Re: [RFC patch v1 2/3] qemu-file: add buffered mode

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> 
> 
> On 27.04.2020 15:14, Dr. David Alan Gilbert wrote:
> > * Denis Plotnikov (dplotnikov@virtuozzo.com) wrote:
> > > The patch adds ability to qemu-file to write the data
> > > asynchronously to improve the performance on writing.
> > > Before, only synchronous writing was supported.
> > > 
> > > Enabling of the asyncronous mode is managed by new
> > > "enabled_buffered" callback.
> > It's a bit invasive isn't it - changes a lot of functions in a lot of
> > places!
> 
> If you mean changing the qemu-file code - yes, it is.

Yeh that's what I worry about; qemu-file is pretty complex as it is.
Especially when it then passes it to the channel code etc

> If you mean changing the qemu-file usage in the code - no.
> The only place to change is the snapshot code when the buffered mode is
> enabled with a callback.
> The change is in 03 patch of the series.

That's fine - that's easy.

> > The multifd code separated the control headers from the data on separate
> > fd's - but that doesn't help your case.
> 
> yes, that doesn't help
> > 
> > Is there any chance you could do this by using the existing 'save_page'
> > hook (that RDMA uses).
> 
> I don't think so. My goal is to improve writing performance of
> the internal snapshot to qcow2 image. The snapshot is saved in qcow2 as
> continuous stream placed in the end of address space.
> To achieve the best writing speed I need a size and base-aligned buffer
> containing the vm state (with ram) which looks like that (related to ram):
> 
> ... | ram page header | ram page | ram page header | ram page | ... and so
> on
> 
> to store the buffer in qcow2 with a single operation.
> 
> 'save_page' would allow me not to store 'ram page' in the qemu-file internal
> structures,
> and write my own ram page storing logic. I think that wouldn't help me a lot
> because:
> 1. I need a page with the ram page header
> 2. I want to reduce the number of io operations
> 3. I want to save other parts of vm state as fast as possible
> 
> May be I can't see the better way of using 'save page' callback.
> Could you suggest anything?

I guess it depends if we care about keeping the format of the snapshot
the same here;  if we were open to changing it, then we could use
the save_page hook to delay the writes, so we'd have a pile of headers
followed by a pile of pages.

> Denis
> > In the cover letter you mention direct qemu_fflush calls - have we got a
> > few too many in some palces that you think we can clean out?
> 
> I'm not sure that some of them are excessive. To the best of my knowlege,
> qemu-file is used for the source-destination communication on migration
> and removing some qemu_fflush-es may break communication logic.

I can't see any obvious places where it's called during the ram
migration; can you try and give me a hint to where you're seeing it ?

> Snapshot is just a special case (if not the only) when we know that we can
> do buffered (cached)
> writings. Do you know any other cases when the buffered (cached) mode could
> be useful?

The RDMA code does it because it's really not good at small transfers,
but maybe generally it would be a good idea to do larger writes if
possible - something that multifd manages.

Dave

> 
> > 
> > Dave
> > 
> > > Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
> > > ---
> > >   include/qemu/typedefs.h |   1 +
> > >   migration/qemu-file.c   | 351 +++++++++++++++++++++++++++++++++++++++++++++---
> > >   migration/qemu-file.h   |   9 ++
> > >   3 files changed, 339 insertions(+), 22 deletions(-)
> > > 
> > > diff --git a/include/qemu/typedefs.h b/include/qemu/typedefs.h
> > > index 88dce54..9b388c8 100644
> > > --- a/include/qemu/typedefs.h
> > > +++ b/include/qemu/typedefs.h
> > > @@ -98,6 +98,7 @@ typedef struct QEMUBH QEMUBH;
> > >   typedef struct QemuConsole QemuConsole;
> > >   typedef struct QEMUFile QEMUFile;
> > >   typedef struct QEMUFileBuffer QEMUFileBuffer;
> > > +typedef struct QEMUFileAioTask QEMUFileAioTask;
> > >   typedef struct QemuLockable QemuLockable;
> > >   typedef struct QemuMutex QemuMutex;
> > >   typedef struct QemuOpt QemuOpt;
> > > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > > index 285c6ef..f42f949 100644
> > > --- a/migration/qemu-file.c
> > > +++ b/migration/qemu-file.c
> > > @@ -29,19 +29,25 @@
> > >   #include "qemu-file.h"
> > >   #include "trace.h"
> > >   #include "qapi/error.h"
> > > +#include "block/aio_task.h"
> > > -#define IO_BUF_SIZE 32768
> > > +#define IO_BUF_SIZE (1024 * 1024)
> > >   #define MAX_IOV_SIZE MIN(IOV_MAX, 64)
> > > +#define IO_BUF_NUM 2
> > > +#define IO_BUF_ALIGNMENT 512
> > > -QEMU_BUILD_BUG_ON(!QEMU_IS_ALIGNED(IO_BUF_SIZE, 512));
> > > +QEMU_BUILD_BUG_ON(!QEMU_IS_ALIGNED(IO_BUF_SIZE, IO_BUF_ALIGNMENT));
> > > +QEMU_BUILD_BUG_ON(IO_BUF_SIZE > INT_MAX);
> > > +QEMU_BUILD_BUG_ON(IO_BUF_NUM <= 0);
> > >   struct QEMUFileBuffer {
> > >       int buf_index;
> > > -    int buf_size; /* 0 when writing */
> > > +    int buf_size; /* 0 when non-buffered writing */
> > >       uint8_t *buf;
> > >       unsigned long *may_free;
> > >       struct iovec *iov;
> > >       unsigned int iovcnt;
> > > +    QLIST_ENTRY(QEMUFileBuffer) link;
> > >   };
> > >   struct QEMUFile {
> > > @@ -60,6 +66,22 @@ struct QEMUFile {
> > >       bool shutdown;
> > >       /* currently used buffer */
> > >       QEMUFileBuffer *current_buf;
> > > +    /*
> > > +     * with buffered_mode enabled all the data copied to 512 byte
> > > +     * aligned buffer, including iov data. Then the buffer is passed
> > > +     * to writev_buffer callback.
> > > +     */
> > > +    bool buffered_mode;
> > > +    /* for async buffer writing */
> > > +    AioTaskPool *pool;
> > > +    /* the list of free buffers, currently used on is NOT there */
> > > +    QLIST_HEAD(, QEMUFileBuffer) free_buffers;
> > > +};
> > > +
> > > +struct QEMUFileAioTask {
> > > +    AioTask task;
> > > +    QEMUFile *f;
> > > +    QEMUFileBuffer *fb;
> > >   };
> > >   /*
> > > @@ -115,10 +137,42 @@ QEMUFile *qemu_fopen_ops(void *opaque, const QEMUFileOps *ops)
> > >       f->opaque = opaque;
> > >       f->ops = ops;
> > > -    f->current_buf = g_new0(QEMUFileBuffer, 1);
> > > -    f->current_buf->buf = g_malloc(IO_BUF_SIZE);
> > > -    f->current_buf->iov = g_new0(struct iovec, MAX_IOV_SIZE);
> > > -    f->current_buf->may_free = bitmap_new(MAX_IOV_SIZE);
> > > +    if (f->ops->enable_buffered) {
> > > +        f->buffered_mode = f->ops->enable_buffered(f->opaque);
> > > +    }
> > > +
> > > +    if (f->buffered_mode && qemu_file_is_writable(f)) {
> > > +        int i;
> > > +        /*
> > > +         * in buffered_mode we don't use internal io vectors
> > > +         * and may_free bitmap, because we copy the data to be
> > > +         * written right away to the buffer
> > > +         */
> > > +        f->pool = aio_task_pool_new(IO_BUF_NUM);
> > > +
> > > +        /* allocate io buffers */
> > > +        for (i = 0; i < IO_BUF_NUM; i++) {
> > > +            QEMUFileBuffer *fb = g_new0(QEMUFileBuffer, 1);
> > > +
> > > +            fb->buf = qemu_memalign(IO_BUF_ALIGNMENT, IO_BUF_SIZE);
> > > +            fb->buf_size = IO_BUF_SIZE;
> > > +
> > > +            /*
> > > +             * put the first buffer to the current buf and the rest
> > > +             * to the list of free buffers
> > > +             */
> > > +            if (i == 0) {
> > > +                f->current_buf = fb;
> > > +            } else {
> > > +                QLIST_INSERT_HEAD(&f->free_buffers, fb, link);
> > > +            }
> > > +        }
> > > +    } else {
> > > +        f->current_buf = g_new0(QEMUFileBuffer, 1);
> > > +        f->current_buf->buf = g_malloc(IO_BUF_SIZE);
> > > +        f->current_buf->iov = g_new0(struct iovec, MAX_IOV_SIZE);
> > > +        f->current_buf->may_free = bitmap_new(MAX_IOV_SIZE);
> > > +    }
> > >       return f;
> > >   }
> > > @@ -190,6 +244,8 @@ static void qemu_iovec_release_ram(QEMUFile *f)
> > >       unsigned long idx;
> > >       QEMUFileBuffer *fb = f->current_buf;
> > > +    assert(!f->buffered_mode);
> > > +
> > >       /* Find and release all the contiguous memory ranges marked as may_free. */
> > >       idx = find_next_bit(fb->may_free, fb->iovcnt, 0);
> > >       if (idx >= fb->iovcnt) {
> > > @@ -221,6 +277,147 @@ static void qemu_iovec_release_ram(QEMUFile *f)
> > >       bitmap_zero(fb->may_free, MAX_IOV_SIZE);
> > >   }
> > > +static void advance_buf_ptr(QEMUFile *f, size_t size)
> > > +{
> > > +    QEMUFileBuffer *fb = f->current_buf;
> > > +    /* must not advance to 0 */
> > > +    assert(size);
> > > +    /* must not overflow buf_index (int) */
> > > +    assert(fb->buf_index + size <= INT_MAX);
> > > +    /* must not exceed buf_size */
> > > +    assert(fb->buf_index + size <= fb->buf_size);
> > > +
> > > +    fb->buf_index += size;
> > > +}
> > > +
> > > +static size_t get_buf_free_size(QEMUFile *f)
> > > +{
> > > +    QEMUFileBuffer *fb = f->current_buf;
> > > +    /* buf_index can't be greated than buf_size */
> > > +    assert(fb->buf_size >= fb->buf_index);
> > > +    return fb->buf_size - fb->buf_index;
> > > +}
> > > +
> > > +static size_t get_buf_used_size(QEMUFile *f)
> > > +{
> > > +    QEMUFileBuffer *fb = f->current_buf;
> > > +    return fb->buf_index;
> > > +}
> > > +
> > > +static uint8_t *get_buf_ptr(QEMUFile *f)
> > > +{
> > > +    QEMUFileBuffer *fb = f->current_buf;
> > > +    /* protects from out of bound reading */
> > > +    assert(fb->buf_index <= IO_BUF_SIZE);
> > > +    return fb->buf + fb->buf_index;
> > > +}
> > > +
> > > +static bool buf_is_full(QEMUFile *f)
> > > +{
> > > +    return get_buf_free_size(f) == 0;
> > > +}
> > > +
> > > +static void reset_buf(QEMUFile *f)
> > > +{
> > > +    QEMUFileBuffer *fb = f->current_buf;
> > > +    fb->buf_index = 0;
> > > +}
> > > +
> > > +static int write_task_fn(AioTask *task)
> > > +{
> > > +    int ret;
> > > +    Error *local_error = NULL;
> > > +    QEMUFileAioTask *t = (QEMUFileAioTask *) task;
> > > +    QEMUFile *f = t->f;
> > > +    QEMUFileBuffer *fb = t->fb;
> > > +    uint64_t pos = f->pos;
> > > +    struct iovec v = (struct iovec) {
> > > +        .iov_base = fb->buf,
> > > +        .iov_len = fb->buf_index,
> > > +    };
> > > +
> > > +    assert(f->buffered_mode);
> > > +
> > > +    /*
> > > +     * Increment file position.
> > > +     * This needs to be here before calling writev_buffer, because
> > > +     * writev_buffer is asynchronous and there could be more than one
> > > +     * writev_buffer started simultaniously. Each writev_buffer should
> > > +     * use its own file pos to write to. writev_buffer may write less
> > > +     * than buf_index bytes but we treat this situation as an error.
> > > +     * If error appeared, further file using is meaningless.
> > > +     * We expect that, the most of the time the full buffer is written,
> > > +     * (when buf_size == buf_index). The only case when the non-full
> > > +     * buffer is written (buf_size != buf_index) is file close,
> > > +     * when we need to flush the rest of the buffer content.
> > > +     */
> > > +    f->pos += fb->buf_index;
> > > +
> > > +    ret = f->ops->writev_buffer(f->opaque, &v, 1, pos, &local_error);
> > > +
> > > +    /* return the just written buffer to the free list */
> > > +    QLIST_INSERT_HEAD(&f->free_buffers, fb, link);
> > > +
> > > +    /* check that we have written everything */
> > > +    if (ret != fb->buf_index) {
> > > +        qemu_file_set_error_obj(f, ret < 0 ? ret : -EIO, local_error);
> > > +    }
> > > +
> > > +    /*
> > > +     * always return 0 - don't use task error handling, relay on
> > > +     * qemu file error handling
> > > +     */
> > > +    return 0;
> > > +}
> > > +
> > > +static void qemu_file_switch_current_buf(QEMUFile *f)
> > > +{
> > > +    /*
> > > +     * if the list is empty, wait until some task returns a buffer
> > > +     * to the list of free buffers.
> > > +     */
> > > +    if (QLIST_EMPTY(&f->free_buffers)) {
> > > +        aio_task_pool_wait_slot(f->pool);
> > > +    }
> > > +
> > > +    /*
> > > +     * sanity check that the list isn't empty
> > > +     * if the free list was empty, we waited for a task complition,
> > > +     * and the pompleted task must return a buffer to a list of free buffers
> > > +     */
> > > +    assert(!QLIST_EMPTY(&f->free_buffers));
> > > +
> > > +    /* set the current buffer for using from the free list */
> > > +    f->current_buf = QLIST_FIRST(&f->free_buffers);
> > > +    reset_buf(f);
> > > +
> > > +    QLIST_REMOVE(f->current_buf, link);
> > > +}
> > > +
> > > +/**
> > > + *  Asynchronously flushes QEMUFile buffer
> > > + *
> > > + * This will flush all pending data. If data was only partially flushed, it
> > > + * will set an error state. The function may return before the data actually
> > > + * written.
> > > + */
> > > +static void flush_buffer(QEMUFile *f)
> > > +{
> > > +    QEMUFileAioTask *t = g_new(QEMUFileAioTask, 1);
> > > +
> > > +    *t = (QEMUFileAioTask) {
> > > +        .task.func = &write_task_fn,
> > > +        .f = f,
> > > +        .fb = f->current_buf,
> > > +    };
> > > +
> > > +    /* aio_task_pool should free t for us */
> > > +    aio_task_pool_start_task(f->pool, (AioTask *) t);
> > > +
> > > +    /* if no errors this will switch the buffer */
> > > +    qemu_file_switch_current_buf(f);
> > > +}
> > > +
> > >   /**
> > >    * Flushes QEMUFile buffer
> > >    *
> > > @@ -241,7 +438,13 @@ void qemu_fflush(QEMUFile *f)
> > >       if (f->shutdown) {
> > >           return;
> > >       }
> > > +
> > > +    if (f->buffered_mode) {
> > > +        return;
> > > +    }
> > > +
> > >       if (fb->iovcnt > 0) {
> > > +        /* this is non-buffered mode */
> > >           expect = iov_size(fb->iov, fb->iovcnt);
> > >           ret = f->ops->writev_buffer(f->opaque, fb->iov, fb->iovcnt, f->pos,
> > >                                       &local_error);
> > > @@ -378,6 +581,7 @@ static ssize_t qemu_fill_buffer(QEMUFile *f)
> > >   void qemu_update_position(QEMUFile *f, size_t size)
> > >   {
> > > +    assert(!f->buffered_mode);
> > >       f->pos += size;
> > >   }
> > > @@ -392,7 +596,18 @@ void qemu_update_position(QEMUFile *f, size_t size)
> > >   int qemu_fclose(QEMUFile *f)
> > >   {
> > >       int ret;
> > > -    qemu_fflush(f);
> > > +
> > > +    if (qemu_file_is_writable(f) && f->buffered_mode) {
> > > +        ret = qemu_file_get_error(f);
> > > +        if (!ret) {
> > > +            flush_buffer(f);
> > > +        }
> > > +        /* wait until all tasks are done */
> > > +        aio_task_pool_wait_all(f->pool);
> > > +    } else {
> > > +        qemu_fflush(f);
> > > +    }
> > > +
> > >       ret = qemu_file_get_error(f);
> > >       if (f->ops->close) {
> > > @@ -408,16 +623,77 @@ int qemu_fclose(QEMUFile *f)
> > >           ret = f->last_error;
> > >       }
> > >       error_free(f->last_error_obj);
> > > -    g_free(f->current_buf->buf);
> > > -    g_free(f->current_buf->iov);
> > > -    g_free(f->current_buf->may_free);
> > > -    g_free(f->current_buf);
> > > +
> > > +    if (f->buffered_mode) {
> > > +        QEMUFileBuffer *fb, *next;
> > > +        /*
> > > +         * put the current back to the free buffers list
> > > +         * to destroy all the buffers in one loop
> > > +         */
> > > +        QLIST_INSERT_HEAD(&f->free_buffers, f->current_buf, link);
> > > +
> > > +        /* destroy all the buffers */
> > > +        QLIST_FOREACH_SAFE(fb, &f->free_buffers, link, next) {
> > > +            QLIST_REMOVE(fb, link);
> > > +            /* looks like qemu_vfree pairs with qemu_memalign */
> > > +            qemu_vfree(fb->buf);
> > > +            g_free(fb);
> > > +        }
> > > +        g_free(f->pool);
> > > +    } else {
> > > +        g_free(f->current_buf->buf);
> > > +        g_free(f->current_buf->iov);
> > > +        g_free(f->current_buf->may_free);
> > > +        g_free(f->current_buf);
> > > +    }
> > > +
> > >       g_free(f);
> > >       trace_qemu_file_fclose();
> > >       return ret;
> > >   }
> > >   /*
> > > + * Copy an external buffer to the intenal current buffer.
> > > + */
> > > +static void copy_buf(QEMUFile *f, const uint8_t *buf, size_t size,
> > > +                     bool may_free)
> > > +{
> > > +    size_t data_size = size;
> > > +    const uint8_t *src_ptr = buf;
> > > +
> > > +    assert(f->buffered_mode);
> > > +    assert(size <= INT_MAX);
> > > +
> > > +    while (data_size > 0) {
> > > +        size_t chunk_size;
> > > +
> > > +        if (buf_is_full(f)) {
> > > +            flush_buffer(f);
> > > +            if (qemu_file_get_error(f)) {
> > > +                return;
> > > +            }
> > > +        }
> > > +
> > > +        chunk_size = MIN(get_buf_free_size(f), data_size);
> > > +
> > > +        memcpy(get_buf_ptr(f), src_ptr, chunk_size);
> > > +
> > > +        advance_buf_ptr(f, chunk_size);
> > > +
> > > +        src_ptr += chunk_size;
> > > +        data_size -= chunk_size;
> > > +        f->bytes_xfer += chunk_size;
> > > +    }
> > > +
> > > +    if (may_free) {
> > > +        if (qemu_madvise((void *) buf, size, QEMU_MADV_DONTNEED) < 0) {
> > > +            error_report("migrate: madvise DONTNEED failed %p %zd: %s",
> > > +                         buf, size, strerror(errno));
> > > +        }
> > > +    }
> > > +}
> > > +
> > > +/*
> > >    * Add buf to iovec. Do flush if iovec is full.
> > >    *
> > >    * Return values:
> > > @@ -454,6 +730,9 @@ static int add_to_iovec(QEMUFile *f, const uint8_t *buf, size_t size,
> > >   static void add_buf_to_iovec(QEMUFile *f, size_t len)
> > >   {
> > >       QEMUFileBuffer *fb = f->current_buf;
> > > +
> > > +    assert(!f->buffered_mode);
> > > +
> > >       if (!add_to_iovec(f, fb->buf + fb->buf_index, len, false)) {
> > >           fb->buf_index += len;
> > >           if (fb->buf_index == IO_BUF_SIZE) {
> > > @@ -469,8 +748,12 @@ void qemu_put_buffer_async(QEMUFile *f, const uint8_t *buf, size_t size,
> > >           return;
> > >       }
> > > -    f->bytes_xfer += size;
> > > -    add_to_iovec(f, buf, size, may_free);
> > > +    if (f->buffered_mode) {
> > > +        copy_buf(f, buf, size, may_free);
> > > +    } else {
> > > +        f->bytes_xfer += size;
> > > +        add_to_iovec(f, buf, size, may_free);
> > > +    }
> > >   }
> > >   void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> > > @@ -482,6 +765,11 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, size_t size)
> > >           return;
> > >       }
> > > +    if (f->buffered_mode) {
> > > +        copy_buf(f, buf, size, false);
> > > +        return;
> > > +    }
> > > +
> > >       while (size > 0) {
> > >           l = IO_BUF_SIZE - fb->buf_index;
> > >           if (l > size) {
> > > @@ -506,15 +794,21 @@ void qemu_put_byte(QEMUFile *f, int v)
> > >           return;
> > >       }
> > > -    fb->buf[fb->buf_index] = v;
> > > -    f->bytes_xfer++;
> > > -    add_buf_to_iovec(f, 1);
> > > +    if (f->buffered_mode) {
> > > +        copy_buf(f, (const uint8_t *) &v, 1, false);
> > > +    } else {
> > > +        fb->buf[fb->buf_index] = v;
> > > +        add_buf_to_iovec(f, 1);
> > > +        f->bytes_xfer++;
> > > +    }
> > >   }
> > >   void qemu_file_skip(QEMUFile *f, int size)
> > >   {
> > >       QEMUFileBuffer *fb = f->current_buf;
> > > +    assert(!f->buffered_mode);
> > > +
> > >       if (fb->buf_index + size <= fb->buf_size) {
> > >           fb->buf_index += size;
> > >       }
> > > @@ -672,10 +966,14 @@ int64_t qemu_ftell_fast(QEMUFile *f)
> > >   {
> > >       int64_t ret = f->pos;
> > >       int i;
> > > -    QEMUFileBuffer *fb = f->current_buf;
> > > -    for (i = 0; i < fb->iovcnt; i++) {
> > > -        ret += fb->iov[i].iov_len;
> > > +    if (f->buffered_mode) {
> > > +        ret += get_buf_used_size(f);
> > > +    } else {
> > > +        QEMUFileBuffer *fb = f->current_buf;
> > > +        for (i = 0; i < fb->iovcnt; i++) {
> > > +            ret += fb->iov[i].iov_len;
> > > +        }
> > >       }
> > >       return ret;
> > > @@ -683,8 +981,12 @@ int64_t qemu_ftell_fast(QEMUFile *f)
> > >   int64_t qemu_ftell(QEMUFile *f)
> > >   {
> > > -    qemu_fflush(f);
> > > -    return f->pos;
> > > +    if (f->buffered_mode) {
> > > +        return qemu_ftell_fast(f);
> > > +    } else {
> > > +        qemu_fflush(f);
> > > +        return f->pos;
> > > +    }
> > >   }
> > >   int qemu_file_rate_limit(QEMUFile *f)
> > > @@ -803,6 +1105,8 @@ ssize_t qemu_put_compression_data(QEMUFile *f, z_stream *stream,
> > >       QEMUFileBuffer *fb = f->current_buf;
> > >       ssize_t blen = IO_BUF_SIZE - fb->buf_index - sizeof(int32_t);
> > > +    assert(!f->buffered_mode);
> > > +
> > >       if (blen < compressBound(size)) {
> > >           return -1;
> > >       }
> > > @@ -827,6 +1131,9 @@ int qemu_put_qemu_file(QEMUFile *f_des, QEMUFile *f_src)
> > >       int len = 0;
> > >       QEMUFileBuffer *fb_src = f_src->current_buf;
> > > +    assert(!f_des->buffered_mode);
> > > +    assert(!f_src->buffered_mode);
> > > +
> > >       if (fb_src->buf_index > 0) {
> > >           len = fb_src->buf_index;
> > >           qemu_put_buffer(f_des, fb_src->buf, fb_src->buf_index);
> > > diff --git a/migration/qemu-file.h b/migration/qemu-file.h
> > > index a9b6d6c..08655d2 100644
> > > --- a/migration/qemu-file.h
> > > +++ b/migration/qemu-file.h
> > > @@ -103,6 +103,14 @@ typedef QEMUFile *(QEMURetPathFunc)(void *opaque);
> > >   typedef int (QEMUFileShutdownFunc)(void *opaque, bool rd, bool wr,
> > >                                      Error **errp);
> > > +/*
> > > + * Enables or disables the buffered mode
> > > + * Existing blocking reads/writes must be woken
> > > + * Returns true if the buffered mode has to be enabled,
> > > + * false if it has to be disabled.
> > > + */
> > > +typedef bool (QEMUFileEnableBufferedFunc)(void *opaque);
> > > +
> > >   typedef struct QEMUFileOps {
> > >       QEMUFileGetBufferFunc *get_buffer;
> > >       QEMUFileCloseFunc *close;
> > > @@ -110,6 +118,7 @@ typedef struct QEMUFileOps {
> > >       QEMUFileWritevBufferFunc *writev_buffer;
> > >       QEMURetPathFunc *get_return_path;
> > >       QEMUFileShutdownFunc *shut_down;
> > > +    QEMUFileEnableBufferedFunc *enable_buffered;
> > >   } QEMUFileOps;
> > >   typedef struct QEMUFileHooks {
> > > -- 
> > > 1.8.3.1
> > > 
> > --
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK