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

[Gleb Natapov]

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@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx>
> >>>>>>
> >>>>>>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@xxxxxxxxxx>
> >>>>>
> >>>>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.
> 
Fair enough. But can you at least hint what is this mysterious guest?

--
			Gleb.
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