Re: [Qemu-block] [Qemu-devel] [PATCH 1/1] raw-posix.c - use max transfer length / max segemnt count only for SCSI passthrough

[Eric Blake]

On 6/30/19 10:08 AM, Maxim Levitsky wrote:
> Regular block devices (/dev/sda*, /dev/nvme*, etc) interface is not limited

The regular block device interface is

or

Regular block devices interfaces are

> by the underlying storage limits, but rather the kernel block layer
> takes care to split the requests that are too large/fragmented.
> 
> Doing so allows us to have less overhead in qemu.
> 
> Signed-off-by: Maxim Levitsky <address@hidden>
> ---
>  block/file-posix.c | 16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index ab05b51a66..66dad34f8a 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1038,15 +1038,13 @@ static void raw_reopen_abort(BDRVReopenState *state)
>      s->reopen_state = NULL;
>  }
>  
> -static int hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
> +static int sg_get_max_transfer_length(BlockDriverState *bs, int fd)
>  {
>  #ifdef BLKSECTGET
>      int max_bytes = 0;
> -    short max_sectors = 0;
> -    if (bs->sg && ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
> +
> +    if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
>          return max_bytes;
> -    } else if (!bs->sg && ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
> -        return max_sectors << BDRV_SECTOR_BITS;
>      } else {
>          return -errno;
>      }
> @@ -1055,7 +1053,7 @@ static int 
> hdev_get_max_transfer_length(BlockDriverState *bs, int fd)
>  #endif
>  }
>  
> -static int hdev_get_max_segments(const struct stat *st)
> +static int sg_get_max_segments(const struct stat *st)
>  {
>  #ifdef CONFIG_LINUX
>      char buf[32];
> @@ -1106,12 +1104,12 @@ static void raw_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>      struct stat st;
>  
>      if (!fstat(s->fd, &st)) {
> -        if (S_ISBLK(st.st_mode) || S_ISCHR(st.st_mode)) {
> -            int ret = hdev_get_max_transfer_length(bs, s->fd);

Is it worth delaying the fstat()...

> +        if (bs->sg) {
> +            int ret = sg_get_max_transfer_length(bs, s->fd);
>              if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
>                  bs->bl.max_transfer = pow2floor(ret);
>              }
> -            ret = hdev_get_max_segments(&st);
> +            ret = sg_get_max_segments(&st);

...until inside the if (bs->sg) condition, to avoid wasted work for
other scenarios?

>              if (ret > 0) {
>                  bs->bl.max_transfer = MIN(bs->bl.max_transfer,
>                                            ret * getpagesize());
> 

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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