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

[Maxim Levitsky <mlevitsk@redhat.com>]

On Wed, 2019-07-03 at 09:50 -0500, Eric Blake wrote:
> 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 <mlevitsk@redhat.com>
> > ---
> >  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 <eblake@redhat.com>
> 

Thank you very much for the review. I'll send a V2 soon.

Best regards,
	Maxim Levitsky