Gregory Haskins wrote: >> >> Is having a read() (or a write()) actually necessary? >> > > Based on what I know: yes. It could be a case of ignorance, however ;) > > Heres why I think its necessary: You need poll to simply tell you when > something is pending. You can't clear the pending status in poll because you > cannot predict the internal access pattern (e.g. I assume it could be polled > multiple times by the kernel without returning immediately to userspace). > Therefore, you need a second method to actually clear the pending "signal", > which I use the read() method for. I can be convinced otherwise, but that > was my original thinking. >
I think you are right, but am cc'ing an expert. Davide, we're using an fd to signal something to userspace, but have nothing to actually read() or write(). Is a read() or write() avoidable? > >>> + >>> + if (indirect_sig && waitqueue_active(&vcpu- >irq.wq)) >>> + wake_up(&vcpu- >irq.wq); >>> } >>> >>> >>> >> Did you check that we can actually deliver signals with this? I think a >> fasync_struct or something like that is necessary, but not sure. >> > > Actually, my signals *didn't* seem to be working, but they werent working > with "send_sig()" either so I just assumed I had a userspace coding problem. > Based on what I read, it seemed like what I did should work if you do a > fcntl(F_SETSIG), etc. But again, it could be ignorance. I am not familiar > with fasync_struct. If you have any pointers, please forward. > > fs/pipe.c. hairy stuff. >> Another implementation option (which I've only thought of now, sorry) is >> to have an ioctl which returns a real eventfd, reducing some code >> duplication. >> > > So based on this, I assume eventfd must be in the kernel already? Cool. It is in 2.6.22-rc1. As is the anonymous inodes source which can be used to retire kvmfs (which will probably break the record for shortest-lived filesystem ever). > Even if its not, I like this idea much better than what I did. There was > still an unresolved problem regarding how I was going to expose the signaling > mechanism to QEMU without giving away the vcpu_fd from the kvmctl library > that this solves nicely. > > With this methodology, I can simply provide a function like > "kvm_vcpu_get_eventfd()" in the library, and return the eventfd directly to > the QEMU process. Then we dont have to worry about layering violations. > Nice! > I hadn't though of it. Looks like a win from all directions. It means we need to package eventfd for the external module, but that's easily done. -- error compiling committee.c: too many arguments to function ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ kvm-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/kvm-devel
