Re: [Qemu-devel] KVM call minutes 2013-01-29

[Paolo Bonzini]

Il 29/01/2013 17:47, Anthony Liguori ha scritto:
> Paolo Bonzini <address@hidden> writes:
> 
>> Il 29/01/2013 16:41, Juan Quintela ha scritto:
>>> * Replacing select(2) so that we will not hit the 1024 fd_set limit in the
>>>   future. (stefan)
>>>
>>>   Add checks for fd's bigger than 1024? multifunction devices uses lot
>>>   of fd's for device.
>>>
>>>   Portability?
>>>   Use glib?  and let it use poll underneath.
>>>   slirp is a problem.
>>>   in the end loop: moving to a glib event loop, how we arrive there is the 
>>> discussion.
>>
>> We can use g_poll while keeping the main-loop.c wrappers around the glib
>> event loop.  Both slirp and iohandler.c access the fd_sets randomly, so
>> we need to remember some state between the fill and poll functions.  We
>> can use two main-loop.c functions:
>>
>> int qemu_add_poll_fd(int fd, int events);
>>
>>   select: writes the events into three fd_sets, returns the file
>>   descriptor itself
>>
>>   poll: writes a GPollFD into a dynamically-sized array (of GPollFDs)
>>   and returns the index in the array.
>>
>> int qemu_get_poll_fd_revents(int index);
>>
>>   select: takes the file descriptor (returned by qemu_add_poll_fd),
>>   makes up revents based on the three fd_sets
>>
>>   poll: takes the index into the array and returns the corresponding
>>   revents
>>
>> iohandler.c can simply store the index into struct IOHandlerRecord, and
>> use it later.  slirp can do the same for struct socket.
>>
>> The select code can be kept for Windows after POSIX switches to poll.
> 
> Doesn't g_poll already do this under the covers for Windows?

No, g_poll is for synchronization objects (like Linux eventfd or
timerfd).  Sockets still require select.  You can tie a socket to a
synchronization object; this way socket events can exit g_poll, and in
fact that's exactly what QEMU does.  But you still need to retrieve the
currently-active events with select, so iohandler.c and slirp (which use
sockets) need to work in terms of select.

Paolo