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

Brian Gerst
2023-02-23T12:11:50Z

On Thu, Feb 23, 2023 at 1:50 AM H. Peter Anvin <hpa@zytor.com> wrote:
>
> 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...

I remember some ancient versions of Xen stored the address of the
descriptor, instead of the contents. But looking at load_direct_gdt()
and load_fixmap_gdt() which both load a descriptor on the stack, I
don't think that is still a concern.

--
Brian Gerst