Re: [RFC PATCH 07/20] Introduce qemu_put_vector() and qemu_put_vector_prepare() to use put_vector() in QEMUFile.

[Yoshiaki Tamura <tamura.yoshiaki@xxxxxxxxxxxxx>]

Anthony Liguori wrote:
> On 04/21/2010 12:57 AM, Yoshiaki Tamura wrote:
> > For fool proof purpose, qemu_put_vector_parepare should be called
> > before qemu_put_vector. Then, if qemu_put_* functions except this is
> > called after qemu_put_vector_prepare, program will abort().
> >
> > Signed-off-by: Yoshiaki Tamura<tamura.yoshiaki@xxxxxxxxxxxxx>
>
> I don't get it. What's this protecting against?

This was introduced to prevent mixing the order of normal write and vector 
write, and flush QEMUFile buffer before handling vectors.
While qemu_put_buffer copies data to QEMUFile buffer, qemu_put_vector() will 
bypass that buffer.

It's just fool proof purpose for what we encountered at beginning, and if the 
user of qemu_put_vector() is careful enough, we can remove 
qemu_put_vectore_prepare().  While writing this message, I started to think that 
just calling qemu_fflush() in qemu_put_vector() would be enough...

> Regards,
>
> Anthony Liguori
>
> > ---
> > hw/hw.h | 2 ++
> > savevm.c | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> > 2 files changed, 55 insertions(+), 0 deletions(-)
> >
> > diff --git a/hw/hw.h b/hw/hw.h
> > index 921cf90..10e6dda 100644
> > --- a/hw/hw.h
> > +++ b/hw/hw.h
> > @@ -77,6 +77,8 @@ void qemu_fflush(QEMUFile *f);
> > int qemu_fclose(QEMUFile *f);
> > void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, int size);
> > void qemu_put_byte(QEMUFile *f, int v);
> > +void qemu_put_vector(QEMUFile *f, QEMUIOVector *qiov);
> > +void qemu_put_vector_prepare(QEMUFile *f);
> > void *qemu_realloc_buffer(QEMUFile *f, int size);
> > void qemu_clear_buffer(QEMUFile *f);
> >
> > diff --git a/savevm.c b/savevm.c
> > index 944e788..22d928c 100644
> > --- a/savevm.c
> > +++ b/savevm.c
> > @@ -180,6 +180,7 @@ struct QEMUFile {
> > uint8_t *buf;
> >
> > int has_error;
> > + int prepares_vector;
> > };
> >
> > typedef struct QEMUFileStdio
> > @@ -557,6 +558,58 @@ void qemu_put_byte(QEMUFile *f, int v)
> > qemu_fflush(f);
> > }
> >
> > +void qemu_put_vector(QEMUFile *f, QEMUIOVector *v)
> > +{
> > + struct iovec *iov;
> > + int cnt;
> > + size_t bufsize;
> > + uint8_t *buf;
> > +
> > + if (qemu_file_get_rate_limit(f) != 0) {
> > + fprintf(stderr,
> > + "Attempted to write vector while bandwidth limit is not zero.\n");
> > + abort();
> > + }
> > +
> > + /* checks prepares vector.
> > + * For fool proof purpose, qemu_put_vector_parepare should be called
> > + * before qemu_put_vector. Then, if qemu_put_* functions except this
> > + * is called after qemu_put_vector_prepare, program will abort().
> > + */
> > + if (!f->prepares_vector) {
> > + fprintf(stderr,
> > + "You should prepare with qemu_put_vector_prepare.\n");
> > + abort();
> > + } else if (f->prepares_vector&& f->buf_index != 0) {
> > + fprintf(stderr, "Wrote data after qemu_put_vector_prepare.\n");
> > + abort();
> > + }
> > + f->prepares_vector = 0;
> > +
> > + if (f->put_vector) {
> > + qemu_iovec_to_vector(v,&iov,&cnt);
> > + f->put_vector(f->opaque, iov, 0, cnt);
> > + } else {
> > + qemu_iovec_to_size(v,&bufsize);
> > + buf = qemu_malloc(bufsize + 1 /* for '\0' */);
> > + qemu_iovec_to_buffer(v, buf);
> > + qemu_put_buffer(f, buf, bufsize);
> > + qemu_free(buf);
> > + }
> > +
> > +}
> > +
> > +void qemu_put_vector_prepare(QEMUFile *f)
> > +{
> > + if (f->prepares_vector) {
> > + /* prepare vector */
> > + fprintf(stderr, "Attempted to prepare vector twice\n");
> > + abort();
> > + }
> > + f->prepares_vector = 1;
> > + qemu_fflush(f);
> > +}
> > +
> > int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size1)
> > {
> > int size, l;



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