Re: [Qemu-devel] [PATCH 6/7] qemu: Implement virtio-pstore device

[Daniel P. Berrange]

On Sat, Jul 30, 2016 at 05:38:27PM +0900, Namhyung Kim wrote:
> Hello,
> 
> On Thu, Jul 28, 2016 at 02:08:41PM +0100, Daniel P. Berrange wrote:
> > On Thu, Jul 28, 2016 at 01:56:07PM +0100, Stefan Hajnoczi wrote:
> > > On Thu, Jul 28, 2016 at 02:39:53PM +0900, Namhyung Kim wrote:
> > > > On Thu, Jul 28, 2016 at 03:02:54AM +0300, Michael S. Tsirkin wrote:
> > > > > On Thu, Jul 28, 2016 at 12:08:30AM +0900, Namhyung Kim wrote:
> > > > > > +static ssize_t virtio_pstore_do_write(VirtIOPstore *s, struct 
> > > > > > iovec *out_sg,
> > > > > > +                                      unsigned int out_num,
> > > > > > +                                      struct virtio_pstore_req 
> > > > > > *req)
> > > > > > +{
> > > > > > +    char path[PATH_MAX];
> > > > > > +    int fd;
> > > > > > +    ssize_t len;
> > > > > > +    unsigned short type;
> > > > > > +    int flags = O_WRONLY | O_CREAT;
> > > > > > +
> > > > > > +    /* we already consume the req */
> > > > > > +    iov_discard_front(&out_sg, &out_num, sizeof(*req));
> > > > > > +
> > > > > > +    virtio_pstore_to_filename(s, path, sizeof(path), req);
> > > > > > +
> > > > > > +    type = le16_to_cpu(req->type);
> > > > > > +
> > > > > > +    if (type == VIRTIO_PSTORE_TYPE_DMESG) {
> > > > > > +        flags |= O_TRUNC;
> > > > > > +    } else if (type == VIRTIO_PSTORE_TYPE_CONSOLE) {
> > > > > > +        flags |= O_APPEND;
> > > > > > +    }
> > > > > > +
> > > > > > +    fd = open(path, flags, 0644);
> > > > > > +    if (fd < 0) {
> > > > > > +        error_report("cannot open %s", path);
> > > > > > +        return -1;
> > > > > > +    }
> > > > > > +    len = writev(fd, out_sg, out_num);
> > > > > > +    close(fd);
> > > > > > +
> > > > > > +    return len;
> > > > > 
> > > > > All this is blocking VM until host io completes.
> > > > 
> > > > Hmm.. I don't know about the internals of qemu.  So does it make guest
> > > > stop?  If so, that's what I want to do for _DMESG. :)  As it's called
> > > > only on kernel oops I think it's admittable.  But for _CONSOLE, it
> > > > needs to do asynchronously.  Maybe I can add a thread to do the work.
> > > 
> > > Please look at include/io/channel.h.  QEMU is event-driven and tends to
> > > use asynchronous I/O instead of spawning threads.  The include/io/ APIs
> > > allow you to do asynchronous I/O in the event loop.
> > 
> > That is true, except for I/O to/from plain files - the QIOChannelFile
> > impl doesn't do anything special (yet) to make that work correctly in
> > non-blocking mode. Of course that could be fixed...
> 
> Yep, I don't know how I can use the QIOChannelFile for async IO.
> AFAICS it's just a wrapper for normal readv/writev.  Who is
> responsible to do these IO when I use the IO channel API?  Also does
> it guarantee that all IOs are processed in order?

I'd suggest just using QIOChannelFile regardless - we need to fix the
blocking behaviour already for sake of the qemu chardev code, and your
code just adds more pressure to fix it. I/O will be done in the order
in which you make the calls, as with regular POSIX I/O funcs you're
currently using.

Regards,
Daniel
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