Re: [PATCH v4 3/4] drm/xe: Assert that VF will never use fixed placement of BOs

[Michal Wajdeczko <michal.wajdeczko@intel.com>]

On 10/20/2025 10:58 PM, Tomasz Lis wrote:
> Most BOs do not care at which offset they will be accessed within
> GGTT or PPGTT. The few which do care, should be only created
> on PF, and mapped within GGTT. On VFs, mapping at fixed offset
> would be problematic, as each VF is granted access to a range of
> GGTT address space.

it's not a matter of being "problematic" but this is just prohibited
as there is no assumption which GGTT range will be assigned to the VF

> 
> So, since fixed addresses of GGTT mapping can only be used on PF,
> we can add an assert which makes sure no attempt of fixed placement
> will happen for a driver probed on a VF.
> 
> The assert will also ensure that VF migration can be properly
> performed without a need for special handling of the fixed placement
> addresses.

this last sentence can be dropped

> 
> Signed-off-by: Tomasz Lis <tomasz.lis@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_bo.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
> index 7b6502081873..8e826a4aa574 100644
> --- a/drivers/gpu/drm/xe/xe_bo.c
> +++ b/drivers/gpu/drm/xe/xe_bo.c
> @@ -2259,6 +2259,12 @@ static int __xe_bo_fixed_placement(struct xe_device *xe,
>  	struct ttm_place *place = bo->placements;
>  	u32 vram_flag, vram_stolen_flags;
>  
> +	/*
> +	 * to allow fixed placement in GGTT of a VF, post-migration fixups
> +	 * would have to include shifting the page ranges

it's not about implementation of fixups, this is just not allowed

> +	 */
> +	xe_assert(xe, !IS_SRIOV_VF(xe) || !(bo->flags & XE_BO_FLAG_GGTT));

but the assert is fine, so with commit/comment updated,

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>

> +
>  	if (flags & (XE_BO_FLAG_USER | XE_BO_FLAG_SYSTEM))
>  		return -EINVAL;
>