-by: Smita Koralahalli <Smita.KoralahalliChannabasappa@....com>
> ---
>  arch/x86/kernel/e820.c    |  2 +-
>  drivers/cxl/acpi.c        |  2 +-
>  drivers/dax/hmem/device.c |  4 +-
>  drivers/dax/hmem/hmem.c   |  7 ++-
>  include/linux/ioport.h    | 13 +++++-
>  kernel/resource.c         | 92 +++++++++++++++++++++++++++++++++------
>  6 files changed, 100 insertions(+), 20 deletions(-)
> 
[..]
> @@ -426,6 +443,26 @@ int walk_iomem_res_desc(unsigned long desc, unsigned long flags, u64 start,
>  }
>  EXPORT_SYMBOL_GPL(walk_iomem_res_desc);
>  
> +#ifdef CONFIG_EFI_SOFT_RESERVE
> +struct resource soft_reserve_resource = {
> +	.name	= "Soft Reserved",
> +	.start	= 0,
> +	.end	= -1,
> +	.desc	= IORES_DESC_SOFT_RESERVED,
> +	.flags	= IORESOURCE_MEM,
> +};
> +EXPORT_SYMBOL_GPL(soft_reserve_resource);

It looks like one of the things you changed from my RFC was the addition
of walk_soft_reserve_res_desc() and region_intersects_soft_reserve().
With those APIs not only does this symbol not need to be exported, but
it also can be static / private to resource.c.

> +
> +int walk_soft_reserve_res_desc(unsigned long desc, unsigned long flags,
> +			       u64 start, u64 end, void *arg,
> +			       int (*func)(struct resource *, void *))
> +{
> +	return walk_res_desc(&soft_reserve_resource, start, end, flags, desc,
> +			     arg, func);
> +}
> +EXPORT_SYMBOL_GPL(walk_soft_reserve_res_desc);
> +#endif
> +
>  /*
>   * This function calls the @func callback against all memory ranges of type
>   * System RAM which are marked as IORESOURCE_SYSTEM_RAM and IORESOUCE_BUSY.
> @@ -648,6 +685,22 @@ int region_intersects(resource_size_t start, size_t size, unsigned long flags,
>  }
>  EXPORT_SYMBOL_GPL(region_intersects);
>  
> +#ifdef CONFIG_EFI_SOFT_RESERVE
> +int region_intersects_soft_reserve(resource_size_t start, size_t size,
> +				   unsigned long flags, unsigned long desc)
> +{
> +	int ret;
> +
> +	read_lock(&resource_lock);
> +	ret = __region_intersects(&soft_reserve_resource, start, size, flags,
> +				  desc);
> +	read_unlock(&resource_lock);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(region_intersects_soft_reserve);
> +#endif
> +
>  void __weak arch_remove_reservations(struct resource *avail)
>  {
>  }
> @@ -966,7 +1019,7 @@ EXPORT_SYMBOL_GPL(insert_resource);
>   * Insert a resource into the resource tree, possibly expanding it in order
>   * to make it encompass any conflicting resources.
>   */
> -void insert_resource_expand_to_fit(struct resource *root, struct resource *new)
> +void __insert_resource_expand_to_fit(struct resource *root, struct resource *new)
>  {
>  	if (new->parent)
>  		return;
> @@ -997,7 +1050,20 @@ void insert_resource_expand_to_fit(struct resource *root, struct resource *new)
>   * to use this interface. The former are built-in and only the latter,
>   * CXL, is a module.
>   */
> -EXPORT_SYMBOL_NS_GPL(insert_resource_expand_to_fit, "CXL");
> +EXPORT_SYMBOL_NS_GPL(__insert_resource_expand_to_fit, "CXL");
> +
> +void insert_resource_expand_to_fit(struct resource *new)
> +{
> +	struct resource *root = &iomem_resource;
> +
> +#ifdef CONFIG_EFI_SOFT_RESERVE
> +	if (new->desc == IORES_DESC_SOFT_RESERVED)
> +		root = &soft_reserve_resource;
> +#endif

I can not say I am entirely happy with this change, I would prefer to
avoid ifdef in C, and I would prefer not to break the legacy semantics
of this function, but it meets the spirit of the original RFC without
introducing a new insert_resource_late(). I assume review feedback
requested this?

> +	__insert_resource_expand_to_fit(root, new);
> +}
> +EXPORT_SYMBOL_GPL(insert_resource_expand_to_fit);

There are no consumers for this export, so it can be dropped.