[v4,4/7] file-posix: try BLKSECTGET on block devices too, do not round to power of 2

bs->sg is only true for character devices, but block devices can also
be used with scsi-block and scsi-generic.  Unfortunately BLKSECTGET
returns bytes in an int for /dev/sgN devices, and sectors in a short
for block devices, so account for that in the code.

The maximum transfer also need not be a power of 2 (for example I have
seen disks with 1280 KiB maximum transfer) so there's no need to pass
the result through pow2floor.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 block/file-posix.c | 44 ++++++++++++++++++++++++--------------------
 1 file changed, 24 insertions(+), 20 deletions(-)

On Tue, Jun 08, 2021 at 03:16:31PM +0200, Paolo Bonzini wrote:
> bs->sg is only true for character devices, but block devices can also
> be used with scsi-block and scsi-generic.  Unfortunately BLKSECTGET
> returns bytes in an int for /dev/sgN devices, and sectors in a short
> for block devices, so account for that in the code.

Gotta love inconsistent and poorly-documented kernel interfaces! (on my
system, 'man -k BLKSECTGET' had no hits)

> 
> The maximum transfer also need not be a power of 2 (for example I have
> seen disks with 1280 KiB maximum transfer) so there's no need to pass
> the result through pow2floor.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/file-posix.c | 44 ++++++++++++++++++++++++--------------------
>  1 file changed, 24 insertions(+), 20 deletions(-)
>

Reviewed-by: Eric Blake <eblake@redhat.com>

On Tue, 2021-06-08 at 15:16 +0200, Paolo Bonzini wrote:
> bs->sg is only true for character devices, but block devices can also
> be used with scsi-block and scsi-generic.  Unfortunately BLKSECTGET
> returns bytes in an int for /dev/sgN devices, and sectors in a short
> for block devices, so account for that in the code.
> 
> The maximum transfer also need not be a power of 2 (for example I have
> seen disks with 1280 KiB maximum transfer) so there's no need to pass
> the result through pow2floor.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  block/file-posix.c | 44 ++++++++++++++++++++++++--------------------
>  1 file changed, 24 insertions(+), 20 deletions(-)
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index c9746d3eb6..1439293f63 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -1149,22 +1149,27 @@ static void raw_reopen_abort(BDRVReopenState *state)
>      s->reopen_state = NULL;
>  }
>  
> -static int sg_get_max_transfer_length(int fd)
> +static int hdev_get_max_hw_transfer(int fd, struct stat *st)
>  {
>  #ifdef BLKSECTGET
> -    int max_bytes = 0;
> -
> -    if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {
> -        return max_bytes;
> +    if (S_ISBLK(st->st_mode)) {
> +        unsigned short max_sectors = 0;
> +        if (ioctl(fd, BLKSECTGET, &max_sectors) == 0) {
> +            return max_sectors * 512;
> +        }
>      } else {
> -        return -errno;
> +        int max_bytes = 0;
> +        if (ioctl(fd, BLKSECTGET, &max_bytes) == 0) {

Again I would use the bs->sg for that.

> +            return max_bytes;
> +        }
>      }
> +    return -errno;
>  #else
>      return -ENOSYS;
>  #endif
>  }
>  
> -static int sg_get_max_segments(int fd)
> +static int hdev_get_max_segments(int fd, struct stat *st)
>  {
>  #ifdef CONFIG_LINUX
>      char buf[32];
> @@ -1173,26 +1178,20 @@ static int sg_get_max_segments(int fd)
>      int ret;
>      int sysfd = -1;
>      long max_segments;
> -    struct stat st;
>  
> -    if (fstat(fd, &st)) {
> -        ret = -errno;
> -        goto out;
> -    }
> -
> -    if (S_ISCHR(st.st_mode)) {
> +    if (S_ISCHR(st->st_mode)) {
>          if (ioctl(fd, SG_GET_SG_TABLESIZE, &ret) == 0) {
>              return ret;
>          }
>          return -ENOTSUP;
>      }
>  
> -    if (!S_ISBLK(st.st_mode)) {
> +    if (!S_ISBLK(st->st_mode)) {
>          return -ENOTSUP;
>      }
>  
>      sysfspath = g_strdup_printf("/sys/dev/block/%u:%u/queue/max_segments",
> -                                major(st.st_rdev), minor(st.st_rdev));
> +                                major(st->st_rdev), minor(st->st_rdev));
>      sysfd = open(sysfspath, O_RDONLY);
>      if (sysfd == -1) {
>          ret = -errno;
> @@ -1229,15 +1228,20 @@ out:
>  static void raw_refresh_limits(BlockDriverState *bs, Error **errp)
>  {
>      BDRVRawState *s = bs->opaque;
> +    struct stat st;
> +
> +    if (fstat(s->fd, &st)) {
> +        return;
> +    }
>  
> -    if (bs->sg) {
> -        int ret = sg_get_max_transfer_length(s->fd);
> +    if (bs->sg || S_ISBLK(st.st_mode)) {
> +        int ret = hdev_get_max_hw_transfer(s->fd, &st);
>  
>          if (ret > 0 && ret <= BDRV_REQUEST_MAX_BYTES) {
> -            bs->bl.max_hw_transfer = pow2floor(ret);
> +            bs->bl.max_hw_transfer = ret;
>          }
>  
> -        ret = sg_get_max_segments(s->fd);
> +        ret = hdev_get_max_segments(s->fd, &st);
>          if (ret > 0) {
>              bs->bl.max_iov = ret;
>          }


Roughly speaking this looks correct, but I might have missed something as well.

This is roughly the same as patches from Tom Yan which I carried in my series

https://www.mail-archive.com/qemu-devel@nongnu.org/msg768258.html
https://www.mail-archive.com/qemu-devel@nongnu.org/msg768262.html


I like a bit more how he created separate functions for /dev/sg and for all other block devices.
Please take a look.

Also not related to this patch, you are missing my fix I did to the VPD limit emulation, please consider taking
it into the series:

https://www.mail-archive.com/qemu-devel@nongnu.org/msg768260.html


Best regards,
	Maxim Levitsky