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SubjectRe: [tip:x86/asm] x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP
On Thu, Feb 26, 2015 at 5:12 AM, tip-bot for Wang Nan <tipbot@zytor.com> wrote:
> Commit-ID: b4d8327024637cb2a1f7910dcb5d0ad7a096f473
> Gitweb: http://git.kernel.org/tip/b4d8327024637cb2a1f7910dcb5d0ad7a096f473
> Author: Wang Nan <wangnan0@huawei.com>
> AuthorDate: Thu, 26 Feb 2015 13:49:39 +0800
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Thu, 26 Feb 2015 12:29:20 +0100
>
> x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP
>
> Before this patch early_trap_init() installs DEBUG_STACK for
> X86_TRAP_BP and X86_TRAP_DB. However, DEBUG_STACK doesn't work
> correctly until cpu_init() <-- trap_init().
>
> This patch passes 0 to set_intr_gate_ist() and
> set_system_intr_gate_ist() instead of DEBUG_STACK to let it use
> same stack as kernel, and installs DEBUG_STACK for them in
> trap_init().
>

Sorry, I'm late to the party. This patch, while technically correct
AFAICT, is really ugly, because the whole point is that it *doesn't*
use ist. In other words:

> + set_intr_gate_ist(X86_TRAP_DB, &debug, 0);

That should be set_intr_gate(X86_TRAP_DB, &debug);

> /* int3 can be called from all */
> - set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
> + set_system_intr_gate_ist(X86_TRAP_BP, &int3, 0);

Similarly, this should be set_system_gate.

> #ifdef CONFIG_X86_32
> set_intr_gate(X86_TRAP_PF, page_fault);
> #endif
> @@ -1005,6 +1013,15 @@ void __init trap_init(void)
> */
> cpu_init();
>
> + /*
> + * X86_TRAP_DB and X86_TRAP_BP have been set
> + * in early_trap_init(). However, DEBUG_STACK works only after
> + * cpu_init() loads TSS. See comments in early_trap_init().

It's not that DEBUG_STACK only works after the TSS is loaded; it's
that the IST mechanism only works after TSS is loaded.

--Andy


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