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x86 ptrace: avoid unnecessary wrmsr

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This avoids using wrmsr on MSR_IA32_DEBUGCTLMSR when it's not needed.
No wrmsr ever needs to be done if noone has ever used block stepping.

Without this change, using ptrace on 2.6.25 on an x86 KVM guest
will tickle KVM's missing support for the MSR and crash the guest
kernel.  Though host KVM is the buggy one, this makes for a regression
in the guest behavior from 2.6.24->2.6.25 that we can easily avoid.

I also corrected some bad whitespace.

Signed-off-by: Roland McGrath <roland@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Roland McGrath 2008-04-03 14:18:55 -07:00 committed by Linus Torvalds
parent 2eccd6f65a
commit 4ba51fd75c
1 changed files with 7 additions and 4 deletions
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11
arch/x86/kernel/step.c
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@ -140,6 +140,9 @@ static int enable_single_step(struct task_struct *child)
*/ */
static void write_debugctlmsr(struct task_struct *child, unsigned long val) static void write_debugctlmsr(struct task_struct *child, unsigned long val)
{ {
if (child->thread.debugctlmsr == val)
return;
child->thread.debugctlmsr = val; child->thread.debugctlmsr = val;
if (child != current) if (child != current)
@ -165,11 +168,11 @@ static void enable_step(struct task_struct *child, bool block)
write_debugctlmsr(child, write_debugctlmsr(child,
child->thread.debugctlmsr | DEBUGCTLMSR_BTF); child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
} else { } else {
write_debugctlmsr(child, write_debugctlmsr(child,
child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF); child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF);
if (!child->thread.debugctlmsr) if (!child->thread.debugctlmsr)
clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR); clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
} }
} }