Re: [PATCH v2 06/16] KVM: PPC: Book3S HV: XIVE: add controls for the EQ configuration

From: David Gibson <david@gibson.dropbear.id.au>
To: "Cédric Le Goater" <clg@kaod.org>
Cc: kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
	Paul Mackerras <paulus@samba.org>,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH v2 06/16] KVM: PPC: Book3S HV: XIVE: add controls for the EQ configuration
Date: Thu, 14 Mar 2019 14:29:27 +1100	[thread overview]
Message-ID: <20190314032927.GP8211@umbus.fritz.box> (raw)
In-Reply-To: <badcd1dc-2eef-590e-b362-0d3dfdf52d34@kaod.org>

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On Wed, Mar 13, 2019 at 09:46:08AM +0100, Cédric Le Goater wrote:
> On 3/13/19 5:03 AM, David Gibson wrote:
> > On Tue, Mar 12, 2019 at 06:00:38PM +0100, Cédric Le Goater wrote:
> >> On 2/25/19 3:39 AM, David Gibson wrote:
> >>> On Fri, Feb 22, 2019 at 12:28:30PM +0100, Cédric Le Goater wrote:
> >>>> These controls will be used by the H_INT_SET_QUEUE_CONFIG and
> >>>> H_INT_GET_QUEUE_CONFIG hcalls from QEMU. They will also be used to
> >>>> restore the configuration of the XIVE EQs in the KVM device and to
> >>>> capture the internal runtime state of the EQs. Both 'get' and 'set'
> >>>> rely on an OPAL call to access from the XIVE interrupt controller the
> >>>> EQ toggle bit and EQ index which are updated by the HW when event
> >>>> notifications are enqueued in the EQ.
> >>>>
> >>>> The value of the guest physical address of the event queue is saved in
> >>>> the XIVE internal xive_q structure for later use. That is when
> >>>> migration needs to mark the EQ pages dirty to capture a consistent
> >>>> memory state of the VM.
> >>>>
> >>>> To be noted that H_INT_SET_QUEUE_CONFIG does not require the extra
> >>>> OPAL call setting the EQ toggle bit and EQ index to configure the EQ,
> >>>> but restoring the EQ state will.
> >>
> >> I think we need to add some kind of flags to differentiate the hcall
> >> H_INT_SET_QUEUE_CONFIG from the restore of the EQ. The hcall does
> >> not need OPAL support call and this could help in the code
> >> transition.
> > 
> > Hrm.  What's the actual difference in the semantics between the two
> > cases.  
> 
> None. 
> 
> But we don't need to set the EQ state in the case of the HCALL and it's 
> (very) practical to run guests with XIVE enabled without the OPAL support. 
> The latter is the main reason clearly.
> 
> Thinking of it, I could test the EQ toggle bit and index passed to KVM 
> and skip the OPAL call which restores the EQ state if they are zero. 
> This is because I know that the OPAL call configuring the EQ resets them. 
> 
> That will do. No need for a flag.

That's a much better idea.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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