Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd

From: Avi Kivity <avi@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Alex Williamson <alex.williamson@redhat.com>,
	gleb@redhat.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v9 1/2] kvm: Use a reserved IRQ source ID for irqfd
Date: Wed, 05 Sep 2012 18:22:57 +0300	[thread overview]
Message-ID: <50476E51.5020005@redhat.com> (raw)
In-Reply-To: <20120905151348.GC11058@redhat.com>

On 09/05/2012 06:13 PM, Michael S. Tsirkin wrote:
> On Wed, Sep 05, 2012 at 05:59:46PM +0300, Avi Kivity wrote:
>> On 09/05/2012 05:51 PM, Michael S. Tsirkin wrote:
>> > On Wed, Sep 05, 2012 at 05:35:43PM +0300, Avi Kivity wrote:
>> >> On 08/22/2012 03:41 AM, Michael S. Tsirkin wrote:
>> >> >> 
>> >> >> I assumed you were pointing out the level vs edge interaction.  If we
>> >> >> call that a userspace bug, I can just drop this.  Thanks,
>> >> >> 
>> >> >> Alex
>> >> > 
>> >> > level is userspace bug I think :)
>> >> 
>> >> I don't see how it's a bug.  Suppose we have a vfio device that shares a
>> >> gsi with an emulated device.  The emulated device naturally uses
>> >> KVM_IRQ_LINE (it has no need to re-sample on ADN), while vfio naturally
>> >> has to use irqfd.
>> > 
>> > Absolutely. But vfio needs to use irqfd with the new flag.
>> > Using existing irqfd for level is a bug.
>> 
>> I see we're not reusing this irq source id for level irqfd.  But I think
>> we should, there's no need for per-gsi irq source id.
> 
> I agree. All resample irqfds are deasserted at the same time,
> tracking them separately gets us nothing.

That's not the reason.  Separate irq source ids only have meanings
within a gsi.  We could have two lines (gsi 3 isid 4) and (gsi 4 isid 4)
that can be toggled independently with no effect on the other gsi.
Within a gsi we do need a separate irq source id usually, but as 2/2
recognizes, AODNs are a special case since we clear all inputs anyway.
The end result is that all AODNs can share a single isid.

> 
>> Plus I'd like to
>> fix the theoretical bug even if it doesn't bite in practice.
>> 
> 
> I'm not sure what the bug is, for edge, and how a separate ID fixes it.
> Could you clarify?

gsi 3 is configured as edge in the ioapic.  It has (unusually) two
inputs: one driven by userspace, the other by irqfd.

cpu 0                    cpu 1
------------------------ -------------------------
irqfd: set to 1
ioapic: recognize edge
inject irq
EOI
                         KVM_IRQ_LINE: set to 1
                         ioapic: ignore
                         KVM_IRQ_LINE: set to 0
irqfd: set to 0

We had two edges with an EOI between them, but injected just on interrupt.

-- 
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