From: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> Sent: Saturday, May 3, 2025 12:15 PM > > From: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> > > 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@xxxxxxxxxxxxx> > Signed-off-by: Yunhong Jiang <yunhong.jiang@xxxxxxxxxxxxxxx> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> > --- > 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@xxxxxxxxxxx>
