linux-kernel - Re: [PATCH 1/4] vdpa: introduce config op to get valid iova range

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Date:   Mon, 10 Aug 2020 08:05:56 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eli Cohen <eli@...lanox.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, rob.miller@...adcom.com,
        lingshan.zhu@...el.com, eperezma@...hat.com, lulu@...hat.com,
        shahafs@...lanox.com, hanand@...inx.com, mhabets@...arflare.com,
        gdawar@...inx.com, saugatm@...inx.com, vmireyno@...vell.com,
        zhangweining@...jie.com.cn
Subject: Re: [PATCH 1/4] vdpa: introduce config op to get valid iova range

On Thu, Aug 06, 2020 at 03:43:54PM +0300, Eli Cohen wrote:
> On Thu, Aug 06, 2020 at 08:29:22AM -0400, Michael S. Tsirkin wrote:
> > On Thu, Aug 06, 2020 at 03:03:55PM +0300, Eli Cohen wrote:
> > > On Wed, Aug 05, 2020 at 08:51:56AM -0400, Michael S. Tsirkin wrote:
> > > > On Wed, Jun 17, 2020 at 11:29:44AM +0800, Jason Wang wrote:
> > > > > This patch introduce a config op to get valid iova range from the vDPA
> > > > > device.
> > > > > 
> > > > > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > > > > ---
> > > > >  include/linux/vdpa.h | 14 ++++++++++++++
> > > > >  1 file changed, 14 insertions(+)
> > > > > 
> > > > > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> > > > > index 239db794357c..b7633ed2500c 100644
> > > > > --- a/include/linux/vdpa.h
> > > > > +++ b/include/linux/vdpa.h
> > > > > @@ -41,6 +41,16 @@ struct vdpa_device {
> > > > >  	unsigned int index;
> > > > >  };
> > > > >  
> > > > > +/**
> > > > > + * vDPA IOVA range - the IOVA range support by the device
> > > > > + * @start: start of the IOVA range
> > > > > + * @end: end of the IOVA range
> > > > > + */
> > > > > +struct vdpa_iova_range {
> > > > > +	u64 start;
> > > > > +	u64 end;
> > > > > +};
> > > > > +
> > > > 
> > > > 
> > > > This is ambiguous. Is end in the range or just behind it?
> > > > How about first/last?
> > > 
> > > It is customary in the kernel to use start-end where end corresponds to
> > > the byte following the last in the range. See struct vm_area_struct
> > > vm_start and vm_end fields
> > 
> > Exactly my point:
> > 
> > include/linux/mm_types.h:       unsigned long vm_end;           /* The first byte after our end address
> > 
> > in this case Jason wants it to be the last byte, not one behind.
> > 
> > 
> Maybe start, size? Not ambiguous, and you don't need to do annoying
> calculations like size = last - start + 1

Size has a bunch of issues: can overlap, can not cover the entire 64 bit
range. The requisite checks are arguably easier to get wrong than
getting the size if you need it.