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

[Michael S. Tsirkin]

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.
> 
> Nadav

It's a quality of implementation question.
It is correct in the same sense that
a NIC dropping each second packet is correct.

If we ship this hack we have to maintain it forever,
so there needs to be a compelling reason beyond
just "because we can".


-- 
MST
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