Re: [Qemu-devel] [PATCH 2/3] qmp: dump-guest-memory: don't spin if non-blocking fd would block

[Luiz Capitulino]

On Mon, 24 Sep 2012 14:27:17 +0800
Wen Congyang <address@hidden> wrote:

> At 09/22/2012 01:07 AM, Luiz Capitulino Wrote:
> > fd_write_vmcore() will indefinitely spin for a non-blocking
> > file-descriptor that would block. However, if the fd is non-blocking,
> > how does it make sense to spin?
> > 
> > Change this behavior to return an error instead.
> > 
> > Note that this can only happen with an fd provided by a management
> > application. The fd opened internally by dump-guest-memory is blocking.
> > 
> > Signed-off-by: Luiz Capitulino <address@hidden>
> > ---
> >  dump.c | 13 +++----------
> >  1 file changed, 3 insertions(+), 10 deletions(-)
> > 
> > diff --git a/dump.c b/dump.c
> > index 2bf8d8d..5eea015 100644
> > --- a/dump.c
> > +++ b/dump.c
> > @@ -100,18 +100,11 @@ static void dump_error(DumpState *s, const char 
> > *reason)
> >  static int fd_write_vmcore(void *buf, size_t size, void *opaque)
> >  {
> >      DumpState *s = opaque;
> > -    int fd = s->fd;
> >      size_t writen_size;
> >  
> > -    /* The fd may be passed from user, and it can be non-blocked */
> > -    while (size) {
> > -        writen_size = qemu_write_full(fd, buf, size);
> > -        if (writen_size != size && errno != EAGAIN) {
> 
> Hmm, if the fd is a blocking fd, errno can't be EAGAIN. So the
> function doesn't spin. What problems do you meet?

The problem is with non-blocking fds, where spinning isn't correct, for
two reasons:

 1. If the fd is non-blocking, that means you don't want to block
    and spinning for a long time will have the same effects

 2. Spinning consumes host resources