RE: [PATCH] hv: use substraction to update ring buffer index

[Long Li]

> -----Original Message-----
> From: Dan Carpenter [mailto:dan.carpenter@xxxxxxxxxx]
> Sent: Thursday, January 05, 2017 3:40 AM
> To: Long Li <longli@xxxxxxxxxxxxx>
> Cc: KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; devel@xxxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] hv: use substraction to update ring buffer index
> 
> On Wed, Jan 04, 2017 at 08:08:22PM -0800, Long Li wrote:
> > From: Long Li <longli@xxxxxxxxxxxxx>
> >
> > The ring buffer code uses %= to calculate index. For x86/64, %=
> > compiles to div, more than 10 times slower than sub.
> >
> > Replace div with sub for this data heavy code path.
> >
> > Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> > ---
> >  drivers/hv/ring_buffer.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c index
> > cd49cb1..f8eee6e 100644
> > --- a/drivers/hv/ring_buffer.c
> > +++ b/drivers/hv/ring_buffer.c
> > @@ -135,7 +135,8 @@ hv_get_next_readlocation_withoffset(struct
> hv_ring_buffer_info *ring_info,
> >  	u32 next = ring_info->ring_buffer->read_index;
> >
> >  	next += offset;
> > -	next %= ring_info->ring_datasize;
> > +	if (next >= ring_info->ring_datasize)
> > +		next -= ring_info->ring_datasize;
> 
> I take it that we trust that offset is roughly correct and not more than 2x
> ring_info->ring_datasize?  I guess there is only one caller so it's probably
> true...

Yes, you are right. It's not possible that we are getting to 2x ring_datasize, because it's not possible to transfer data more than ring_datasize over ring buffer.

> 
> >
> >  	return next;
> >  }
> > @@ -179,7 +180,8 @@ static u32 hv_copyfrom_ringbuffer(
> >  	memcpy(dest, ring_buffer + start_read_offset, destlen);
> >
> >  	start_read_offset += destlen;
> > -	start_read_offset %= ring_buffer_size;
> > +	if (start_read_offset >= ring_buffer_size)
> > +		start_read_offset -= ring_buffer_size;
> 
> I totally don't understand the original code here.  We do the memset and
> then we verify that we are not copying beyond the end of the ring buffer?  If
> feels like we should verify that offset + destlen aren't more than
> ring_buffer_size before we do the memcpy().

The ring buffer pages are mapped to wraparound 2x virtual address space. Please see hv_ringbuffer_init(). The call to vmap() setup this virtual address space. So we can use memcpy across the last page.

> 
> regards,
> dan carpenter
> 

Thanks for reviewing!

Long