Re: [PATCH 5/9] migration/qemu-file: Add qemu_file_get_to_fd()

[Avihai Horon]

On 5/16/2022 2:31 PM, Juan Quintela wrote:
> External email: Use caution opening links or attachments
>
>
> Avihai Horon <avihaih@nvidia.com> wrote:
> > Add new function qemu_file_get_to_fd() that allows reading data from
> > QEMUFile and writing it straight into a given fd.
> >
> > This will be used later in VFIO migration code.
> >
> > Signed-off-by: Avihai Horon <avihaih@nvidia.com>
> > ---
> >   migration/qemu-file.c | 34 ++++++++++++++++++++++++++++++++++
> >   migration/qemu-file.h |  1 +
> >   2 files changed, 35 insertions(+)
> >
> > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > index 1479cddad9..cad3d32eb3 100644
> > --- a/migration/qemu-file.c
> > +++ b/migration/qemu-file.c
> > @@ -867,3 +867,37 @@ QIOChannel *qemu_file_get_ioc(QEMUFile *file)
> >   {
> >       return file->has_ioc ? QIO_CHANNEL(file->opaque) : NULL;
> >   }
> > +
> > +/*
> > + * Read size bytes from QEMUFile f and write them to fd.
> > + */
> > +int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size)
> > +{
> > +    while (size) {
> > +        size_t pending = f->buf_size - f->buf_index;
> > +        ssize_t rc;
> > +
> > +        if (!pending) {
> > +            rc = qemu_fill_buffer(f);
> > +            if (rc < 0) {
> > +                return rc;
> > +            }
> > +            if (rc == 0) {
> > +                return -1;
> > +            }
> > +            continue;
> > +        }
> > +
> > +        rc = write(fd, f->buf + f->buf_index, MIN(pending, size));
> > +        if (rc < 0) {
> > +            return rc;
> > +        }
> > +        if (rc == 0) {
> > +            return -1;
> > +        }
> > +        f->buf_index += rc;
> > +        size -= rc;
> > +    }
> > +
> > +    return 0;
> > +}
> Is there a really performance difference to just use:
>
> uint8_t buffer[size];
>
> qemu_get_buffer(f, buffer, size);
> write(fd, buffer, size);
>
> Or telling it otherwise, what sizes are we talking here?

It depends on the device, but It can range from a few MBs to several GBs 
AFAIK.

Thanks.

> Thanks, Juan.
>
>
> > diff --git a/migration/qemu-file.h b/migration/qemu-file.h
> > index 3f36d4dc8c..dd26037450 100644
> > --- a/migration/qemu-file.h
> > +++ b/migration/qemu-file.h
> > @@ -162,6 +162,7 @@ int qemu_file_shutdown(QEMUFile *f);
> >   QEMUFile *qemu_file_get_return_path(QEMUFile *f);
> >   void qemu_fflush(QEMUFile *f);
> >   void qemu_file_set_blocking(QEMUFile *f, bool block);
> > +int qemu_file_get_to_fd(QEMUFile *f, int fd, size_t size);
> >
> >   void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
> >   void ram_control_after_iterate(QEMUFile *f, uint64_t flags);