linux-kernel - RE: [PATCH v3 09/13] x86/realmode: Make the location of the trampoline configurable

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Message-ID:
 <SN6PR02MB4157B62D4DAB55DBFDB72315D49FA@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Tue, 20 May 2025 01:30:33 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
	<conor+dt@...nel.org>, Rob Herring <robh@...nel.org>, "K. Y. Srinivasan"
	<kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, Wei Liu
	<wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
CC: "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>, Saurabh Sengar
	<ssengar@...ux.microsoft.com>, Chris Oo <cho@...rosoft.com>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>, "Kirill A.
 Shutemov" <kirill.shutemov@...ux.intel.com>, "linux-acpi@...r.kernel.org"
	<linux-acpi@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
	Ricardo Neri <ricardo.neri@...el.com>
Subject: RE: [PATCH v3 09/13] x86/realmode: Make the location of the
 trampoline configurable

From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> Sent: Saturday, May 3, 2025 12:15 PM
> 
> From: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
> 
> x86 CPUs boot in real mode. This mode uses 20-bit memory addresses (16-bit
> registers plus 4-bit segment selectors). This implies that the trampoline
> must reside under the 1MB memory boundary.
> 
> There are platforms in which the firmware boots the secondary CPUs,
> switches them to long mode and transfers control to the kernel. An example
> of such mechanism is the ACPI Multiprocessor Wakeup Structure.
> 
> In this scenario there is no restriction to locate the trampoline under 1MB
> memory. Moreover, certain platforms (for example, Hyper-V VTL guests) may
> not have memory available for allocation under 1MB.
> 
> Add a new member to struct x86_init_resources to specify the upper bound
> for the location of the trampoline memory. Keep the default upper bound of
> 1MB to conserve the current behavior.
> 
> Originally-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Yunhong Jiang <yunhong.jiang@...ux.intel.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> ---
> Changes since v2:
>  - Edited the commit message for clarity.
>  - Minor tweaks to comments.
>  - Removed the option to not reserve the first 1MB of memory as it is
>    not needed.
> 
> Changes since v1:
>  - Added this patch using code that Thomas suggested:
>    https://lore.kernel.org/lkml/87a5ho2q6x.ffs@tglx/
> ---
>  arch/x86/include/asm/x86_init.h | 3 +++
>  arch/x86/kernel/x86_init.c      | 3 +++
>  arch/x86/realmode/init.c        | 7 +++----
>  3 files changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/x86_init.h b/arch/x86/include/asm/x86_init.h
> index 36698cc9fb44..e770ce507a87 100644
> --- a/arch/x86/include/asm/x86_init.h
> +++ b/arch/x86/include/asm/x86_init.h
> @@ -31,12 +31,15 @@ struct x86_init_mpparse {
>   *				platform
>   * @memory_setup:		platform specific memory setup
>   * @dmi_setup:			platform specific DMI setup
> + * @realmode_limit:		platform specific address limit for the real mode trampoline
> + *				(default 1M)
>   */
>  struct x86_init_resources {
>  	void (*probe_roms)(void);
>  	void (*reserve_resources)(void);
>  	char *(*memory_setup)(void);
>  	void (*dmi_setup)(void);
> +	unsigned long realmode_limit;
>  };
> 
>  /**
> diff --git a/arch/x86/kernel/x86_init.c b/arch/x86/kernel/x86_init.c
> index 0a2bbd674a6d..a25fd7282811 100644
> --- a/arch/x86/kernel/x86_init.c
> +++ b/arch/x86/kernel/x86_init.c
> @@ -9,6 +9,7 @@
>  #include <linux/export.h>
>  #include <linux/pci.h>
>  #include <linux/acpi.h>
> +#include <linux/sizes.h>
> 
>  #include <asm/acpi.h>
>  #include <asm/bios_ebda.h>
> @@ -69,6 +70,8 @@ struct x86_init_ops x86_init __initdata = {
>  		.reserve_resources	= reserve_standard_io_resources,
>  		.memory_setup		= e820__memory_setup_default,
>  		.dmi_setup		= dmi_setup,
> +		/* Has to be under 1M so we can execute real-mode AP code. */
> +		.realmode_limit		= SZ_1M,
>  	},
> 
>  	.mpparse = {
> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
> index ed5c63c0b4e5..01155f995b2b 100644
> --- a/arch/x86/realmode/init.c
> +++ b/arch/x86/realmode/init.c
> @@ -46,7 +46,7 @@ void load_trampoline_pgtable(void)
> 
>  void __init reserve_real_mode(void)
>  {
> -	phys_addr_t mem;
> +	phys_addr_t mem, limit = x86_init.resources.realmode_limit;
>  	size_t size = real_mode_size_needed();
> 
>  	if (!size)
> @@ -54,10 +54,9 @@ void __init reserve_real_mode(void)
> 
>  	WARN_ON(slab_is_available());
> 
> -	/* Has to be under 1M so we can execute real-mode AP code. */
> -	mem = memblock_phys_alloc_range(size, PAGE_SIZE, 0, 1<<20);
> +	mem = memblock_phys_alloc_range(size, PAGE_SIZE, 0, limit);
>  	if (!mem)
> -		pr_info("No sub-1M memory is available for the trampoline\n");
> +		pr_info("No memory below %pa for the real-mode trampoline\n", &limit);
>  	else
>  		set_real_mode_mem(mem);
> 
> --
> 2.43.0

Reviewed-by: Michael Kelley <mhklinux@...look.com>