On Thu, Aug 27, 2009 at 11:05:30AM +0200, Paolo Bonzini wrote:
> > Ok, so why not using the eventfd counter as state?
> > On the device side:
> >
> > void write_state(int sfd, int state) {
> > u64 cnt;
> >
> > /* Clear the current state, sfd is in non-blocking mode */
> > read(sfd,&cnt, sizeof(cnt));
> > /* Writes new state */
> > cnt = 1 + !!state;
> > write(sfd,&cnt, sizeof(cnt));
> > }
>
> It's interesting [no sarcasm intended, mind] that EFD_SEMAPHORE was
> added exactly to avoid a read+write combination for the case of
> decrementing a value. Here it's the same, just it's about the case of
> writing a *given* value. What about having EFD_STATE simply mean "do
> not use a counter, just write the value" without affecting the way
> read works, and use
>
> /* Writes new state */
> cnt = 1 + !!state;
> write(sfd,&cnt, sizeof(cnt));
That would work for kvm.
> See below?
>
> Paolo
>
> > On the hypervisor side:
> >
> > int read_state(int sfd) {
> > u64 cnt;
> >
> > read(sfd,&cnt, sizeof(cnt));
> > return state - 1;
> > }
>
>
> ------------- 8<-- ---------------
> Subject: [PATCH] eventfd: new EFD_ABSOLUTE flag
>
> This implements a new EFD_ABSOLUTE flag for eventfd.
> This changes eventfd behaviour so that write simply
> stores the value written, and is always non-blocking.
>
> Untested (I just modified Michael's patch, and given
> the simpler code I'm not sure it's now worthwhile
> introducing the inline functions), but otherwise
>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> ---
> fs/eventfd.c | 13 ++++++++-----
> include/linux/eventfd.h | 4 ++--
> 2 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/fs/eventfd.c b/fs/eventfd.c
> index 347a0e0..7b279e3 100644
> --- a/fs/eventfd.c
> +++ b/fs/eventfd.c
> @@ -31,8 +31,6 @@
>
> static inline int eventfd_writeable(struct eventfd_ctx *ctx, u64 n)
> {
> - return ULLONG_MAX - n > ctx->count;
> - return (ctx->flags & EFD_ABSOLUTE) || (ULLONG_MAX - n > ctx->count);
> }
>
> static inline int eventfd_overflow(struct eventfd_ctx *ctx, u64 cnt)
> @@ -42,10 +40,14 @@
>
> static inline void eventfd_dowrite(struct eventfd_ctx *ctx, u64 ucnt)
> {
> - if (eventfd_writeable(ctx, ucnt))
> - ucnt = ULLONG_MAX - ctx->count;
> + if (ctx->flags & EFD_ABSOLUTE)
> + ctx->count = ucnt;
> + else {
> + if (ULLONG_MAX - ctx->count < ucnt)
> + ucnt = ULLONG_MAX - ctx->count;
>
> - ctx->count += ucnt;
> + ctx->count += ucnt;
> + }
> }
>
> static inline u64 eventfd_doread(struct eventfd_ctx *ctx)
> diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> index 3b85ba6..78ff649 100644
> --- a/include/linux/eventfd.h
> +++ b/include/linux/eventfd.h
> @@ -19,11 +19,12 @@
> * shared O_* flags.
> */
> #define EFD_SEMAPHORE (1 << 0)
> +#define EFD_ABSOLUTE (1 << 1)
> #define EFD_CLOEXEC O_CLOEXEC
> #define EFD_NONBLOCK O_NONBLOCK
>
> #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
> -#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
> +#define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE | EFD_ABSOLUTE)
>
> #ifdef CONFIG_EVENTFD
> --
> 1.6.2.5
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