linux-kernel - Re: [patch V2 35/44] x86/idt: Switch early trap init to IDT tables

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Date:   Fri, 25 Aug 2017 22:24:27 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [patch V2 35/44] x86/idt: Switch early trap init to IDT tables

On 08/25/2017 05:47 PM, Thomas Gleixner wrote:
>  
> -/* Set of traps needed for early debugging. */
> -void __init early_trap_init(void)
> -{
> -	/*
> -	 * Don't use IST to set DEBUG_STACK as it doesn't work until TSS
> -	 * is ready in cpu_init() <-- trap_init(). Before trap_init(),
> -	 * CPU runs at ring 0 so it is impossible to hit an invalid
> -	 * stack.  Using the original stack works well enough at this
> -	 * early stage. DEBUG_STACK will be equipped after cpu_init() in
> -	 * trap_init().
> -	 */
> -	set_intr_gate(X86_TRAP_DB, debug);
> -	/* int3 can be called from all */
> -	set_system_intr_gate(X86_TRAP_BP, &int3);
> -#ifdef CONFIG_X86_32
> -	set_intr_gate(X86_TRAP_PF, page_fault);
> -#endif
> -	load_idt(&idt_descr);

This breaks 64-bit Xen PV guests: idt_setup_early_handler() is called
from x86_64_start_kernel(), which is not executed on those guests.

The patch below fixes it.

-boris


diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 03fb07d..ed0fb8a 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1374,6 +1374,7 @@ asmlinkage __visible void __init
xen_start_kernel(void)
        i386_start_kernel();
 #else
        cr4_init_shadow(); /* 32b kernel does this in i386_start_kernel() */
+       load_idt(&idt_descr);
        x86_64_start_reservations((char *)__pa_symbol(&boot_params));
 #endif
 }