linux-kernel - RE: [PATCH] KVM: x86: Add lowest-priority support for vt-d posted-interrupts

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Date:	Wed, 25 Nov 2015 01:58:26 +0000
From:	"Wu, Feng" <feng.wu@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krcmár <rkrcmar@...hat.com>
CC:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wu, Feng" <feng.wu@...el.com>
Subject: RE: [PATCH] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts



> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@...hat.com]
> Sent: Tuesday, November 24, 2015 10:38 PM
> To: Radim Krcmár <rkrcmar@...hat.com>; Wu, Feng <feng.wu@...el.com>
> Cc: kvm@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d posted-
> interrupts
> 
> 
> 
> On 24/11/2015 15:35, Radim Krcmár wrote:
> > > Thanks for your guys' review. Yes, we can introduce a module option
> > > for it. According to Radim's comments above, we need use the
> > > same policy for PI and non-PI lowest-priority interrupts, so here is the
> > > question: for vector hashing, it is easy to apply it for both non-PI and PI
> > > case, however, for Round-Robin, in non-PI case, the round robin counter
> > > is used and updated when the interrupt is injected to guest, but for
> > > PI case, the interrupt is injected to guest totally by hardware, software
> > > cannot control it while interrupt delivery, we can only decide the
> > > destination vCPU for the PI interrupt in the initial configuration
> > > time (guest update vMSI -> QEMU -> KVM). Do you guys have any good
> > > suggestion to do round robin for PI lowest-priority? Seems Round robin
> > > is not a good way for PI lowest-priority interrupts. Any comments
> > > are appreciated!
> >
> > It's meaningless to try dynamic algorithms with PI so if we allow both
> > lowest priority algorithms, I'd let PI handle any lowest priority only
> > with vector hashing.  (It's an ugly compromise.)
> 
> For now, I would just keep the 4.4 behavior, i.e. disable PI unless
> there is a single destination || vector hashing is enabled.  We can flip
> the switch later.

Okay, let me try to understand this clearly:
- We will have a new KVM command line parameter to indicate whether
  vector hashing is enabled.
- If it is not enabled, for PI, we can only support single destination lowest
  priority interrupts, for non-PI, we continue to use RR.
- If it is enabled, for PI and non-PI we use vector hashing for both of them.

Is this the case you have in mind? Thanks a lot!

Thanks,
Feng

> 
> Paolo