Re: Save paused cpu ctx

From: "Jan Beulich" <JBeulich@suse.com>
To: aisaila@bitdefender.com, Razvan Cojocaru <rcojocaru@bitdefender.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, xen-devel@lists.xen.org
Subject: Re: Save paused cpu ctx
Date: Wed, 19 Sep 2018 06:13:48 -0600	[thread overview]
Message-ID: <5BA23D7C02000078001E9E4C@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <fe2ad7c8-e42e-7851-25a4-9268a0f2cc1f@bitdefender.com>

>>> On 19.09.18 at 13:11, <rcojocaru@bitdefender.com> wrote:
> The scenario is that we are trying to query the state of a VCPU (please
> note: just query). That means that we're only interested in getting some
> coherent VCPU state via the XEN_DOMCTL_gethvmcontext_partial domctl.
> 
> We don't care if said state ends up being saved for the migration stream
> or not, so in that respect the answer to Roger's question is: no size
> increase or difference whatsoever.

"We don't care" is too little. _We_ care that state for offline vCPU-s does
not make it into the migration stream. And at this point I think you mean
"no size increase or difference whatsoever _intended_", since removing
the check in question would result in a size increase afaict.

> All we want to do is to be able to query the state of any VCPU in the
> valid range of VCPUs assigned to the domain, online or not. We believe
> being able to query them is reasonable, and the SDM states that they do
> have a state (whatever it happens to be: the init state, after reset, etc.).

I didn't know the SDM stated anything about offline vCPU-s. There's
(according to my way of looking at things) no bare hardware equivalent
to this state, which means whatever the SDM says is not applicable.

> For example, please look at this XenServer-only patch:
> 
> https://github.com/xenserver/xen.pg/blob/XS-7.1.x/master/x86-domctl-Don-t-pa 
> use-the-whole-domain-if-only-gett.patch

That's what iirc Alexandru's series started from.

> Now about the "the agent should do something else if the VCPU is down"
> objection, that's not possible with the current Xen code: -ENOENT is
> returned both when the VCPU is down _and_ when the VCPU index is out of
> bounds (so if I query the state of VCPU 10 on a 2-VCPUs guest).

If there are no other ways to inquire vCPU state, I'm sure we can
provide some. I also don't view disambiguating the error code as
an impossible thing to do.

Jan

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