[PATCH V6] drm/xe/mmap: Add mmap support for PCI memory barrier

[Matthew Auld]

On 25/10/2024 12:05, Tejas Upadhyay wrote:
> In order to avoid having userspace to use MI_MEM_FENCE,
> we are adding a mechanism for userspace to generate a
> PCI memory barrier with low overhead (avoiding IOCTL call
> as well as writing to VRAM will adds some overhead).
> 
> This is implemented by memory-mapping a page as uncached
> that is backed by MMIO on the dGPU and thus allowing userspace
> to do memory write to the page without invoking an IOCTL.
> We are selecting the MMIO so that it is not accessible from
> the PCI bus so that the MMIO writes themselves are ignored,
> but the PCI memory barrier will still take action as the MMIO
> filtering will happen after the memory barrier effect.
> 
> When we detect special defined offset in mmap(), We are mapping
> 4K page which contains the last of page of doorbell MMIO range
> to userspace for same purpose.
> 
> For user to query special offset we are adding special flag in
> mmap_offset ioctl which needs to be passed as follows,
> struct drm_xe_gem_mmap_offset mmo = {
>          .handle = 0, /* this must be 0 */
>          .flags = DRM_XE_MMAP_OFFSET_FLAG_PCI_BARRIER,
> };
> igt_ioctl(fd, DRM_IOCTL_XE_GEM_MMAP_OFFSET, &mmo);
> map = mmap(NULL, size, PROT_WRITE, MAP_SHARED, fd, mmo);
> 
> Note: Test coverage for this is added by IGT
>        https://patchwork.freedesktop.org/series/140368/  here.
>        UMD implementing test, once PR is ready will attach with
>        this patch.
> 
> V6(MAuld)
>    - Move physical mmap to fault handler
>    - Modify kernel-doc and attach UMD PR when ready
> V5(MAuld)
>    - Return invalid early in case of non 4K PAGE_SIZE
>    - Format kernel-doc and add note for 4K PAGE_SIZE HW limit
> V4(MAuld)
>    - Add kernel-doc for uapi change
>    - Restrict page size to 4K
> V3(MAuld)
>    - Remove offset defination from UAPI to be able to change later
>    - Edit commit message for special flag addition
> V2(MAuld)
>    - Add fault handler with dummy page to handle unplug device
>    - Add Build check for special offset to be below normal start page
>    - Test d3hot, mapping seems to be valid in d3hot as well
>    - Add more info to commit message
> 
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Michal Mrozek <michal.mrozek at intel.com>
> Signed-off-by: Tejas Upadhyay <tejas.upadhyay at intel.com>

Reviewed-by: Matthew Auld <matthew.auld at intel.com>