Re: [PATCH v7 2/2] kvm: KVM_EOIFD, an eventfd for EOIs

[Alex Williamson <alex.williamson@redhat.com>]

On Sun, 2012-08-12 at 11:36 +0300, Avi Kivity wrote:
> On 08/09/2012 10:26 PM, 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.
> 
> You can look up the gsi while registering the eoifd, so it's accessible
> as eoifd->gsi instead of eoifd->source->gsi.  The irqfd can go away
> while the eoifd is still active, but is this a problem?

In my opinion, no, but Michael disagrees.

> > 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?
> 
> When we start being unhappy we can increase that number.  On the other
> hand more locks and lists makes me unhappy now.

Yep, good point.  My latest version removes the source ID object lock
and list (and objects).  I still have a lock and list for the ack
notification, but it's hard not to unless we combine them into one
mega-irqfd ioctl as Michael suggests.

> > 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.
> 
> There are, in one userspace, just three gsis available for PCI links, so
> you're compressing the source id space by 3.

I imagine there's a way to put each PCI interrupt pin on a GSI, but
still only 4, not a great expansion of source ID space.  I like
Michael's idea of re-using source IDs if we run out better.

> > 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.
> 
> We can fix those kinds of problems by adding another layer of
> indirection.  But I doubt they will be needed.  I don't see people
> assigning 60 legacy devices to one guest.

Yep, we can ignore it for now and put it in the hands of userspace to
re-use IDs if needed.

> > 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,
> 
> Another option is to push the responsibility for allocating IDs for the
> association to userspace.  Let userspace both create the irqfd and the
> eoifd with the same ID, the kernel matches them at registration time and
> copies the gsi/sourceid from the first to the second eventfd.

Aside from the copying gsi/sourceid bit, you've just described my latest
attempt at this series.  Specifying both a sourceid and gsi also allows
userspace to make better use of the sourceid address space (use more
than one gsi if userspace wants the complexity of managing them).
Thanks,

Alex