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

[Yoshiaki Tamura <tamura.yoshiaki@lab.ntt.co.jp>]

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@lab.ntt.co.jp>
>
> 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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