Re: [PATCH v3 08/19] drm/xe/svm: Add xe_svm_ranges_zap_ptes_in_range() for PTE zapping

[Matthew Brost <matthew.brost@intel.com>]

On Thu, May 29, 2025 at 08:36:28AM +0530, Ghimiray, Himal Prasad wrote:
> 
> 
> On 29-05-2025 04:45, Matthew Brost wrote:
> > On Tue, May 27, 2025 at 10:09:52PM +0530, Himal Prasad Ghimiray wrote:
> > > Introduce xe_svm_ranges_zap_ptes_in_range(), a function to zap page table
> > > entries (PTEs) for all SVM ranges within a user-specified address range.
> > > 
> > > Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
> > > ---
> > >   drivers/gpu/drm/xe/xe_svm.c | 43 +++++++++++++++++++++++++++++++++++++
> > >   drivers/gpu/drm/xe/xe_svm.h |  7 ++++++
> > >   2 files changed, 50 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
> > > index 59e73187114d..a4d53c24fcbc 100644
> > > --- a/drivers/gpu/drm/xe/xe_svm.c
> > > +++ b/drivers/gpu/drm/xe/xe_svm.c
> > > @@ -1006,6 +1006,49 @@ int xe_svm_range_get_pages(struct xe_vm *vm, struct xe_svm_range *range,
> > >   	return err;
> > >   }
> > > +/**
> > > + * xe_svm_ranges_zap_ptes_in_range - clear ptes of svm ranges in input range
> > > + * @vm: Pointer to the xe_vm structure
> > > + * @start: Start of the input range
> > > + * @end: End of the input range
> > > + *
> > > + * This function removes the page table entries (PTEs) associated
> > > + * with the svm ranges within the given input start amnd end
> > > + *
> > > + * Return: tile_mask for which gt's need to be tlb invalidated.
> > > + */
> > > +u8 xe_svm_ranges_zap_ptes_in_range(struct xe_vm *vm, u64 start, u64 end)
> > > +{
> > > +	struct drm_gpusvm_notifier *notifier;
> > > +	struct xe_svm_range *range;
> > > +	u64 adj_start, adj_end;
> > > +	struct xe_tile *tile;
> > > +	u8 tile_mask = 0;
> > > +	u8 id;
> > > +
> > > +	down_write(&vm->svm.gpusvm.notifier_lock);
> > 
> > xe_svm_notifier_lock
> 
> xe_pt_zap_ptes_range needs write_lock, whereas xe_svm_notifier_lock/unlock
> provides read lock.

Hmm, I think the assert in xe_pt_zap_ptes_range is actually wrong. I
likely just added the in notifier assertion because that was the only
user of it. We want to guarantee that only 1 KMD thread is issuing a zap
or modifying the PTEs at a time.

- The notifier lock in read mode guarantees that an invalidation
  from MMU notifier doesn't race here.

- The VM lock in write mode guarantees no one is modifying the page
  tables.

- The notifier lock in write mode guarantees no one is modifying the
  page tables and invalidation from madvise doesn't race.

I think this complex condition can expressed in lockdep by:

lockdep_assert(lockdep_is_held_type(notifier_lock, 0) ||
	       (lockdep_is_held_type(notifier_lock, 1) &&
		lockdep_is_held_type(vm_lock, 0)));

If this works, a comment explaining above is probably warrented.

If the above doesn't work or we deemed this to complex, maybe it fine to
just take the notifier lock in write mode...

I suggest we get another opinion here, perhaps from Thomas.

Matt

>  >
> > > +
> > > +	drm_gpusvm_for_each_notifier(notifier, &vm->svm.gpusvm, start, end) {
> > > +		struct drm_gpusvm_range *r = NULL;
> > > +
> > > +		adj_start = max(start, notifier->itree.start);
> > > +		adj_end = min(end, notifier->itree.last + 1);
> > > +		drm_gpusvm_for_each_range(r, notifier, adj_start, adj_end) {
> > > +			range = to_xe_range(r);
> > > +			for_each_tile(tile, vm->xe, id) {
> > > +				if (xe_pt_zap_ptes_range(tile, vm, range)) {
> > > +					tile_mask |= BIT(id);
> > > +					range->tile_invalidated |= BIT(id);
> > > +				}
> > > +			}
> > > +		}
> > > +	}
> > > +
> > > +	up_write(&vm->svm.gpusvm.notifier_lock);
> > > +
> > 
> > xe_svm_notifier_unlock
> > 
> > Matt
> > 
> > > +	return tile_mask;
> > > +}
> > > +
> > >   #if IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR)
> > >   static struct drm_pagemap_device_addr
> > > diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
> > > index 19ce4f2754a7..af8f285b6caa 100644
> > > --- a/drivers/gpu/drm/xe/xe_svm.h
> > > +++ b/drivers/gpu/drm/xe/xe_svm.h
> > > @@ -91,6 +91,7 @@ bool xe_svm_range_validate(struct xe_vm *vm,
> > >   u64 xe_svm_find_vma_start(struct xe_vm *vm, u64 addr, u64 end,  struct xe_vma *vma);
> > > +u8 xe_svm_ranges_zap_ptes_in_range(struct xe_vm *vm, u64 start, u64 end);
> > >   /**
> > >    * xe_svm_range_has_dma_mapping() - SVM range has DMA mapping
> > >    * @range: SVM range
> > > @@ -305,6 +306,12 @@ u64 xe_svm_find_vma_start(struct xe_vm *vm, u64 addr, u64 end, struct xe_vma *vm
> > >   	return ULONG_MAX;
> > >   }
> > > +static inline
> > > +u8 xe_svm_ranges_zap_ptes_in_range(struct xe_vm *vm, u64 start, u64 end)
> > > +{
> > > +	return 0;
> > > +}
> > > +
> > >   #define xe_svm_assert_in_notifier(...) do {} while (0)
> > >   #define xe_svm_range_has_dma_mapping(...) false
> > > -- 
> > > 2.34.1
> > > 
>