linux-kernel - Re: [PATCH-RFC 2/2] eventfd: EFD

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Message-ID: <4A76FDB2.7080706@redhat.com>
Date:	Mon, 03 Aug 2009 18:09:38 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	davidel@...ilserver.org, gleb@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH-RFC 2/2] eventfd: EFD_STATE flag

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?

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

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