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

Michael Kelley
2025-05-20T01:30:36Z

From: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> Sent: Saturday, May 3, 2025 12:15 PM
>
> From: Yunhong Jiang <yunhong.jiang@linux.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@linutronix.de>
> Signed-off-by: Yunhong Jiang <yunhong.jiang@linux.intel.com>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.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@outlook.com>