Linux Virtualization - Jul 2016 - [Qemu-devel] [PATCH 6/7] qemu: Implement virtio-pstore device

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.

Stefan
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 473 bytes
Desc: not available
URL:
<http://lists.linuxfoundation.org/pipermail/virtualization/attachments/20160728/e35e9141/attachment.sig>

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...

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|

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?

Thanks,
Namhyung