Message-ID: <20120813220004.GC15639@redhat.com>
Date:	Tue, 14 Aug 2012 01:00:04 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, jan.kiszka@...mens.com
Subject: Re: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs

On Mon, Aug 13, 2012 at 03:23:24PM -0600, Alex Williamson wrote:
> On Sun, 2012-08-12 at 12:33 +0300, Michael S. Tsirkin wrote:
> > On Thu, Aug 09, 2012 at 01:26:15PM -0600, Alex Williamson wrote:
> > > On Mon, 2012-08-06 at 13:40 +0300, Avi Kivity wrote:
> > > > On 08/06/2012 01:38 PM, Avi Kivity wrote:
> > > > 
> > > > > Regarding the implementation, instead of a linked list, would an array
> > > > > of counters parallel to the bitmap make it simpler?
> > > > 
> > > > Or even, replace the bitmap with an array of counters.
> > > 
> > > I'm not sure a counter array is what we're really after.  That gives us
> > > reference counting for the irq source IDs, but not the key->gsi lookup.
> > > It also highlights another issue, that we have a limited set of source
> > > IDs.  Looks like we have BITS_PER_LONG IDs, with two already used, one
> > > for the shared userspace ID and another for the PIT.  How happy are we
> > > going to be with a limit of 62 level interrupts in use at one time?
> > > 
> > > It's arguably a reasonable number since the most virtualization friendly
> > > devices (sr-iov VFs) don't even support this kind of interrupt.  It's
> > > also very wasteful allocating an entire source ID for a single GSI
> > > within that source ID.  PCI supports interrupts A, B, C, and D, which,
> > > in the most optimal config, each go to different GSIs.  So we could
> > > theoretically be more efficient in our use and allocation of irq source
> > > IDs if we tracked use by the source ID, gsi pair.
> > > 
> > > That probably makes it less practical to replace anything at the top
> > > level with a counter array.  The key that we pass back is currently the
> > > actual source ID, but we don't specify what it is, so we could split it
> > > and have it encode a 16bit source ID plus 16 bit GSI.  It could also be
> > > an idr entry.
> > > 
> > > 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.
> 
> This is effectively what I meant when I suggested we either need to a)
> pull eoifd into irqfd or b) implement them as modular components.  I
> chose to implement b) because I think that non-irqfd related ack
> notification to userspace will be useful and a) does not provide that.
> So this interface enables exactly the use case for device assignment and
> no more.  I feel like this is the start of an ioctl that will be quickly
> deprecated, but if that's the direction we want to go, I'll write the
> code.  Thanks,
> 
> Alex

Sorry I wrote this before I knew we really do not need
the deassert on ack at all, existing irqfd is fine for level.


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MST
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