RE: [PATCH v4] drm/doc: add rfc section for small BAR uapi

> -----Original Message-----
> From: Tvrtko Ursulin <[email protected]>
> Sent: Tuesday, May 17, 2022 6:49 AM
> To: Auld, Matthew <[email protected]>; [email protected]
> Cc: [email protected]; Thomas Hellström
> <[email protected]>; Landwerlin, Lionel G
> <[email protected]>; Bloomfield, Jon <[email protected]>;
> Daniel Vetter <[email protected]>; Justen, Jordan L
> <[email protected]>; Kenneth Graunke <[email protected]>;
> Abodunrin, Akeem G <[email protected]>; mesa-
> [email protected]
> Subject: Re: [PATCH v4] drm/doc: add rfc section for small BAR uapi
> 
> 
> On 17/05/2022 11:52, Matthew Auld wrote:
> > Add an entry for the new uapi needed for small BAR on DG2+.
> >
> > v2:
> >    - Some spelling fixes and other small tweaks. (Akeem & Thomas)
> >    - Rework error capture interactions, including no longer needing
> >      NEEDS_CPU_ACCESS for objects marked for capture. (Thomas)
> >    - Add probed_cpu_visible_size. (Lionel)
> > v3:
> >    - Drop the vma query for now.
> >    - Add unallocated_cpu_visible_size as part of the region query.
> >    - Improve the docs some more, including documenting the expected
> >      behaviour on older kernels, since this came up in some offline
> >      discussion.
> > v4:
> >    - Various improvements all over. (Tvrtko)
> 
> You can ignore my previous reply, the clarifications from v4 read good to me.
> 
> Acked-by: Tvrtko Ursulin <[email protected]>

The patch looks good to me as well...

Acked-by: Akeem G Abodunrin <[email protected]>

> 
> Regards,
> 
> Tvrtko
> 
> >
> > Signed-off-by: Matthew Auld <[email protected]>
> > Cc: Thomas Hellström <[email protected]>
> > Cc: Lionel Landwerlin <[email protected]>
> > Cc: Tvrtko Ursulin <[email protected]>
> > Cc: Jon Bloomfield <[email protected]>
> > Cc: Daniel Vetter <[email protected]>
> > Cc: Jon Bloomfield <[email protected]>
> > Cc: Jordan Justen <[email protected]>
> > Cc: Kenneth Graunke <[email protected]>
> > Cc: Akeem G Abodunrin <[email protected]>
> > Cc: [email protected]
> > ---
> >   Documentation/gpu/rfc/i915_small_bar.h   | 189
> +++++++++++++++++++++++
> >   Documentation/gpu/rfc/i915_small_bar.rst |  47 ++++++
> >   Documentation/gpu/rfc/index.rst          |   4 +
> >   3 files changed, 240 insertions(+)
> >   create mode 100644 Documentation/gpu/rfc/i915_small_bar.h
> >   create mode 100644 Documentation/gpu/rfc/i915_small_bar.rst
> >
> > diff --git a/Documentation/gpu/rfc/i915_small_bar.h
> > b/Documentation/gpu/rfc/i915_small_bar.h
> > new file mode 100644
> > index 000000000000..c676640b23ef
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.h
> > @@ -0,0 +1,189 @@
> > +/**
> > + * struct __drm_i915_memory_region_info - Describes one region as
> > +known to the
> > + * driver.
> > + *
> > + * Note this is using both struct drm_i915_query_item and struct
> drm_i915_query.
> > + * For this new query we are adding the new query id
> > +DRM_I915_QUERY_MEMORY_REGIONS
> > + * at &drm_i915_query_item.query_id.
> > + */
> > +struct __drm_i915_memory_region_info {
> > +	/** @region: The class:instance pair encoding */
> > +	struct drm_i915_gem_memory_class_instance region;
> > +
> > +	/** @rsvd0: MBZ */
> > +	__u32 rsvd0;
> > +
> > +	/**
> > +	 * @probed_size: Memory probed by the driver (-1 = unknown)
> > +	 *
> > +	 * Note that it should not be possible to ever encounter a zero value
> > +	 * here, also note that no current region type will ever return -1 here.
> > +	 * Although for future region types, this might be a possibility. The
> > +	 * same applies to the other size fields.
> > +	 */
> > +	__u64 probed_size;
> > +
> > +	/**
> > +	 * @unallocated_size: Estimate of memory remaining (-1 = unknown)
> > +	 *
> > +	 * Requires CAP_PERFMON or CAP_SYS_ADMIN to get reliable
> accounting.
> > +	 * Without this (or if this is an older kernel) the value here will
> > +	 * always equal the @probed_size. Note this is only currently tracked
> > +	 * for I915_MEMORY_CLASS_DEVICE regions (for other types the value
> here
> > +	 * will always equal the @probed_size).
> > +	 */
> > +	__u64 unallocated_size;
> > +
> > +	union {
> > +		/** @rsvd1: MBZ */
> > +		__u64 rsvd1[8];
> > +		struct {
> > +			/**
> > +			 * @probed_cpu_visible_size: Memory probed by the
> driver
> > +			 * that is CPU accessible. (-1 = unknown).
> > +			 *
> > +			 * This will be always be <= @probed_size, and the
> > +			 * remainder (if there is any) will not be CPU
> > +			 * accessible.
> > +			 *
> > +			 * On systems without small BAR, the @probed_size will
> > +			 * always equal the @probed_cpu_visible_size, since all
> > +			 * of it will be CPU accessible.
> > +			 *
> > +			 * Note this is only tracked for
> > +			 * I915_MEMORY_CLASS_DEVICE regions (for other
> types the
> > +			 * value here will always equal the @probed_size).
> > +			 *
> > +			 * Note that if the value returned here is zero, then
> > +			 * this must be an old kernel which lacks the relevant
> > +			 * small-bar uAPI support (including
> > +			 *
> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS), but on
> > +			 * such systems we should never actually end up with a
> > +			 * small BAR configuration, assuming we are able to
> load
> > +			 * the kernel module. Hence it should be safe to treat
> > +			 * this the same as when @probed_cpu_visible_size ==
> > +			 * @probed_size.
> > +			 */
> > +			__u64 probed_cpu_visible_size;
> > +
> > +			/**
> > +			 * @unallocated_cpu_visible_size: Estimate of CPU
> > +			 * visible memory remaining (-1 = unknown).
> > +			 *
> > +			 * Note this is only tracked for
> > +			 * I915_MEMORY_CLASS_DEVICE regions (for other
> types the
> > +			 * value here will always equal the
> > +			 * @probed_cpu_visible_size).
> > +			 *
> > +			 * Requires CAP_PERFMON or CAP_SYS_ADMIN to get
> reliable
> > +			 * accounting.  Without this the value here will always
> > +			 * equal the @probed_cpu_visible_size. Note this is only
> > +			 * currently tracked for I915_MEMORY_CLASS_DEVICE
> > +			 * regions (for other types the value here will also
> > +			 * always equal the @probed_cpu_visible_size).
> > +			 *
> > +			 * If this is an older kernel the value here will be
> > +			 * zero, see also @probed_cpu_visible_size.
> > +			 */
> > +			__u64 unallocated_cpu_visible_size;
> > +		};
> > +	};
> > +};
> > +
> > +/**
> > + * struct __drm_i915_gem_create_ext - Existing gem_create behaviour,
> > +with added
> > + * extension support using struct i915_user_extension.
> > + *
> > + * Note that new buffer flags should be added here, at least for the
> > +stuff that
> > + * is immutable. Previously we would have two ioctls, one to create
> > +the object
> > + * with gem_create, and another to apply various parameters, however
> > +this
> > + * creates some ambiguity for the params which are considered
> > +immutable. Also in
> > + * general we're phasing out the various SET/GET ioctls.
> > + */
> > +struct __drm_i915_gem_create_ext {
> > +	/**
> > +	 * @size: Requested size for the object.
> > +	 *
> > +	 * The (page-aligned) allocated size for the object will be returned.
> > +	 *
> > +	 * Note that for some devices we have might have further minimum
> > +	 * page-size restrictions (larger than 4K), like for device local-memory.
> > +	 * However in general the final size here should always reflect any
> > +	 * rounding up, if for example using the
> I915_GEM_CREATE_EXT_MEMORY_REGIONS
> > +	 * extension to place the object in device local-memory. The kernel will
> > +	 * always select the largest minimum page-size for the set of possible
> > +	 * placements as the value to use when rounding up the @size.
> > +	 */
> > +	__u64 size;
> > +
> > +	/**
> > +	 * @handle: Returned handle for the object.
> > +	 *
> > +	 * Object handles are nonzero.
> > +	 */
> > +	__u32 handle;
> > +
> > +	/**
> > +	 * @flags: Optional flags.
> > +	 *
> > +	 * Supported values:
> > +	 *
> > +	 * I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS - Signal to the
> kernel that
> > +	 * the object will need to be accessed via the CPU.
> > +	 *
> > +	 * Only valid when placing objects in I915_MEMORY_CLASS_DEVICE,
> and only
> > +	 * strictly required on configurations where some subset of the device
> > +	 * memory is directly visible/mappable through the CPU (which we also
> > +	 * call small BAR), like on some DG2+ systems. Note that this is quite
> > +	 * undesirable, but due to various factors like the client CPU, BIOS etc
> > +	 * it's something we can expect to see in the wild. See struct
> > +	 * __drm_i915_memory_region_info.probed_cpu_visible_size for how
> to
> > +	 * determine if this system applies.
> > +	 *
> > +	 * Note that one of the placements MUST be
> I915_MEMORY_CLASS_SYSTEM, to
> > +	 * ensure the kernel can always spill the allocation to system memory,
> > +	 * if the object can't be allocated in the mappable part of
> > +	 * I915_MEMORY_CLASS_DEVICE.
> > +	 *
> > +	 * Also note that since the kernel only supports flat-CCS on objects
> > +	 * that can *only* be placed in I915_MEMORY_CLASS_DEVICE, we
> therefore
> > +	 * don't support I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS
> together with
> > +	 * flat-CCS.
> > +	 *
> > +	 * Without this hint, the kernel will assume that non-mappable
> > +	 * I915_MEMORY_CLASS_DEVICE is preferred for this object. Note that
> the
> > +	 * kernel can still migrate the object to the mappable part, as a last
> > +	 * resort, if userspace ever CPU faults this object, but this might be
> > +	 * expensive, and so ideally should be avoided.
> > +	 *
> > +	 * On older kernels which lack the relevant small-bar uAPI support (see
> > +	 * also struct
> __drm_i915_memory_region_info.probed_cpu_visible_size),
> > +	 * usage of the flag will result in an error, but it should NEVER be
> > +	 * possible to end up with a small BAR configuration, assuming we can
> > +	 * also successfully load the i915 kernel module. In such cases the
> > +	 * entire I915_MEMORY_CLASS_DEVICE region will be CPU accessible,
> and as
> > +	 * such there are zero restrictions on where the object can be placed.
> > +	 */
> > +#define I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS (1 << 0)
> > +	__u32 flags;
> > +
> > +	/**
> > +	 * @extensions: The chain of extensions to apply to this object.
> > +	 *
> > +	 * This will be useful in the future when we need to support several
> > +	 * different extensions, and we need to apply more than one when
> > +	 * creating the object. See struct i915_user_extension.
> > +	 *
> > +	 * If we don't supply any extensions then we get the same old
> gem_create
> > +	 * behaviour.
> > +	 *
> > +	 * For I915_GEM_CREATE_EXT_MEMORY_REGIONS usage see
> > +	 * struct drm_i915_gem_create_ext_memory_regions.
> > +	 *
> > +	 * For I915_GEM_CREATE_EXT_PROTECTED_CONTENT usage see
> > +	 * struct drm_i915_gem_create_ext_protected_content.
> > +	 */
> > +#define I915_GEM_CREATE_EXT_MEMORY_REGIONS 0 #define
> > +I915_GEM_CREATE_EXT_PROTECTED_CONTENT 1
> > +	__u64 extensions;
> > +};
> > diff --git a/Documentation/gpu/rfc/i915_small_bar.rst
> > b/Documentation/gpu/rfc/i915_small_bar.rst
> > new file mode 100644
> > index 000000000000..a322481cea8b
> > --- /dev/null
> > +++ b/Documentation/gpu/rfc/i915_small_bar.rst
> > @@ -0,0 +1,47 @@
> > +==========================
> > +I915 Small BAR RFC Section
> > +==========================
> > +Starting from DG2 we will have resizable BAR support for device
> > +local-memory(i.e I915_MEMORY_CLASS_DEVICE), but in some cases the
> > +final BAR size might still be smaller than the total probed_size. In
> > +such cases, only some subset of I915_MEMORY_CLASS_DEVICE will be CPU
> > +accessible(for example the first 256M), while the remainder is only accessible
> via the GPU.
> > +
> > +I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS flag
> > +----------------------------------------------
> > +New gem_create_ext flag to tell the kernel that a BO will require CPU access.
> > +This becomes important when placing an object in
> > +I915_MEMORY_CLASS_DEVICE, where underneath the device has a small
> > +BAR, meaning only some portion of it is CPU accessible. Without this
> > +flag the kernel will assume that CPU access is not required, and
> > +prioritize using the non-CPU visible portion of
> I915_MEMORY_CLASS_DEVICE.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > +   :functions: __drm_i915_gem_create_ext
> > +
> > +probed_cpu_visible_size attribute
> > +---------------------------------
> > +New struct__drm_i915_memory_region attribute which returns the total
> > +size of the CPU accessible portion, for the particular region. This
> > +should only be applicable for I915_MEMORY_CLASS_DEVICE. We also
> > +report the unallocated_cpu_visible_size, alongside the unallocated_size.
> > +
> > +Vulkan will need this as part of creating a separate VkMemoryHeap
> > +with the VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT set, to represent the
> > +CPU visible portion, where the total size of the heap needs to be
> > +known. It also wants to be able to give a rough estimate of how memory can
> potentially be allocated.
> > +
> > +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
> > +   :functions: __drm_i915_memory_region_info
> > +
> > +Error Capture restrictions
> > +--------------------------
> > +With error capture we have two new restrictions:
> > +
> > +    1) Error capture is best effort on small BAR systems; if the pages are not
> > +    CPU accessible, at the time of capture, then the kernel is free to skip
> > +    trying to capture them.
> > +
> > +    2) On discrete we now reject error capture on recoverable contexts. In the
> > +    future the kernel may want to blit during error capture, when for example
> > +    something is not currently CPU accessible.
> > diff --git a/Documentation/gpu/rfc/index.rst
> > b/Documentation/gpu/rfc/index.rst index 91e93a705230..5a3bd3924ba6
> > 100644
> > --- a/Documentation/gpu/rfc/index.rst
> > +++ b/Documentation/gpu/rfc/index.rst
> > @@ -23,3 +23,7 @@ host such documentation:
> >   .. toctree::
> >
> >       i915_scheduler.rst
> > +
> > +.. toctree::
> > +
> > +    i915_small_bar.rst