Re: [PATCH v4 2/2] file-posix: add sg_get_max_segments that actually works with sg

[Tom Yan]

In commit 867eccfed84f96b54f4a432c510a02c2ce03b430, Levitsky appears
to have assumed that the only "SCSI Passthrough" is `-device
scsi-generic`, while the fact is there's also `-device scsi-block`
(passthrough without the sg driver). Unlike `-device scsi-hd`, getting
max_sectors is necessary to it (more precisely, hw_max_sectors might
what matters, but BLKSECTGET reports max_sectors, so).

I'm unsure about how qemu-nbd works, but the commit clearly wasn't the
right approach to fix the original issue it addresses. (It should for
example ignore the max_transfer if it will never matter in to it, or
overrides it in certain cases; when I glimpsed over
https://bugzilla.redhat.com/show_bug.cgi?id=1647104, I don't see how
it could be file-posix problem when it is reporting the right thing,
regardless of whether "removing" the code helps.)

I don't think we want to "mark" `-device scsi-block` as sg either. It
will probably bring even more unexpected problems, because they are
literally different sets of things behind the scene / in the kernel.

On Fri, 4 Sep 2020 at 10:09, Tom Yan <tom.ty89@gmail.com> wrote:
>
> sg devices have different major/minor than their corresponding
> block devices. Using sysfs to get max segments never really worked
> for them.
>
> Fortunately the sg driver provides an ioctl to get sg_tablesize,
> which is apparently equivalent to max segments.
>
> Signed-off-by: Tom Yan <tom.ty89@gmail.com>
> ---
>  block/file-posix.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/block/file-posix.c b/block/file-posix.c
> index 411a23cf99..9e37594145 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1178,6 +1178,21 @@ static int sg_get_max_transfer_length(int fd)
>  #endif
>  }
>
> +static int sg_get_max_segments(int fd)
> +{
> +#ifdef SG_GET_SG_TABLESIZE
> +    long max_segments = 0;
> +
> +    if (ioctl(fd, SG_GET_SG_TABLESIZE, &max_segments) == 0) {
> +        return max_segments;
> +    } else {
> +        return -errno;
> +    }
> +#else
> +    return -ENOSYS;
> +#endif
> +}
> +
>  static int get_max_transfer_length(int fd)
>  {
>  #if defined(BLKSECTGET) && defined(BLKSSZGET)
> @@ -1268,7 +1283,7 @@ static void hdev_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>          bs->bl.max_transfer = pow2floor(ret);
>      }
>
> -    ret = get_max_segments(s->fd);
> +    ret = bs->sg ? sg_get_max_segments(s->fd) : 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);
> --
> 2.28.0
>