Re: [Qemu-devel] [PATCH 1/3] qemu-char: Introduce Memory driver

[Luiz Capitulino <lcapitulino@redhat.com>]

On Thu, 11 Nov 2010 17:32:06 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Luiz Capitulino <lcapitulino@redhat.com> writes:
> 
> > On Thu, 11 Nov 2010 16:30:26 +0100
> > Markus Armbruster <armbru@redhat.com> wrote:
> >
> >> Luiz Capitulino <lcapitulino@redhat.com> writes:
> >> 
> >> > This driver handles in-memory chardev operations. That's, all writes
> >> > to this driver are stored in an internal buffer and it doesn't talk
> >> > to the external world in any way.
> >> >
> >> > Right now it's very simple: it supports only writes. But it can be
> >> > easily extended to support more operations.
> >> >
> >> > This is going to be used by the monitor's "HMP passthrough via QMP"
> >> > feature, which needs to run monitor handlers without a backing
> >> > device.
> >> >
> >> > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> >> > ---
> >> >  qemu-char.c |   66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >> >  qemu-char.h |    6 +++++
> >> >  2 files changed, 72 insertions(+), 0 deletions(-)
> >> >
> >> > diff --git a/qemu-char.c b/qemu-char.c
> >> > index 88997f9..896df14 100644
> >> > --- a/qemu-char.c
> >> > +++ b/qemu-char.c
> >> > @@ -2275,6 +2275,72 @@ static CharDriverState *qemu_chr_open_socket(QemuOpts *opts)
> >> >      return NULL;
> >> >  }
> >> >  
> >> > +/***********************************************************/
> >> > +/* Memory chardev */
> >> > +typedef struct {
> >> > +    size_t outbuf_size;
> >> > +    size_t outbuf_capacity;
> >> > +    uint8_t *outbuf;
> >> > +} MemoryDriver;
> >> > +
> >> > +static int mem_chr_write(CharDriverState *chr, const uint8_t *buf, int len)
> >> > +{
> >> > +    MemoryDriver *d = chr->opaque;
> >> > +
> >> > +    /* TODO: the QString implementation has the same code, we should
> >> > +     * introduce a generic way to do this in cutils.c */
> >> > +    if (d->outbuf_capacity < d->outbuf_size + len) {
> >> > +        /* grown outbuf */
> >> 
> >> Used to say "grow" (sans n) here.  Intentional change?
> >
> > Hum, no. I think I've squashed an older commit while rebasing (but this seems
> > to be the only problem).
> >
> >> 
> >> > +        d->outbuf_capacity += len;
> >> > +        d->outbuf_capacity *= 2;
> >> > +        d->outbuf = qemu_realloc(d->outbuf, d->outbuf_capacity);
> >> > +    }
> >> > +
> >> > +    memcpy(d->outbuf + d->outbuf_size, buf, len);
> >> > +    d->outbuf_size += len;
> >> > +
> >> > +    return len;
> >> > +}
> >> > +
> >> > +#define DEFAULT_BUF_SIZE 4096
> >> 
> >> It's the *initial* buffer size, isn't it?
> >
> > Yes.
> 
> Could we make the name reflect that then?
> 
> >> Doubt it's worth a #define (there's just one user), but that's a matter
> >> of taste.
> >> 
> >> > +
> >> > +void qemu_chr_init_mem(CharDriverState *chr)
> >> > +{
> >> > +    MemoryDriver *d;
> >> > +
> >> > +    d = qemu_malloc(sizeof(*d));
> >> > +    d->outbuf_size = 0;
> >> > +    d->outbuf_capacity = DEFAULT_BUF_SIZE;
> >> > +    d->outbuf = qemu_mallocz(d->outbuf_capacity);
> >> > +
> >> > +    memset(chr, 0, sizeof(*chr));
> >> > +    chr->opaque = d;
> >> > +    chr->chr_write = mem_chr_write;
> >> > +}
> >> > +
> >> > +/* assumes the stored data is a string */
> >> 
> >> What else could it be?  Worrying about embedded '\0's?
> >
> > Yes, as the driver itself doesn't interpret the contents of its
> > buffer.
> 
> What happens if there are embedded '\0's?

The string will be shorter than expected? And what if it contains
non-printable characters?

It's just a cautionary comment to help the user identify such problems, I think
we're making a whole argument about a quite minor thing.

> 
> >> > +QString *qemu_chr_mem_to_qs(CharDriverState *chr)
> >> > +{
> >> > +    MemoryDriver *d = chr->opaque;
> >> > +
> >> > +    if (d->outbuf_size == 0) {
> >> > +        return NULL;
> >> > +    }
> >> 
> >> Did you forget to change this?  We agreed to return an empty QString
> >> when chr contains an empty string.
> >
> > I've changed my mind and forgot to mention it: I thought that we would
> > need to return NULL on error conditions, but turns out that this function
> > never fails.
> >
> > So, I do think it's better to let it that way for two reasons:
> >
> >  1. An empty has at least the '\0' character, but in this case the buffer
> >     is really empty
> 
> qstring_from_substr() copies the contents of the buffer (any length
> works, including 0), then appends a '\0'.  I'm afraid I don't get the
> problem here...
> 
> >  2. Returning an empty string for this case will add unneeded complexity
> >     to the caller, ie. checking if the QString's length is 0 and decref'ing it
> 
> I strongly recommend not to screw up the interface of a generally useful
> function like qemu_chr_mem_to_qs() just to make its initial user
> marginally simpler.

Okay, found a different way of doing this that should make us both happy.

Very exciting changes in v3!

> 
> If you decide not to follow my recommendation, please document the
> unusual mapping of empty string to null pointer in a function comment.
> 
> >> > +
> >> > +    return qstring_from_substr((char *) d->outbuf, 0, d->outbuf_size - 1);
> >> > +}
> >> > +
> >> > +/* NOTE: this driver can not be closed with qemu_chr_close()! */
> >> > +void qemu_chr_close_mem(CharDriverState *chr)
> >> > +{
> >> > +    MemoryDriver *d = chr->opaque;
> >> > +
> >> > +    qemu_free(d->outbuf);
> >> > +    qemu_free(chr->opaque);
> >> > +    chr->opaque = NULL;
> >> > +    chr->chr_write = NULL;
> >> > +}
> >> > +
> >> 
> >> Unlike normal character drivers, this one can't be closed with
> >> qemu_chr_close().  It probably explodes if you try.  Please add a
> >> suitable assertion to qemu_chr_close() to document the fact, and to
> >> ensure misuse fails in a controlled, obvious manner.
> >
> > Ah forgot, but that can be done as a separate patch, so if I don't respin
> > this series I'll send an additional patch for that.
> 
> Okay.

Btw, what should we assert() for? We're going to have to access QTAILQ
member I guess.

> 
> >> 
> >> >  QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename)
> >> >  {
> >> >      char host[65], port[33], width[8], height[8];
> >> > diff --git a/qemu-char.h b/qemu-char.h
> >> > index 18ad12b..c4e55b4 100644
> >> > --- a/qemu-char.h
> >> > +++ b/qemu-char.h
> >> > @@ -6,6 +6,7 @@
> >> >  #include "qemu-option.h"
> >> >  #include "qemu-config.h"
> >> >  #include "qobject.h"
> >> > +#include "qstring.h"
> >> >  
> >> >  /* character device */
> >> >  
> >> > @@ -100,6 +101,11 @@ CharDriverState *qemu_chr_open_eventfd(int eventfd);
> >> >  
> >> >  extern int term_escape_char;
> >> >  
> >> > +/* memory chardev */
> >> > +void qemu_chr_init_mem(CharDriverState *chr);
> >> > +void qemu_chr_close_mem(CharDriverState *chr);
> >> > +QString *qemu_chr_mem_to_qs(CharDriverState *chr);
> >> > +
> >> >  /* async I/O support */
> >> >  
> >> >  int qemu_set_fd_handler2(int fd,
> >> 
>