Message-ID: <502A462A.1000600@redhat.com>
Date: Tue, 14 Aug 2012 15:35:54 +0300
From: Avi Kivity <avi@redhat.com>
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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,
        jan.kiszka@siemens.com
Subject: Re: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs
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On 08/12/2012 12:33 PM, Michael S. Tsirkin wrote:
>> 
>> Michael, would the interface be more acceptable to you if we added
>> separate ioctls to allocate and free some representation of an irq
>> source ID, gsi pair?  For instance, an ioctl might return an idr entry
>> for an irq source ID/gsi object which would then be passed as a
>> parameter in struct kvm_irqfd and struct kvm_eoifd so that the object
>> representing the source id/gsi isn't magically freed on it's own.  This
>> would also allow us to deassign/close one end and reconfigure it later.
>> Thanks,
>> 
>> Alex
> 
> It's acceptable to me either way. I was only pointing out that as
> designed, the interface looks simple at first but then you find out some
> subtle limitations which are implementation driven. This gives
> an overall feeling the abstraction is too low level.
> 
> If we compare to the existing irqfd, isn't the difference
> simply that irqfd deasserts immediately ATM, while we
> want to delay this until later?
> 
> If yes, then along the lines that you proposed, and combining with my
> idea of tracking deasserts, how do you like the following:
> 
> /* Keep line asserted until guest has handled the interrupt. */
> #define KVM_IRQFD_FLAG_DEASSERT_ON_ACK (1 << 1)
> /* Notify after line is deasserted. */
> #define KVM_IRQFD_FLAG_DEASSERT_EVENTFD (2 << 1)
> 
> 	struct kvm_irqfd {
> 		__u32 fd;
> 		__u32 gsi;
> 		__u32 flags;
> 		/* eventfd to notify when line is deasserted */
> 		__u32 deassert_eventfd;
> 		__u8  pad[16];
> 	};
> 
> now the only limitation is that KVM_IRQFD_FLAG_DEASSERT_ON_ACK is only
> effective for level interrupts.
> 
> Notes about lifetime of objects:
> 	- closing deassert_eventfd does nothing (we can keep
> 	  reference to it from irqfd so no need for
>           complex polling/flushing scheme)
> 	- closing irqfd or deasserting dis-associates
> 	  deassert_eventfd automatically
> 	- source id is internal to irqfd and goes away with it
> 
> it looks harder to misuse and fits what we want to do nicely,
> and needs less code to implement.
> 
> Avi, what do you think?

I think given all the complexity in the separate ioctl approach that
this makes sense.  There are no lifetime issues or code to match the two
eventfds.  Alex, would this API simplify the code?

Yet another option was raised in the past, and that was exiling ioapic
and pic to userspace.  This moves the entire issue to userspace.  The
cost is a new interface that implements the APIC bus (betweem APIC and
IOAPIC) and the INTACK sequence (between APIC and PIC), and potential
for performance regressions due to the PIC, IOAPIC, and PIT being in
userspace.  We would still have to keep the IOAPIC/PIC in the kernel,
but no new features would be added.

However, this is a huge job.  We could discuss this to death too but I
have the feeling the end result will be to choose the shorter path --
adding irqackfd/deassertfd/whateverwecallitfd.


-- 
error compiling committee.c: too many arguments to function
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