Re: [PATCH] drm/i915/gen9: Set PIN_ZONE_4G end to 4GB - 1 page

[Chris Wilson <chris@chris-wilson.co.uk>]

On Mon, Jan 11, 2016 at 11:39:27AM +0000, Michel Thierry wrote:
> Kernel and userspace are able to handle 4GB (1<<32) address space range,
> but "A32 Stateless Model" is not. According to documentation, A32 accesses
> are based on General State Base Address and bound checking is in place.
> Because size field (instruction State Base Address) limitation, it is not
> possible to address full 4GB memory region.
> 
> A32 Stateless Model is used by some libraries and without this patch, the
> last page of 4GB address space is not accessible in 32bit processes.
> 
> Reported-by: Artur Harasimiuk <artur.harasimiuk@intel.com>
> Signed-off-by: Michel Thierry <michel.thierry@intel.com>

That seems reasonably in line with the flag, and a caveat I think we
live with (GGTT which is already 4G max can't use the last page, even if
we hadn't already given it away).

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 6c60e04..ddc21d4 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -3488,7 +3488,7 @@ i915_gem_object_bind_to_vm(struct drm_i915_gem_object *obj,
>  	if (flags & PIN_MAPPABLE)
>  		end = min_t(u64, end, dev_priv->gtt.mappable_end);
>  	if (flags & PIN_ZONE_4G)
> -		end = min_t(u64, end, (1ULL << 32));
> +		end = min_t(u64, end, (1ULL << 32) - PAGE_SIZE);

Bah, I came here expecting to find a mistake with brackets and u64
promotion. Spoilsport.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre
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