[Intel-gfx] [PATCH v2] drm/doc: add rfc section for small BAR uapi
Tvrtko Ursulin
tvrtko.ursulin at linux.intel.com
Thu Apr 28 08:55:10 UTC 2022
On 27/04/2022 18:36, Matthew Auld wrote:
> On 27/04/2022 09:36, Tvrtko Ursulin wrote:
>>
>> On 20/04/2022 18:13, 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)
>>>
>>> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
>>> Cc: Thomas Hellström <thomas.hellstrom at linux.intel.com>
>>> Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>>> Cc: Jon Bloomfield <jon.bloomfield at intel.com>
>>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>>> Cc: Jordan Justen <jordan.l.justen at intel.com>
>>> Cc: Kenneth Graunke <kenneth at whitecape.org>
>>> Cc: Akeem G Abodunrin <akeem.g.abodunrin at intel.com>
>>> Cc: mesa-dev at lists.freedesktop.org
>>> ---
>>> Documentation/gpu/rfc/i915_small_bar.h | 190 +++++++++++++++++++++++
>>> Documentation/gpu/rfc/i915_small_bar.rst | 58 +++++++
>>> Documentation/gpu/rfc/index.rst | 4 +
>>> 3 files changed, 252 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..7bfd0cf44d35
>>> --- /dev/null
>>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>>> @@ -0,0 +1,190 @@
>>> +/**
>>> + * 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) */
>>> + __u64 probed_size;
>>> +
>>> + /** @unallocated_size: Estimate of memory remaining (-1 =
>>> unknown) */
>>> + __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.
>>> + */
>>> + __u64 probed_cpu_visible_size;
>>
>> Would unallocated_cpu_visible_size be useful, to follow the total
>> unallocated_size?
>
> Make sense. But I don't think unallocated_size has actually been
> properly wired up yet. It still just gives the same value as
> probed_size. IIRC for unallocated_size we still need a real
> user/usecase/umd, before wiring that up for real with the existing avail
> tracking. Once we have that we can also add unallocated_cpu_visible_size.
So this does nothing at the moment:
info.unallocated_size = mr->avail;
Right, it is set to "mem->avail = mem->total;" at region init time and I
indeed can't find it ever getting modified. Okay.
>> Btw, have we ever considered whether unallocated_size should require
>> CAP_SYS_ADMIN/PERFMON or something?
>
> Note sure. But just in case we do add it for real at some point, why the
> added restriction?
To avoid a side channel, albeit perhaps a very weak one. For engine
utilization we require CAP_SYS_PERFMON, but that is implied by the perf
core API. It's open for discussion. I guess it may make sense to limit
it also because it is questionable the field(s) are even useful.
>
>>
>>> + };
>>> + };
>>> +};
>>> +
>>> +/**
>>> + * 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), likefor 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.
>>> + */
>>> + __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 platforms where only some of the
>>> device
>>> + * memory is directly visible or mappable through the CPU, like
>>> on DG2+.
>>> + *
>>> + * One of the placements MUST also be I915_MEMORY_CLASS_SYSTEM, to
>>> + * ensure we can always spill the allocation tosystem memory, if we
>>> + * can't place the object in the mappable part of
>>> + * I915_MEMORY_CLASS_DEVICE.
>>> + *
>>> + * 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.
>>> + */
>>
>> So "needs_cpu_access" flag could almost be viewed as a sub-region
>> placement priority? What I mean is this:
>>
>> 1)
>> placements=device,system flags=
>>
>> This results in placement priorities: device, device_cpu_mappable,
>> system.
>
> Yup.
>
>>
>> 2)
>> placements=device,system flags=needs_cpu_access
>>
>> This results in placement priorities: device_cpu_mappable, device,
>> system.
>
> Here it would only be: device_cpu_mappable, system. We would completely
> ignore "device" in this case.
>
>>
>> Is this correct?
>>
>> The benefit of the flag is that i915 can place the object to the right
>> place from the start instead of on the first CPU access? Is that worth
>> it or is there more to it?
>
> Yeah, the object will only be placed somewhere that is also CPU
> mappable, with the flag set.
Hm, wouldn't it be more efficient to be able to migrate it over to
non-mappable in cases when mappable is over-subscribed?
>>> +#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 usagesee
>>> + * 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;
>>> +};
>>> +
>>> +#define DRM_I915_QUERY_VMA_INFO 5
>>> +
>>> +/**
>>> + * struct __drm_i915_query_vma_info
>>> + *
>>> + * Given a vm and GTT address, lookup the corresponding vma,
>>> returning its set
>>> + * of attributes.
>>> + *
>>> + * .. code-block:: C
>>> + *
>>> + * struct drm_i915_query_vma_info info = {};
>>> + * struct drm_i915_query_item item = {
>>> + * .data_ptr = (uintptr_t)&info,
>>> + * .query_id = DRM_I915_QUERY_VMA_INFO,
>>> + * };
>>> + * struct drm_i915_query query = {
>>> + * .num_items = 1,
>>> + * .items_ptr = (uintptr_t)&item,
>>> + * };
>>> + * int err;
>>> + *
>>> + * // Unlike some other types of queries, there is noneed to
>>> first query
>>> + * // the size of the data_ptr blob here, since we already know
>>> ahead of
>>> + * // time how big this needs to be.
>>> + * item.length = sizeof(info);
>>> + *
>>> + * // Next we fill in the vm_id and ppGTT address of the vma we wish
>>> + * // to query, before then firing off the query.
>>> + * info.vm_id = vm_id;
>>> + * info.offset = gtt_address;
>>> + * err = ioctl(fd, DRM_IOCTL_I915_QUERY, &query);
>>> + * if (err || item.length < 0) ...
>>> + *
>>> + * // If all went well we can now inspect the returned attributes.
>>> + * if (info.attributes & DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE) ...
>>> + */
>>> +struct __drm_i915_query_vma_info {
>>> + /**
>>> + * @vm_id: The given vm id that contains the vma. The id is the
>>> value
>>> + * returned by the DRM_I915_GEM_VM_CREATE. See struct
>>> + * drm_i915_gem_vm_control.vm_id.
>>> + */
>>> + __u32 vm_id;
>>> + /** @pad: MBZ. */
>>> + __u32 pad;
>>> + /**
>>> + * @offset: The corresponding ppGTT address of the vma which the
>>> kernel
>>> + * will use to perform the lookup.
>>> + */
>>> + __u64 offset;
>>> + /**
>>> + * @attributes: The returned attributes for thegiven vma.
>>> + *
>>> + * Possible values:
>>> + *
>>> + * DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE - Set ifthe pages backing
>>> the
>>> + * vma are currently CPU accessible. If this isnot set then the
>>> vma is
>>> + * currently backed by I915_MEMORY_CLASS_DEVICEmemory, which the
>>> CPU
>>> + * cannot directly access(this is only possibleon discrete
>>> devices with
>>> + * a small BAR). Attempting to MMAP and fault such an object will
>>> + * require the kernel first synchronising any GPU work tied to the
>>> + * object, before then migrating the pages, either to the CPU
>>> accessible
>>> + * part of I915_MEMORY_CLASS_DEVICE, or
>>> I915_MEMORY_CLASS_SYSTEM, if the
>>> + * placements permit it. See
>>> I915_GEM_CREATE_EXT_FLAG_NEEDS_CPU_ACCESS.
>>> + *
>>> + * Note that this is inherently racy.
>>> + */
>>> +#define DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE (1<<0)
>>> + __u64 attributes;
>>> + /** @rsvd: MBZ */
>>> + __u32 rsvd[4];
>>> +};
>>> diff --git a/Documentation/gpu/rfc/i915_small_bar.rst
>>> b/Documentation/gpu/rfc/i915_small_bar.rst
>>> new file mode 100644
>>> index 000000000000..be3d9bcdd86d
>>> --- /dev/null
>>> +++ b/Documentation/gpu/rfc/i915_small_bar.rst
>>> @@ -0,0 +1,58 @@
>>> +==========================
>>> +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.
>>> +
>>> +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.
>>> +
>>> +.. kernel-doc:: Documentation/gpu/rfc/i915_small_bar.h
>>> + :functions: __drm_i915_memory_region_info
>>> +
>>> +DRM_I915_QUERY_VMA_INFO query
>>> +-----------------------------
>>> +Query the attributes of some vma. Given a vm and GTT offset, find the
>>> +respective vma, and return its set of attributes. For now we only
>>> support
>>> +DRM_I915_QUERY_VMA_INFO_CPU_VISIBLE, which is set if the object/vma is
>>> +currently placed in memory that is accessible by the CPU. This
>>> should always be
>>> +set on devices where the CPU probed_cpu_visible_size of
>>> I915_MEMORY_CLASS_DEVICE
>>> +matches the probed_size. If this is not set then CPU faulting the
>>> object will
>>> +likely first require migrating the pages.
>>
>> I think there should be justification for the new query documented as
>> well. (Why on top of what.)
>
> Yeah, I'm wondering now if we can just drop this part of the uapi, for
> now at least, and focus on landing the new flag stuff first.
>
>>
>> Without it personally I can't immediately understand why the
>> disconnect between the object based and VMA based API. Userspace has
>> to do some intervening operations like either execbuf, or vm bind in
>> the future, to make this query usable after object creation. So
>> question is why wouldn't it know already which placements it allowed
>> and so would i915 auto-migrate or not for this particular object. No?
>> Or in other words why this wouldn't be an object based query since the
>> question it is answering is about the object backing store and not the
>> VMA.
>
> Yeah, just using the object handle or so I guess would also work. Thanks
> for the comments.
I saw other folks have said the same so omitting for now sounds good to
me indeed.
Regards,
Tvrtko
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