Re: [PATCH 0/2] eventfd: new EFD_STATE flag

From: Avi Kivity
Date: Wed Aug 26 2009 - 06:41:35 EST

On 08/26/2009 01:29 PM, Michael S. Tsirkin wrote:
How we wanted to solve it with EFD_STATE: Share a separate eventfd
between each device and the hypervisor. device sets state to either 0
or 1. hypervisor polls all eventfds, reads interrupt line on changes,
calculates the interrupt level and updates guest.

Alternative solution: shared memory where each device writes interrupt
line value. This makes setup more complex (need to share around much more
than just an fd), and makes access from interrupt impossible unless we
lock the memory (and locking userspace memory introduces yet another set
of issues).

For completeness:

If the device is implemented in the same process as the hypervisor, an eventfd isn't really needed, as there is an ioctl which performs the same operation.

An important class of device implementations is real devices that are assigned to a guest. We would like to forward the interrupt directly from the host interrupt handler to qemu. Currently, we have a kvm-specific interrupt handler that forwards the interrupt using kvm-specific interfaces. We would like to use a generic interrupt handler implemented by uio, so we want a generic interrupt transfer mechanism.

uio already supports edge-triggered interrupts using an eventfd-like mechanism. So it makes sense to extend uio to support real eventfds, and to make it also support level-triggered interrupts.

We can work around the lack of state eventfd by having userspace wait on whatever mechanism uio uses to make the interrupt state visible, and then use the ioctl mentioned above to inform the hypervisor of this state. But it's faster and nicer to give both components an eventfd and let them communicate directly.

error compiling committee.c: too many arguments to function

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