Messages in this thread | | | Date | Fri, 31 May 2019 13:48:39 +0200 | From | Roman Penyaev <> | Subject | Re: [PATCH 1/1] eventfd new tag EFD_VPOLL: generate epoll events |
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On 2019-05-31 12:45, Renzo Davoli wrote: > HI Roman, > > On Fri, May 31, 2019 at 11:34:08AM +0200, Roman Penyaev wrote: >> On 2019-05-27 15:36, Renzo Davoli wrote: >> > Unfortunately this approach cannot be applied to >> > poll/select/ppoll/pselect/epoll. >> >> If you have to override other systemcalls, what is the problem to >> override >> poll family? It will add, let's say, 50 extra code lines complexity >> to your >> userspace code. All you need is to be woken up by *any* event and >> check >> one mask variable, in order to understand what you need to do: read or >> write, >> basically exactly what you do in your eventfd modification, but only >> in >> userspace. > > This approach would not scale. If I want to use both a (user-space) > network stack > and a (emulated) device (or more stacks and devices) which > (overridden) poll would I use? > > The poll of the first stack is not able to to deal with the third > device.
Since each such a stack has a set of read/write/etc functions you always can extend you stack with another call which returns you event mask, specifying what exactly you have to do, e.g.:
nfds = epoll_wait(epollfd, events, MAX_EVENTS, -1); for (n = 0; n < nfds; ++n) { struct sock *sock;
sock = events[n].data.ptr; events = sock->get_events(sock, &events[n]);
if (events & EPOLLIN) sock->read(sock); if (events & EPOLLOUT) sock->write(sock); }
With such a virtual table you can mix all userspace stacks and even with normal sockets, for which 'get_events' function can be declared as
static poll_t kernel_sock_get_events(struct sock *sock, struct epoll_event *ev) { return ev->events; }
Do I miss something?
>> > > Why can it not be less than 64? >> > This is the imeplementation of 'write'. The 64 bits include the >> > 'command' >> > EFD_VPOLL_ADDEVENTS, EFD_VPOLL_DELEVENTS or EFD_VPOLL_MODEVENTS (in the >> > most >> > significant 32 bits) and the set of events (in the lowest 32 bits). >> >> Do you really need add/del/mod semantics? Userspace still has to keep >> mask >> somewhere, so you can have one simple command, which does: >> ctx->count = events; >> in kernel, so no masks and this games with bits are needed. That will >> simplify API. > > It is true, at the price to have more complex code in user space. > Other system calls could have beeen implemented as "set the value", > instead there are > ADD/DEL modification flags. > I mean for example sigprocmask (SIG_BLOCK, SIG_UNBLOCK, SIG_SETMASK), > or even epoll_ctl. > While poll requires the program to keep the struct pollfd array stored > somewhere, > epoll is more powerful and flexible as different file descriptors can > be added > and deleted by different modules/components. > > If I have two threads implementing the send and receive path of a > socket in a user-space
Eventually you come up with such a lock to protect your tcp or whatever state machine. Or you have a real example where read and write paths can work completely independently?
-- Roman
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