Re: [PATCH 1/5] file-posix: split hdev_refresh_limits from raw_refresh_limits

[Tom Yan]

Actually I made a mistake in this. BLKSECTGET (the one in the block
layer) returns the number of "sectors", which is "defined" as 512-byte
block. So we shouldn't use BLKSSZGET here, but simply 512 (1 << 9).
See logical_to_sectors() in sd.h of the kernel.

On Thu, 5 Nov 2020 at 01:32, Maxim Levitsky <mlevitsk@redhat.com> wrote:
>
> From: Tom Yan <tom.ty89@gmail.com>
>
> We can and should get max transfer length and max segments for all host
> devices / cdroms (on Linux).
>
> Also use MIN_NON_ZERO instead when we clamp max transfer length against
> max segments.
>
> Signed-off-by: Tom Yan <tom.ty89@gmail.com>
> Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
> ---
>  block/file-posix.c | 61 ++++++++++++++++++++++++++++++++++------------
>  1 file changed, 45 insertions(+), 16 deletions(-)
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index c63926d592..6581f41b2b 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1162,6 +1162,12 @@ static void raw_reopen_abort(BDRVReopenState *state)
>
>  static int sg_get_max_transfer_length(int fd)
>  {
> +    /*
> +     * BLKSECTGET for /dev/sg* character devices incorrectly returns
> +     * the max transfer size in bytes (rather than in blocks).
> +     * Also note that /dev/sg* doesn't support BLKSSZGET ioctl.
> +     */
> +
>  #ifdef BLKSECTGET
>      int max_bytes = 0;
>
> @@ -1175,7 +1181,24 @@ static int sg_get_max_transfer_length(int fd)
>  #endif
>  }
>
> -static int sg_get_max_segments(int fd)
> +static int get_max_transfer_length(int fd)
> +{
> +#if defined(BLKSECTGET) && defined(BLKSSZGET)
> +    int sect = 0;
> +    int ssz = 0;
> +
> +    if (ioctl(fd, BLKSECTGET, &sect) == 0 &&
> +        ioctl(fd, BLKSSZGET, &ssz) == 0) {
> +        return sect * ssz;
> +    } else {
> +        return -errno;
> +    }
> +#else
> +    return -ENOSYS;
> +#endif
> +}
> +
> +static int get_max_segments(int fd)
>  {
>  #ifdef CONFIG_LINUX
>      char buf[32];
> @@ -1230,23 +1253,29 @@ static void raw_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>  {
>      BDRVRawState *s = bs->opaque;
>
> -    if (bs->sg) {
> -        int ret = sg_get_max_transfer_length(s->fd);
> +    raw_probe_alignment(bs, s->fd, errp);
> +    bs->bl.min_mem_alignment = s->buf_align;
> +    bs->bl.opt_mem_alignment = MAX(s->buf_align, qemu_real_host_page_size);
> +}
>
> -        if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
> -            bs->bl.max_transfer = pow2floor(ret);
> -        }
> +static void hdev_refresh_limits(BlockDriverState *bs, Error **errp)
> +{
> +    BDRVRawState *s = bs->opaque;
>
> -        ret = sg_get_max_segments(s->fd);
> -        if (ret > 0) {
> -            bs->bl.max_transfer = MIN(bs->bl.max_transfer,
> -                                      ret * qemu_real_host_page_size);
> -        }
> +    int ret = bs->sg ? sg_get_max_transfer_length(s->fd) :
> +                       get_max_transfer_length(s->fd);
> +
> +    if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
> +        bs->bl.max_transfer = pow2floor(ret);
>      }
>
> -    raw_probe_alignment(bs, s->fd, errp);
> -    bs->bl.min_mem_alignment = s->buf_align;
> -    bs->bl.opt_mem_alignment = MAX(s->buf_align, qemu_real_host_page_size);
> +    ret = get_max_segments(s->fd);
> +    if (ret > 0) {
> +        bs->bl.max_transfer = MIN_NON_ZERO(bs->bl.max_transfer,
> +                                           ret * qemu_real_host_page_size);
> +    }
> +
> +    raw_refresh_limits(bs, errp);
>  }
>
>  static int check_for_dasd(int fd)
> @@ -3600,7 +3629,7 @@ static BlockDriver bdrv_host_device = {
>      .bdrv_co_pdiscard       = hdev_co_pdiscard,
>      .bdrv_co_copy_range_from = raw_co_copy_range_from,
>      .bdrv_co_copy_range_to  = raw_co_copy_range_to,
> -    .bdrv_refresh_limits = raw_refresh_limits,
> +    .bdrv_refresh_limits = hdev_refresh_limits,
>      .bdrv_io_plug = raw_aio_plug,
>      .bdrv_io_unplug = raw_aio_unplug,
>      .bdrv_attach_aio_context = raw_aio_attach_aio_context,
> @@ -3724,7 +3753,7 @@ static BlockDriver bdrv_host_cdrom = {
>      .bdrv_co_preadv         = raw_co_preadv,
>      .bdrv_co_pwritev        = raw_co_pwritev,
>      .bdrv_co_flush_to_disk  = raw_co_flush_to_disk,
> -    .bdrv_refresh_limits = raw_refresh_limits,
> +    .bdrv_refresh_limits = hdev_refresh_limits,
>      .bdrv_io_plug = raw_aio_plug,
>      .bdrv_io_unplug = raw_aio_unplug,
>      .bdrv_attach_aio_context = raw_aio_attach_aio_context,
> --
> 2.26.2
>