On 11/29/2012 08:04 AM, Marcelo Tosatti wrote: > On Wed, Nov 28, 2012 at 08:54:14PM +0800, Xiao Guangrong wrote: >> vmcs->cpu indicates whether it exists on the target cpu, -1 means the vmcs >> does not exist on any vcpu >> >> If vcpu load vmcs with vmcs.cpu = -1, it can be directly added to cpu's percpu >> list. The list can be corrupted if the cpu prefetch the vmcs's list before >> reading vmcs->cpu. Meanwhile, we should remove vmcs from the list before >> making vmcs->vcpu == -1 be visible >> >> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> >> --- >> arch/x86/kvm/vmx.c | 17 +++++++++++++++++ >> 1 files changed, 17 insertions(+), 0 deletions(-) >> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >> index 29e8f42..6056d88 100644 >> --- a/arch/x86/kvm/vmx.c >> +++ b/arch/x86/kvm/vmx.c >> @@ -1002,6 +1002,15 @@ static void __loaded_vmcs_clear(void *arg) >> if (per_cpu(current_vmcs, cpu) == loaded_vmcs->vmcs) >> per_cpu(current_vmcs, cpu) = NULL; >> list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link); >> + >> + /* >> + * we should ensure updating loaded_vmcs->loaded_vmcss_on_cpu_link >> + * is before setting loaded_vmcs->vcpu to -1 which is done in >> + * loaded_vmcs_init. Otherwise, other cpu can see vcpu = -1 fist >> + * then adds the vmcs into percpu list before it is deleted. >> + */ >> + smp_wmb(); >> + > > Neither loads nor stores are reordered with like operations (see > section 8.2.3.2 of intel's volume 3). This behaviour makes the barrier > not necessary. Ouch, yes, you are right. My memory is wrong. It seems only later-read can be reordered with early-write. But if 'read vs read' and 'write vs write' can be guaranteed by CPU, smp_wmb() and smp_rmb() should only be a complier barrier, so i think we can add the barriers to improve the readable and the portable. And anyway, the current code missed complier-barrier. > > However, i agree access to loaded_vmcs is not obviously safe. I can't > tell its safe with vmm_exclusive = 0 (where vcpu->cpu can change at any > time). If vmm_exclusive = 0, the vmcs can removed from percpu list when vcpu is scheduled out. The list is not broken. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
