[PATCH v2 08/15] drm/panthor: Add the MMU/VM logical block
Steven Price
steven.price at arm.com
Wed Aug 30 14:12:43 UTC 2023
On 29/08/2023 16:33, Boris Brezillon wrote:
> On Mon, 14 Aug 2023 16:53:09 +0100
> Steven Price <steven.price at arm.com> wrote:
>
>>> +
>>> +/**
>>> + * struct panthor_vm_op_ctx - VM operation context
>>> + *
>>> + * With VM operations potentially taking place in a dma-signaling path, we
>>> + * need to make sure everything that might require resource allocation is
>>> + * pre-allocated upfront. This is what this operation context is far.
>>> + *
>>> + * We also collect resources that have been freed, so we can release them
>>> + * asynchronously, and let the VM_BIND scheduler process the next VM_BIND
>>> + * request.
>>> + */
>>> +struct panthor_vm_op_ctx {
>>> + /** @rsvd_page_tables: Pages reserved for the MMU page table update. */
>>> + struct {
>>> + /** @count: Number of pages reserved. */
>>> + u32 count;
>>> +
>>> + /** @ptr: Point to the first unused page in the @pages table. */
>>> + u32 ptr;
>>> +
>>> + /**
>>> + * @page: Array of pages that can be used for an MMU page table update.
>>> + *
>>> + * After an VM operation, there might be free pages left in this array.
>>> + * They should be returned to the pt_cache as part of the op_ctx cleanup.
>>> + */
>>> + void **pages;
>>> + } rsvd_page_tables;
>>
>> Two questions:
>>
>> 1) Would a mempool simplify the implementation? It looks like a
>> reasonable match.
>
> Not sure what you mean by mempool,
See include/linux/mempool.h
> but I'm using a kmem_cache here for
> all page table allocations. The pages that are passed to
> panthor_vm_op_ctx::rsvd_page_tables::pages are allocated from this
> pool. It's just that for each VM operation we pre-allocate page-tables,
> and release those that were not used when the operation is done (we
> over-allocate for the worst case scenario).
The mempool could, potentially, replace the rsvd_page_tables structure.
The kmem_cache you would still want as that's per-driver.
>>
>> 2) Does it really make sense to have a separate pool of memory for every
>> operation? Instead of having a separate pool for each operation, it
>> would be possible to just keep track of the total number needed for all
>> outstanding operations. Then a single (per device or maybe per-VM if
>> necessary) mempool could be resized to ensure it has the right amount of
>> space.
>
> The pool is per-driver (see the global pt_cache). rsvd_page_tables just
> holds pages needed for a specific VM operation. To be more specific, it
> holds pages for the worst case (page table tree is empty, except for the
> root page table).
What I'm wondering is to we need to keep the pages for each operation in
separate pools. So instead of having a rsvd_page_tables for each
operation, can we have one global one which is sized appropriately for
all operations that are in flight for the device? The operations are
serialized so there's no contention. Or at least a per-VM pool if we can
operate on multiple VMs at once.
>>
>> I'm also a little wary that the VM_BIND infrastructure could potentially
>> be abused to trigger a large amount of kernel allocation as it allocates
>> up-front for the worst case but those pages are not charged to the
>> process (AFAICT). But I haven't fully got my head round that yet.
>
> Yep, that's problematic, indeed. I considered allocating page tables
> as GEM objects, but the overhead of a GEM object is quite big
> (hundreds of bytes of meta-data) compared to the size of a page table
> (4k), and kmem_cache was just super convenient for this page table
> cache :-).
I think page tables as GEM objects is likely to be overkill, we
obviously also have to be careful not to allow user space to get access
to the contents - whereas GEM objects are usually to provide user space
access ;) I'm not sure quite what the best solution here is, clearly one
'solution' is to just cap the number of outstanding VM_BINDs.
>>
>>> +
>>> + /** @flags: Combination of drm_panthor_vm_bind_op_flags. */
>>> + u32 flags;
>>> +
>>> + /** @va: Virtual range targeted by the VM operation. */
>>> + struct {
>>> + /** @addr: Start address. */
>>> + u64 addr;
>>> +
>>> + /** @range: Range size. */
>>> + u64 range;
>>> + } va;
>>> +
>>> + /**
>>> + * @returned_vmas: List of panthor_vma objects returned after a VM operation.
>>> + *
>>> + * For unmap operations, this will contain all VMAs that were covered by the
>>> + * specified VA range.
>>> + *
>>> + * For map operations, this will contain all VMAs that previously mapped to
>>> + * the specified VA range.
>>> + *
>>> + * Those VMAs, and the resources they point to will be released as part of
>>> + * the op_ctx cleanup operation.
>>> + */
>>> + struct list_head returned_vmas;
>>> +
>>> + /** @map: Fields specific to a map operation. */
>>> + struct {
>>> + /** @gem: GEM object information. */
>>> + struct {
>>> + /** @obj: GEM object to map. */
>>> + struct drm_gem_object *obj;
>>> +
>>> + /** @offset: Offset in the GEM object. */
>>> + u64 offset;
>>> + } gem;
>>> +
>>> + /**
>>> + * @sgt: sg-table pointing to pages backing the GEM object.
>>> + *
>>> + * This is gathered at job creation time, such that we don't have
>>> + * to allocate in ::run_job().
>>> + */
>>> + struct sg_table *sgt;
>>> +
>>> + /**
>>> + * @prev_vma: Pre-allocated VMA object to deal with a remap situation.
>>> + *
>>> + * If the map request covers a region that's inside another VMA, the
>>> + * previous VMA will be split, requiring instantiation of a maximum of
>>> + * two new VMA objects.
>>> + */
>>> + struct panthor_vma *prev_vma;
>>> +
>>> + /**
>>> + * @new_vma: The new VMA object that will be inserted to the VA tree.
>>> + */
>>> + struct panthor_vma *new_vma;
>>> +
>>> + /**
>>> + * @next_vma: Pre-allocated VMA object to deal with a remap situation.
>>> + *
>>> + * See @prev_vma.
>>> + */
>>> + struct panthor_vma *next_vma;
>>
>> It's probably premature optimization, but it feels like having a cache
>> of these VMA structures might be an idea.
>
> If it's needed, I'll probably go for a kmem_cache, but I need to
> check if it's worth it first (if the closest kmalloc cache is
> significantly biffer than the struct size).
>
>> I'm also struggling to
>> understand how both a new prev and new next VMA are needed - but I
>> haven't dug into the GPU VA manager.
>
> prev/next are for mapping splits: an object is already mapped, and a new
> object is mapped in the middle of this pre-existing mapping. In that
> case, we need 2 vma object for the preceeding and succeeding mappings,
> since the old mapping object will be released.
>
> new_vma is for the new mapping.
Yeah, looking into the GPU VA manager I see now. My problem was that I
assumed in the case of a split one of the original mappings would simply
be resized, so you'd only need one new VMA (plus the one being added).
But AFAICT that resize doesn't happen and instead new VMA are created.
>>
>>> + } map;
>>> +};
>>> +
>
> [...]
>
>>> +/**
>>> + * panthor_vm_active() - Flag a VM as active
>>> + * @VM: VM to flag as active.
>>> + *
>>> + * Assigns an address space to a VM so it can be used by the GPU/MCU.
>>> + *
>>> + * Return: 0 on success, a negative error code otherwise.
>>> + */
>>> +int panthor_vm_active(struct panthor_vm *vm)
>>> +{
>>> + struct panthor_device *ptdev = vm->ptdev;
>>> + struct io_pgtable_cfg *cfg = &io_pgtable_ops_to_pgtable(vm->pgtbl_ops)->cfg;
>>> + int ret = 0, as, cookie;
>>> + u64 transtab, transcfg;
>>> +
>>> + if (!drm_dev_enter(&ptdev->base, &cookie))
>>> + return -ENODEV;
>>> +
>>> + mutex_lock(&ptdev->mmu->as.slots_lock);
>>> +
>>> + as = vm->as.id;
>>> + if (as >= 0) {
>>> + u32 mask = panthor_mmu_as_fault_mask(ptdev, as);
>>> +
>>> + if (ptdev->mmu->as.faulty_mask & mask) {
>>> + /* Unhandled pagefault on this AS, the MMU was
>>> + * disabled. We need to re-enable the MMU after
>>> + * clearing+unmasking the AS interrupts.
>>> + */
>>> + gpu_write(ptdev, MMU_INT_CLEAR, mask);
>>> + ptdev->mmu->as.faulty_mask &= ~mask;
>>> + gpu_write(ptdev, MMU_INT_MASK, ~ptdev->mmu->as.faulty_mask);
>>> + goto out_enable_as;
>>> + }
>>> +
>>> + goto out_unlock;
>>> + }
>>> +
>>> + /* Check for a free AS */
>>> + if (vm->for_mcu) {
>>> + drm_WARN_ON(&ptdev->base, ptdev->mmu->as.alloc_mask & BIT(0));
>>> + as = 0;
>>> + } else {
>>> + as = ffz(ptdev->mmu->as.alloc_mask | BIT(0));
>>> + }
>>> +
>>> + if (!(BIT(as) & ptdev->gpu_info.as_present)) {
>>> + struct panthor_vm *lru_vm;
>>> +
>>> + lru_vm = list_first_entry_or_null(&ptdev->mmu->as.lru_list,
>>> + struct panthor_vm,
>>> + as.lru_node);
>>> + if (drm_WARN_ON(&ptdev->base, !lru_vm)) {
>>> + ret = -EBUSY;
>>> + goto out_unlock;
>>> + }
>>> +
>>> + list_del_init(&lru_vm->as.lru_node);
>>> + as = lru_vm->as.id;
>>
>> Should this not set lru_vm->as.id = -1, so that the code knows the VM no
>> longer has an address space?
>
> Good catch!
>
>>
>>> + } else {
>>> + set_bit(as, &ptdev->mmu->as.alloc_mask);
>>> + }
>>> +
>>> + /* Assign the free or reclaimed AS to the FD */
>>> + vm->as.id = as;
>>> + ptdev->mmu->as.slots[as].vm = vm;
>>> +
>>> +out_enable_as:
>>> + transtab = cfg->arm_lpae_s1_cfg.ttbr;
>>> + transcfg = AS_TRANSCFG_PTW_MEMATTR_WB |
>>> + AS_TRANSCFG_PTW_RA |
>>> + AS_TRANSCFG_ADRMODE_AARCH64_4K;
>>> + if (ptdev->coherent)
>>> + transcfg |= AS_TRANSCFG_PTW_SH_OS;
>>> +
>>> + ret = panthor_mmu_as_enable(vm->ptdev, vm->as.id, transtab, transcfg, vm->memattr);
>>> +
>>> +out_unlock:
>>> + mutex_unlock(&ptdev->mmu->as.slots_lock);
>>> + drm_dev_exit(cookie);
>>> + return ret;
>>> +}
>>> +
>
> [...]
>
>>> +
>>> +static void panthor_mmu_irq_handler(struct panthor_device *ptdev, u32 status)
>>> +{
>>> + status = panthor_mmu_fault_mask(ptdev, status);
>>> + while (status) {
>>> + u32 as = ffs(status | (status >> 16)) - 1;
>>> + u32 mask = panthor_mmu_as_fault_mask(ptdev, as);
>>> + u32 new_int_mask;
>>> + u64 addr;
>>> + u32 fault_status;
>>> + u32 exception_type;
>>> + u32 access_type;
>>> + u32 source_id;
>>> +
>>> + fault_status = gpu_read(ptdev, AS_FAULTSTATUS(as));
>>> + addr = gpu_read(ptdev, AS_FAULTADDRESS_LO(as));
>>> + addr |= (u64)gpu_read(ptdev, AS_FAULTADDRESS_HI(as)) << 32;
>>> +
>>> + /* decode the fault status */
>>> + exception_type = fault_status & 0xFF;
>>> + access_type = (fault_status >> 8) & 0x3;
>>> + source_id = (fault_status >> 16);
>>> +
>>> + /* Page fault only */
>>
>> This comment makes no sense - it looks like it's copied over from panfrost.
>
> Uh, it made sense before I dropped map/alloc-on-fault :-).
:)
>>
>> If I understand correctly we don't (currently) support growing on page
>> fault - and it's not really needed now the MCU can handle the tiler heaps.
>
> Exaclty. Map/alloc on fault is a bit challenging because of the whole
> 'we have to guarantee that a job is done in finite time, and we must
> make sure fence signaling is not blocked on allocation'. Given
> drm_gem_get_pages() doesn't do non-blocking allocations, I thought it'd
> be preferable to postpone map-on-fault until we actually decide we need
> it. Note that i915 seems to have some sort of non-blocking page
> allocator in shmem_sg_alloc_table()[1].
Agreed, the intention is definitely to move away from map/alloc-on-fault
- handling page faults from the GPU on the CPU is expensive even without
the can-of-worms of fence signalling.
Steve
>>
>>> + mutex_lock(&ptdev->mmu->as.slots_lock);
>>> +
>>> + new_int_mask =
>>> + panthor_mmu_fault_mask(ptdev, ~ptdev->mmu->as.faulty_mask);
>>> +
>>> + /* terminal fault, print info about the fault */
>>> + drm_err(&ptdev->base,
>>> + "Unhandled Page fault in AS%d at VA 0x%016llX\n"
>>> + "raw fault status: 0x%X\n"
>>> + "decoded fault status: %s\n"
>>> + "exception type 0x%X: %s\n"
>>> + "access type 0x%X: %s\n"
>>> + "source id 0x%X\n",
>>> + as, addr,
>>> + fault_status,
>>> + (fault_status & (1 << 10) ? "DECODER FAULT" : "SLAVE FAULT"),
>>> + exception_type, panthor_exception_name(ptdev, exception_type),
>>> + access_type, access_type_name(ptdev, fault_status),
>>> + source_id);
>>> +
>>> + /* Ignore MMU interrupts on this AS until it's been
>>> + * re-enabled.
>>> + */
>>> + ptdev->mmu->irq.mask = new_int_mask;
>>> + gpu_write(ptdev, MMU_INT_MASK, new_int_mask);
>>> +
>>> + /* Disable the MMU to kill jobs on this AS. */
>>> + panthor_mmu_as_disable(ptdev, as);
>>> + mutex_unlock(&ptdev->mmu->as.slots_lock);
>>> +
>>> + status &= ~mask;
>>> + }
>>> +}
>>> +PANTHOR_IRQ_HANDLER(mmu, MMU, panthor_mmu_irq_handler);
>>> +
>
> [...]
>
>>> +
>>> +/**
>>> + * panthor_mmu_unplug() - Unplug the MMU logic
>>> + * @ptdev: Device.
>>> + *
>>> + * No access to the MMU regs should be done after this function is called.
>>> + * We suspend the IRQ and disable all VMs to guarantee that.
>>> + */
>>> +void panthor_mmu_unplug(struct panthor_device *ptdev)
>>> +{
>>> + if (ptdev->mmu->irq.irq > 0)
>>
>> In what situation is this not true? AFAICT the driver probe will fail if
>> the IRQ can't be obtained.
>
> Right, I'll drop this test.
>
> [1]https://elixir.bootlin.com/linux/v6.5/source/drivers/gpu/drm/i915/gem/i915_gem_shmem.c#L63
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