From: Roland McGrath Date: Thu, 3 Apr 2008 21:18:55 +0000 (-0700) Subject: x86 ptrace: avoid unnecessary wrmsr X-Git-Tag: v2.6.25-rc9~74 X-Git-Url: http://www.pilppa.org/gitweb/gitweb.cgi?a=commitdiff_plain;h=4ba51fd75cc3789be83f0d6f878dabbb0cb19bca;p=linux-2.6-omap-h63xx.git x86 ptrace: avoid unnecessary wrmsr 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 Cc: Ingo Molnar Cc: Thomas Gleixner Signed-off-by: Linus Torvalds --- diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c index 9d406cdc847..071ff479823 100644 --- a/arch/x86/kernel/step.c +++ b/arch/x86/kernel/step.c @@ -140,6 +140,9 @@ static int enable_single_step(struct task_struct *child) */ static void write_debugctlmsr(struct task_struct *child, unsigned long val) { + if (child->thread.debugctlmsr == val) + return; + child->thread.debugctlmsr = val; if (child != current) @@ -165,11 +168,11 @@ static void enable_step(struct task_struct *child, bool block) write_debugctlmsr(child, child->thread.debugctlmsr | DEBUGCTLMSR_BTF); } else { - write_debugctlmsr(child, - child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF); + write_debugctlmsr(child, + child->thread.debugctlmsr & ~DEBUGCTLMSR_BTF); - if (!child->thread.debugctlmsr) - clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR); + if (!child->thread.debugctlmsr) + clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR); } }