Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

Nadav Amit
2014-06-19T12:28:41Z

On 6/19/14, 3:17 PM, Michael S. Tsirkin wrote:
> On Thu, Jun 19, 2014 at 03:10:21PM +0300, Nadav Amit wrote:
>> On 6/19/14, 3:07 PM, Gleb Natapov wrote:
>>> On Thu, Jun 19, 2014 at 02:52:20PM +0300, Nadav Amit wrote:
>>>> On 6/19/14, 2:23 PM, Gleb Natapov wrote:
>>>>> On Thu, Jun 19, 2014 at 01:53:36PM +0300, Nadav Amit wrote:
>>>>>>
>>>>>> On Jun 19, 2014, at 1:18 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
>>>>>>
>>>>>>> On Wed, Jun 18, 2014 at 02:46:01PM -0400, Gabriel L. Somlo wrote:
>>>>>>>> On Wed, Jun 18, 2014 at 10:59:14AM -0700, Eric Northup wrote:
>>>>>>>>> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@cs.technion.ac.il> wrote:
>>>>>>>>>> mwait and monitor are currently handled as nop. Considering this behavior, they
>>>>>>>>>> should still be handled correctly, i.e., check execution conditions and generate
>>>>>>>>>> exceptions when required. mwait and monitor may also be executed in real-mode
>>>>>>>>>> and are not handled in that case. This patch performs the emulation of
>>>>>>>>>> monitor-mwait according to Intel SDM (other than checking whether interrupt can
>>>>>>>>>> be used as a break event).
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Nadav Amit <namit@cs.technion.ac.il>
>>>>>>>>
>>>>>>>> How about this instead (details in the commit log below) ? Please let
>>>>>>>> me know what you think, and if you'd prefer me to send it out as a
>>>>>>>> separate patch rather than a reply to this thread.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> --Gabriel
>>>>>>>
>>>>>>> If there's an easy workaround, I'm inclined to agree.
>>>>>>> We can always go back to Gabriel's patch (and then we'll need
>>>>>>> Nadav's one too) but if we release a kernel with this
>>>>>>> support it becomes an ABI and we can't go back.
>>>>>>>
>>>>>>> So let's be careful here, and revert the hack for 3.16.
>>>>>>>
>>>>>>>
>>>>>>> Acked-by: Michael S. Tsirkin <mst@redhat.com>
>>>>>>>
>>>>>> Personally, I got a custom guest which requires mwait for executing correctly.
>>>>> Can you elaborate on this guest a little bit. With nop implementation
>>>>> for mwait the guest will hog a host cpu. Do you consider this to be
>>>>> "executing correctly?"
>>>>>
>>>>> --
>>>>
>>>> mwait is not as "clean" as it may appear. It encounters false wake-ups due
>>>> to a variety of reasons, and any code need to recheck the wake-up condition
>>>> afterwards. Actually, some CPUs had bugs that caused excessive wake-ups that
>>>> degraded performance considerably (Nehalem, if I am not mistaken).
>>>> Therefore, handling mwait as nop is logically correct (although it may
>>>> degrade performance).
>>>>
>>>> For the reference, if you look at the SDM 8.10.4, you'll see:
>>>> "Multiple events other than a write to the triggering address range can
>>>> cause a processor that executed MWAIT to wake up. These include events that
>>>> would lead to voluntary or involuntary context switches, such as..."
>>>>
>>>> Note the words "include" in the sentence "These include events". Software
>>>> has no way of controlling whether it gets false wake-ups and cannot rely on
>>>> the wake-up as indication to anything.
>>>>
>>> That's all well and good and I didn't say that nop is not a valid
>>> mwait implementation, it is, though there is a big difference between
>>> "encounters false wake-ups" and never sleeps. What I asked is do you
>>> consider your guest hogging host cpu to be "executing correctly?". What
>>> this guest is doing that such behaviour is tolerated and shouldn't it
>>> be better to just poll for a condition you are waiting for instead of
>>> executing expensive vmexits. This will also hog 100% host cpu, but will
>>> be actually faster.
>>>
>> You are correct, but unfortunately I have no control over the guest
>> workload. In this specific workload I do not care about performance but only
>> about correctness.
>>
>> Nadav
>
> No one prevents you from patching your kernel to run this workload. But
> is this of use to anyone else? If yes why?
>
I do not say it should be the default behavior, and I can try to push to
qemu some setting to turn it on by demand.

Anyhow, I believe there are cases you may want mwait support - either an
OS X guest which was not modified to run without mwait, or for debugging
the monitor-mwait flow of a guest OS.

I am not going to argue too much. Since I was under the impression there
are needs for mwait, other than mine, I thought it would make all of our
lives easier to have a better implementation.

Nadav