On Thu, Jul 13, 2023 at 11:10:54AM -0600, Alex Williamson wrote: > On Thu, 13 Jul 2023 12:05:36 +0200 > Christian Brauner <brauner@xxxxxxxxxx> wrote: > > > Hey everyone, > > > > This simplifies the eventfd_signal() and eventfd_signal_mask() helpers > > by removing the count argument which is effectively unused. > > We have a patch under review which does in fact make use of the > signaling value: > > https://lore.kernel.org/all/20230630155936.3015595-1-jaz@xxxxxxxxxxxx/ Huh, thanks for the link. Quoting from https://patchwork.kernel.org/project/kvm/patch/20230307220553.631069-1-jaz@xxxxxxxxxxxx/#25266856 > Reading an eventfd returns an 8-byte value, we generally only use it > as a counter, but it's been discussed previously and IIRC, it's possible > to use that value as a notification value. So the goal is to pipe a specific value through eventfd? But it is explicitly a counter. The whole thing is written around a counter and each write and signal adds to the counter. The consequences are pretty well described in the cover letter of v6 https://lore.kernel.org/all/20230630155936.3015595-1-jaz@xxxxxxxxxxxx/ > Since the eventfd counter is used as ACPI notification value > placeholder, the eventfd signaling needs to be serialized in order to > not end up with notification values being coalesced. Therefore ACPI > notification values are buffered and signalized one by one, when the > previous notification value has been consumed. But isn't this a good indication that you really don't want an eventfd but something that's explicitly designed to associate specific data with a notification? Using eventfd in that manner requires serialization, buffering, and enforces ordering. I have no skin in the game aside from having to drop this conversion which I'm fine to do if there are actually users for this btu really, that looks a lot like abusing an api that really wasn't designed for this.
