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Re: [Qemu-devel] [PATCH v2] qmp: add query-cpus-fast

From: Eduardo Habkost
Subject: Re: [Qemu-devel] [PATCH v2] qmp: add query-cpus-fast
Date: Fri, 9 Feb 2018 12:27:35 -0200
User-agent: Mutt/1.9.1 (2017-09-22)

On Fri, Feb 09, 2018 at 08:49:49AM -0500, Luiz Capitulino wrote:
> On Fri, 9 Feb 2018 08:56:19 +0100
> Viktor Mihajlovski <address@hidden> wrote:
> > On 08.02.2018 21:33, Eduardo Habkost wrote:
> > > On Thu, Feb 08, 2018 at 11:17:32AM -0500, Luiz Capitulino wrote:
> > > [...]  
> > >> The "halted" field is somewhat controversial. On the one hand,
> > >> it offers a convenient way to know if a guest CPU is idle or
> > >> running. On the other hand, it's a field that can change many
> > >> times a second. In fact, the halted state can change even
> > >> before query-cpus-fast has returned. This makes one wonder if
> > >> this field should be dropped all together. Having the "halted"
> > >> field as optional gives a better option for dropping it in
> > >> the future, since we can just stop returning it.  
> > > 
> > > I'd just drop it, unless we find a use case where it's really
> > > useful.
> I don't think there's any, unless for debugging purposes.
> I'm keeping it mainly for s390. Viktor, libvirt is still using
> this field in s390, no?
> Dropping halted and having management software still using query-cpus
> because of halted would be a total failure of query-cpus-fast.

If I understood correctly, the CpuInfoS390::cpu_state field added
by Viktor in another patch[1] would replace "halted" for the s390

I'm assuming QEMU will be able to return that field without
interrupting the VCPUs.  Viktor, is that correct?

[1] https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg02032.html

> > > Also, the code that sets/clears cpu->halted is target-specific,
> > > so I wouldn't be so sure that simply checking for
> > > !kvm_irqchip_in_kernel() is enough on all targets.
> I checked the code and had the impression it was enough, but
> I don't have experience with other archs. So, would be nice
> if other archs maintainers could review this. I'll try to ping them.

I think we need to take a step back and rethink:

1) What the field is supposed to mean?  The semantics of "halted"
   are completely unclear.  What exactly we want to communicate
   to libvirt/management?
2) On which cases the information (whatever it means) is really
   useful/important?  If you are excluding cases with in-kernel
   irqchip, you are already excluding most users.

> > Right, the present patch effectively disables halted anyway (including
> > s390). 
> No, it doesn't. It only disables halted for archs that require going
> to the kernel to get it.

It disables it for all architectures that implement in-kernel
irqchip: x86, arm, s390.

The only existing user of "halted" is s390-specific code in
libvirt, and your patch won't return it on s390, so nobody seems
to benefit from it.

> > So it may be cleaner to just drop it right now.
> > Assuming the presence of architecure-specific data, libvirt can derive a
> > halted state (or an equivalent thereof) from query-cpus-fast returned
> > information.
> This is a different proposal. You're proposing moving the halted state
> to a CPU-specific field. This is doable if that's what we want.

I think it's a valid approach to return a target-specific field
first, and later try to come up with a generic (and clearly
defined) abstraction to represent the same information (either
inside QEMU or inside libvirt).


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