Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

Avi Kivity
2009-08-03T15:04:24Z

On 07/28/2009 08:55 PM, Michael S. Tsirkin wrote:
> This implements a new EFD_STATE flag for eventfd.
> When set, this flag changes eventfd behaviour in the following way:
> - write simply stores the value written, and is always non-blocking
> - read unblocks when the value written changes, and
> returns the value written
>
> Motivation: we'd like to use eventfd in qemu to pass interrupts from
> (emulated or assigned) devices to guest. For level interrupts, the
> counter supported currently by eventfd is not a good match: we really
> need to set interrupt to a level, typically 0 or 1, and give the guest
> ability to see the last value written.
>
>
> @@ -31,37 +31,59 @@ struct eventfd_ctx {
> * issue a wakeup.
> */
> __u64 count;
> + /*
> + * When EF_STATE flag is set, eventfd behaves differently:
> + * value written gets stored in "count", read will copy
> + * "count" to "state".
> + */
> + __u64 state;
> unsigned int flags;
> };
>

Why not write the new value into ->count directly?

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