Re: [PATCH v3] drm/doc: add rfc section for small BAR uapi

On 17/05/2022 12:23, Tvrtko Ursulin wrote:
>
> On 17/05/2022 09:55, Lionel Landwerlin wrote:
>> On 17/05/2022 11:29, Tvrtko Ursulin wrote:
>>>
>>> On 16/05/2022 19:11, 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.
>>>>
>>>> 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   | 164 
>>>> +++++++++++++++++++++++
>>>>   Documentation/gpu/rfc/i915_small_bar.rst |  47 +++++++
>>>>   Documentation/gpu/rfc/index.rst          |   4 +
>>>>   3 files changed, 215 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..4079d287750b
>>>> --- /dev/null
>>>> +++ b/Documentation/gpu/rfc/i915_small_bar.h
>>>> @@ -0,0 +1,164 @@
>>>> +/**
>>>> + * 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;
>>>
>>> Is -1 possible today or when it will be? For system memory it 
>>> appears zeroes are returned today so that has to stay I think. Does 
>>> it effectively mean userspace has to consider both 0 and -1 as 
>>> unknown is the question.
>>
>>
>> I raised this on v2. As far as I can tell there are no situation 
>> where we would get -1.
>>
>> Is it really probed_size=0 on smem?? It's not the case on the 
>> internal branch.
>
> My bad, I misread the arguments to intel_memory_region_create while 
> grepping:
>
> struct intel_memory_region *i915_gem_shmem_setup(struct 
> drm_i915_private *i915,
>                          u16 type, u16 instance)
> {
>     return intel_memory_region_create(i915, 0,
>                       totalram_pages() << PAGE_SHIFT,
>                       PAGE_SIZE, 0, 0,
>                       type, instance,
>                       &shmem_region_ops);
>
> I saw "0, 0" and wrongly assumed that would be the data, since it 
> matched with my mental model and the comment against unallocated_size 
> saying it's only tracked for device memory.
>
> Although I'd say it is questionable for i915 to return this data. I 
> wonder it use case is possible where it would even be wrong but don't 
> know. I guess the cat is out of the bag now.


Not sure how questionable that is. There are a bunch of tools reporting 
the amount of memory available (free, top, htop, etc...).

It might not be totalram_pages() but probably something close to it.

Having a non 0 & non -1 value is useful.


-Lionel


>
> If the situation is -1 for unknown and some valid size (not zero) I 
> don't think there is a problem here.
>
> Regards,
>
> Tvrtko
>
>> Anv is not currently handling that case.
>>
>>
>> I would very much like to not deal with 0 for smem.
>>
>> It really makes it easier for userspace rather than having to fish 
>> information from 2 different places and on top of dealing with 
>> multiple kernel versions.
>>
>>
>> -Lionel
>>
>>
>>>
>>>> +
>>>> +    /**
>>>> +     * @unallocated_size: Estimate of memory remaining (-1 = unknown)
>>>> +     *
>>>> +     * Note this is only currently tracked for 
>>>> I915_MEMORY_CLASS_DEVICE
>>>> +     * regions, and also requires CAP_PERFMON or CAP_SYS_ADMIN to get
>>>> +     * reliable accounting. Without this(or if this an older 
>>>> kernel) the
>>>
>>> s/if this an/if this is an/
>>>
>>> Also same question as above about -1.
>>>
>>>> +     * value here will always match 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).
>>>
>>> Also question about -1. In this case this could be done since the 
>>> field is yet to be added but I am curious if it ever can be -1.
>>>
>>>> +             *
>>>> +             * 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 that if the value returned here is zero, then
>>>> +             * this must be an old kernel which lacks the relevant
>>>> +             * small-bar uAPI support(including
>>>
>>> I have noticed you prefer no space before parentheses throughout the 
>>> text so I guess it's just my preference to have it. Very nitpicky 
>>> even if I am right so up to you.
>>>
>>>> +             * 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 currently tracked for
>>>> +             * I915_MEMORY_CLASS_DEVICE regions, and also requires
>>>> +             * CAP_PERFMON or CAP_SYS_ADMIN to get reliable
>>>> +             * accounting. Without this the value here will always
>>>> +             * equal the @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.
>>>
>>> Is it defined how rounding up works when a list of regions is given 
>>> (like smem+lmem) and should that be explicitly mentioned here?
>>>
>>>> +     */
>>>> +    __u64 size;
>>>
>>> Blank line here (etc below) maybe to match the previous doc block?
>>>
>>> Regards,
>>>
>>> Tvrtko
>>>
>>>> +    /**
>>>> +     * @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, where usage of this flag results in an 
>>>> error, since
>>>> +     * we lack the relevant small BAR uAPI(see also struct
>>>> +     * __drm_i915_memory_region_info.probed_cpu_visible_size) 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
>>
>>