Re: [Qemu-devel] [RFC PATCH RDMA support v4: 09/10] check for QMP string and bypass nonblock() calls

[Paolo Bonzini]

Il 18/03/2013 21:37, Michael R. Hines ha scritto:
>>
>> +    if(!migrate_use_rdma(f)) {
>> +        int fd = qemu_get_fd(f);
>> +        assert(fd != -1);
>> +        socket_set_nonblock(fd);
>> Is this because qemu_get_fd(f) returns -1 for RDMA?  Then, you can
>> instead put socket_set_nonblock under an if(fd != -1).
> 
> Yes, I proposed doing that check (for -1) in a previous RFC,
> but you told me to remove it and make a separate patch =)
> 
> Is it OK to keep it in this patch?

Yes---this is a separate patch.  Apologies if you had the if(fd != -1)
before. :)  In fact, both the if(fd != -1) and the
if(!migrate_use_rdma(f)) are bad, but I prefer to eliminate as many uses
as possible of migrate_use_rdma.

The reason why they are bad, is that we try to operate on the socket in
a non-blocking manner, so that the monitor keeps working during incoming
migration.  We do it with non-blocking sockets because incoming
migration does not (yet?) have a separate thread, and is not a
bottleneck (the VM is not running, so it's not a problem to hold the big
QEMU lock for extended periods of time).

Does librdmacm support non-blocking operation, similar to select() or
poll()?  Perhaps we can add support for that later.

Paolo