Re: [PATCH 4/6] x86/smpbppt: Remove early_gdt_descr on 64-bit

H. Peter Anvin
2023-02-23T06:50:35Z

On February 22, 2023 2:12:59 PM PST, Brian Gerst <brgerst@gmail.com> wrote:
>Build the GDT descriptor on the stack instead.
>
>Signed-off-by: Brian Gerst <brgerst@gmail.com>
>---
> arch/x86/kernel/acpi/sleep.c | 2 --
> arch/x86/kernel/head_64.S | 19 +++++++------------
> 2 files changed, 7 insertions(+), 14 deletions(-)
>
>diff --git a/arch/x86/kernel/acpi/sleep.c b/arch/x86/kernel/acpi/sleep.c
>index 008fda8b1982..6538ddb55f28 100644
>--- a/arch/x86/kernel/acpi/sleep.c
>+++ b/arch/x86/kernel/acpi/sleep.c
>@@ -114,8 +114,6 @@ int x86_acpi_suspend_lowlevel(void)
> #else /* CONFIG_64BIT */
> #ifdef CONFIG_SMP
> current->thread.sp = (unsigned long)temp_stack + sizeof(temp_stack);
>- early_gdt_descr.address =
>- (unsigned long)get_cpu_gdt_rw(smp_processor_id());
> initial_gs = per_cpu_offset(smp_processor_id());
> /* Force the startup into boot mode */
> saved_smpboot_ctrl = xchg(&smpboot_control, 0);
>diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
>index f7905ba4b992..0dd57d573a0e 100644
>--- a/arch/x86/kernel/head_64.S
>+++ b/arch/x86/kernel/head_64.S
>@@ -245,7 +245,7 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> #ifdef CONFIG_SMP
> /*
> * Is this the boot CPU coming up? If so everything is available
>- * in initial_gs and early_gdt_descr.
>+ * in initial_gs.
> */
> movl smpboot_control(%rip), %edx
> testl $STARTUP_SECONDARY, %edx
>@@ -313,11 +313,6 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> /* Save it for GS BASE setup */
> movq %rbx, initial_gs(%rip)
>
>- /* Calculate the GDT address */
>- movq $gdt_page, %rcx
>- addq %rbx, %rcx
>- movq %rcx, early_gdt_descr_base(%rip)
>-
> movq %rbx, %rdx
> #else
> xorl %edx, %edx
>@@ -339,7 +334,12 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
> * addresses where we're currently running on. We have to do that here
> * because in 32bit we couldn't load a 64bit linear address.
> */
>- lgdt early_gdt_descr(%rip)
>+ subq $16, %rsp
>+ movw $(GDT_SIZE-1), (%rsp)
>+ leaq gdt_page(%rdx), %rax
>+ movq %rax, 2(%rsp)
>+ lgdt (%rsp)
>+ addq $16, %rsp
>
> /* set up data segments */
> xorl %eax,%eax
>@@ -754,11 +754,6 @@ SYM_DATA_END(level1_fixmap_pgt)
>
> .data
> .align 16
>-
>-SYM_DATA(early_gdt_descr, .word GDT_ENTRIES*8-1)
>-SYM_DATA_LOCAL(early_gdt_descr_base, .quad INIT_PER_CPU_VAR(gdt_page))
>-
>- .align 16
> SYM_DATA(smpboot_control, .long 0)
>
> .align 16

This is known to break at least some hypervisors, probably old by now, but...