[tip: x86/asm] x86/asm: Add DB flag to 32-bit percpu GDT entry

[tip-bot2 for Vegard Nossum]

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     bc90aefa99f74452d549d503a3f1cbf3adc9c6bb
Gitweb:        https://git.kernel.org/tip/bc90aefa99f74452d549d503a3f1cbf3adc9c6bb
Author:        Vegard Nossum <vegard.nossum@xxxxxxxxxx>
AuthorDate:    Tue, 19 Dec 2023 16:12:00 +01:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Wed, 20 Dec 2023 10:57:51 +01:00

x86/asm: Add DB flag to 32-bit percpu GDT entry

The D/B size flag for the 32-bit percpu GDT entry was not set.

The Intel manual (vol 3, section 3.4.5) only specifies the meaning of
this flag for three cases:

 1) code segments used for %cs -- doesn't apply here

 2) stack segments used for %ss -- doesn't apply

 3) expand-down data segments -- but we don't have the expand-down flag
    set, so it also doesn't apply here

The flag likely doesn't do anything here, although the manual does also
say: "This flag should always be set to 1 for 32-bit code and data
segments [...]" so we should probably do it anyway.

Signed-off-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20231219151200.2878271-6-vegard.nossum@xxxxxxxxxx
---
 arch/x86/kernel/setup_percpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/setup_percpu.c b/arch/x86/kernel/setup_percpu.c
index f2583de..b30d6e1 100644
--- a/arch/x86/kernel/setup_percpu.c
+++ b/arch/x86/kernel/setup_percpu.c
@@ -106,7 +106,7 @@ void __init pcpu_populate_pte(unsigned long addr)
 static inline void setup_percpu_segment(int cpu)
 {
 #ifdef CONFIG_X86_32
-	struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32 & ~_DESC_DB,
+	struct desc_struct d = GDT_ENTRY_INIT(DESC_DATA32,
 					      per_cpu_offset(cpu), 0xFFFFF);
 
 	write_gdt_entry(get_cpu_gdt_rw(cpu), GDT_ENTRY_PERCPU, &d, DESCTYPE_S);