Subject: Re: [PATCH] KVM: x86: Add lowest-priority support for vt-d
 posted-interrupts
To: =?UTF-8?Q?Radim_Krcm=c3=a1r?= <rkrcmar@redhat.com>,
        "Wu, Feng" <feng.wu@intel.com>
References: <1447037208-75615-1-git-send-email-feng.wu@intel.com>
 <20151116190314.GA12245@potion.brq.redhat.com> <564AF645.5010506@redhat.com>
 <E959C4978C3B6342920538CF579893F00AE87B93@SHSMSX104.ccr.corp.intel.com>
 <20151124143557.GA24676@potion.brq.redhat.com>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <56547662.7020804@redhat.com>
Date: Tue, 24 Nov 2015 15:38:26 +0100
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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.

Paolo
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