[Xen-devel] [PATCH for-4.8] x86/cpu: Reposition stack alignment check

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

As identified during review, using BUG_ON() before `lidt` will result in a
triple fault, even on APs.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wei.liu2@xxxxxxxxxx>
---
 xen/arch/x86/cpu/common.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 06ee401..16cf001 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -643,11 +643,6 @@ void load_system_tables(void)
                .limit = (IDT_ENTRIES * sizeof(idt_entry_t)) - 1,
        };
 
-       /* Bottom-of-stack must be 16-byte aligned! */
-       BUILD_BUG_ON((sizeof(struct cpu_info) -
-                     offsetof(struct cpu_info, guest_cpu_user_regs.es)) & 0xf);
-       BUG_ON(system_state != SYS_STATE_early_boot && (stack_bottom & 0xf));
-
        /* Main stack for interrupts/exceptions. */
        tss->rsp0 = stack_bottom;
        tss->bitmap = IOBMP_INVALID_OFFSET;
@@ -672,6 +667,15 @@ void load_system_tables(void)
        asm volatile ("lidt %0"  : : "m"  (idtr) );
        asm volatile ("ltr  %w0" : : "rm" (TSS_ENTRY << 3) );
        asm volatile ("lldt %w0" : : "rm" (0) );
+
+       /*
+         * Bottom-of-stack must be 16-byte aligned!
+         *
+         * Defer checks until exception support is sufficiently set up.
+         */
+       BUILD_BUG_ON((sizeof(struct cpu_info) -
+                     offsetof(struct cpu_info, guest_cpu_user_regs.es)) & 0xf);
+       BUG_ON(system_state != SYS_STATE_early_boot && (stack_bottom & 0xf));
 }
 
 /*
-- 
2.1.4


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