Re: [PATCH v3 09/14] migration: add qemu_get_buffer_at

[Daniel P . Berrangé]

On Fri, Oct 28, 2022 at 01:39:09PM +0300, Nikolay Borisov wrote:
> Restoring a 'fixed-ram' enabled migration stream would require reading
> from specific offsets in the file so add a helper to QEMUFile that uses
> the newly introduced qio_channel_file_preadv.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  migration/qemu-file.c | 23 +++++++++++++++++++++++
>  migration/qemu-file.h |  1 +
>  2 files changed, 24 insertions(+)
> 
> diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> index d0e0ba6150f7..b24972d5728d 100644
> --- a/migration/qemu-file.c
> +++ b/migration/qemu-file.c
> @@ -564,6 +564,29 @@ void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, 
> size_t buflen, off_t po
>      return;
>  }
>  
> +
> +size_t qemu_get_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen, 
> off_t pos)
> +{
> +    Error *err = NULL;
> +    struct iovec iov = { .iov_base = (char *)buf, .iov_len = buflen };
> +    ssize_t ret;
> +
> +    if (f->last_error) {
> +        return 0;
> +    }
> +
> +    ret = qio_channel_io_preadv(f->ioc, &iov, 1, pos, &err);

If we have a qio_channel_io_preadv that does NOT use iovecs,
then this code gets simpler, as the iovec wrapping can be
hidden in the QIOChannel code.

> +    if (ret == -1) {
> +         goto error;
> +    }
> +
> +    return (size_t)ret;
> +
> + error:
> +    qemu_file_set_error_obj(f, -EIO, err);
> +    return 0;
> +}
> +
>  void qemu_set_offset(QEMUFile *f, off_t off, int whence)
>  {
>      Error *err = NULL;
> diff --git a/migration/qemu-file.h b/migration/qemu-file.h
> index 33cfc07b81d1..ab10c3ad7e42 100644
> --- a/migration/qemu-file.h
> +++ b/migration/qemu-file.h
> @@ -151,6 +151,7 @@ void qemu_file_set_blocking(QEMUFile *f, bool block);
>  void qemu_set_offset(QEMUFile *f, off_t off, int whence);
>  off_t qemu_get_offset(QEMUFile *f);
>  void qemu_put_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen, 
> off_t pos);
> +size_t qemu_get_buffer_at(QEMUFile *f, const uint8_t *buf, size_t buflen, 
> off_t pos);
>  
>  void ram_control_before_iterate(QEMUFile *f, uint64_t flags);
>  void ram_control_after_iterate(QEMUFile *f, uint64_t flags);
> -- 
> 2.34.1
> 

With regards,
Daniel
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