[Intel-gfx] [Beignet] Preventing zero GPU virtual address allocation

[Zou, Nanhai]

We don't need MAP_FIXED, we just want to avoid address 0 to be allocated.

Though I think using MAP_FIXED is overkill, will bring much unnecessary complexity on both kernel and beignet side.
I don't mind if people can provide stable MAP_FIXED patches to resolve this problem a few months or years later.

At that time, kernel driver can revert the reserve page 0 patch.
Before that reserve page 0 can benefit all the Beignet user without breaking anything.

As I know, on CPU side, many arches with flexible address space like IA64 have reserved virtual page 0 to address this problem,
I don't see why this is non sense.

Thanks
Zou Nanhai



> -----Original Message-----
> From: Chris Wilson [mailto:chris at chris-wilson.co.uk]
> Sent: Friday, March 06, 2015 4:39 PM
> To: Zou, Nanhai
> Cc: Daniel Vetter; Song, Ruiling; Vetter, Daniel; intel-gfx at lists.freedesktop.org;
> Yang, Rong R; beignet at lists.freedesktop.org; Weinehall, David
> Subject: Re: [Beignet] [Intel-gfx] Preventing zero GPU virtual address allocation
> 
> On Fri, Mar 06, 2015 at 02:11:18AM +0000, Zou, Nanhai wrote:
> > I don't understand why we need a complex solution when there is already a
> simple solution with patch.
> > What is the drawback of reserving page 0?
> > Before we going to that complex solution, could we just reserve page zero?
> > It is simple and straight forward.
> 
> Because it is a nonsense ABI constraint. If you want the equivalent of
> MAP_FIXED, we should give you MAP_FIXED.
> -Chris
> 
> --
> Chris Wilson, Intel Open Source Technology Centre