On Wed, Nov 05, 2008 at 11:41:04AM +0100, Alexander Graf wrote:
> X86 CPUs need to have some magic happening to enable the virtualization
> extensions on them. This magic can result in unpleasant results for
> users, like blocking other VMMs from working (vmx) or using invalid TLB
> entries (svm).
>
> Currently KVM activates virtualization when the respective kernel module
> is loaded. This blocks us from autoloading KVM modules without breaking
> other VMMs.
>
> To circumvent this problem at least a bit, this patch introduces on
> demand activation of virtualization. This means, that instead
> virtualization is enabled on creation of the first virtual machine
> and disabled on removal of the last one.
>
> So using this, KVM can be easily autoloaded, while keeping other
> hypervisors usable.
>
> v2 adds returns to non-x86 hardware_enables and adds IA64 change
> v3 changes:
> - use spin_lock instead of atomics
> - put locking to new functions hardware_{en,dis}able_all that get called
> on VM creation/destruction
> - remove usage counter checks where not necessary
> - return -EINVAL for IA64 slot < 0 case
>
> Signed-off-by: Alexander Graf <[EMAIL PROTECTED]>
> ---
<snip>
>
> -static void hardware_enable(void *junk)
> +static void hardware_enable(void *_r)
> {
> int cpu = raw_smp_processor_id();
> + int r;
> +
> + /* If enabling a previous CPU failed already, let's not continue */
> + if (_r && *((int*)_r))
> + return;
>
> if (cpu_isset(cpu, cpus_hardware_enabled))
> return;
> + r = kvm_arch_hardware_enable(NULL);
> + if (_r)
> + *((int*)_r) = r;
> + if (r) {
> + printk(KERN_INFO "kvm: enabling virtualization on "
> + "CPU%d failed\n", cpu);
> + return;
> + }
> +
> cpu_set(cpu, cpus_hardware_enabled);
> - kvm_arch_hardware_enable(NULL);
> +}
Doesn't on_each_cpu() run the function in parallel on all CPUs? If so,
there is a race between checking *_r and setting *_r.
> +
> +static int hardware_enable_all(void)
> +{
> + int r = 0;
> +
> + spin_lock(&kvm_usage_lock);
> + kvm_usage_count++;
> + if (kvm_usage_count == 1)
> + on_each_cpu(hardware_enable, &r, 1);
> + spin_unlock(&kvm_usage_lock);
> +
> + return r;
> }
>
<snip>
--
Eduardo
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