Re: [PATCH 4/5] KVM: Lazify fpu activation and deactivation

[Avi Kivity <avi@redhat.com>]

(actually copying Lucas).

On 01/14/2010 03:11 PM, Avi Kivity wrote:
> On 12/30/2009 06:25 PM, Avi Kivity wrote:
>> Defer fpu deactivation as much as possible - if the guest fpu is 
>> loaded, keep
>> it loaded until the next heavyweight exit (where we are forced to 
>> unload it).
>> This reduces unnecessary exits.
>>
>> We also defer fpu activation on clts; while clts signals the intent 
>> to use the
>> fpu, we can't be sure the guest will actually use it.
>>
>
> ...
>
>
>> @@ -4988,6 +4988,10 @@ void kvm_put_guest_fpu(struct kvm_vcpu *vcpu)
>>           return;
>>
>>       vcpu->guest_fpu_loaded = 0;
>> +    if (vcpu->fpu_active) {
>> +        vcpu->fpu_active = 0;
>> +        kvm_x86_ops->fpu_deactivate(vcpu);
>> +    }
>>       kvm_fx_save(&vcpu->arch.guest_fx_image);
>>       kvm_fx_restore(&vcpu->arch.host_fx_image);
>>       ++vcpu->stat.fpu_reload;
>
> This is broken badly; kvm_put_guest_fpu() can be called from preempt 
> notifier context, that is during normal execution of vcpu processing.  
> Code which modifies the same variables as ->fpu_deactivate() or that 
> depends on ->fpu_active will break.
>
> I fixed this by calling ->fpu_deactivate() from a synchronous context 
> using vcpu->requests, like we do everywhere else.
>
> Strangely, autotest only caught this on AMD and even it took a while.  
> Lucas, can you integrate something like the following into autotest, 
> so we exercise the preemption code harder?
>
> #!/usr/bin/python
>
> import sys, os, re, random, ctypes, time
>
> tasks = sys.argv[1:]
>
> threads = [int(t)
>            for k in tasks
>            for t in os.listdir('/proc/%s/task' % (k,))]
>
> cpus = [int(c[3:])
>         for c in os.listdir('/sys/devices/system/cpu')
>         if re.match(r'cpu[0-9]+', c)]
>
> rand = random.Random()
>
> sched_setaffinity = ctypes.CDLL('libc.so.6').sched_setaffinity
>
> while True:
>     pid = rand.choice(threads)
>     cpu = rand.choice(cpus)
>     mask = 1 << cpu
>     sched_setaffinity(ctypes.c_int(pid), ctypes.c_size_t(4), 
> ctypes.byref(ctypes.c_int(mask)))
>     try:
>         time.sleep(0.01)
>     except:
>         break
>
>


-- 
error compiling committee.c: too many arguments to function