[Intel-xe] [PATCH] drm/xe: Enable scratch page when page fault is enabled

[Chang, Yu bruce]

> -----Original Message-----
> From: Welty, Brian <brian.welty at intel.com>
> Sent: Tuesday, August 29, 2023 4:59 PM
> To: Chang, Yu bruce <yu.bruce.chang at intel.com>; intel-
> xe at lists.freedesktop.org
> Cc: Zeng, Oak <oak.zeng at intel.com>; Vishwanathapura, Niranjana
> <niranjana.vishwanathapura at intel.com>; Summers, Stuart
> <stuart.summers at intel.com>; Brost, Matthew <matthew.brost at intel.com>
> Subject: Re: [PATCH] drm/xe: Enable scratch page when page fault is enabled
> 
> 
> 
> On 8/29/2023 4:16 PM, Chang, Bruce wrote:
> > The PVC HW has a limitation that the page fault due to invalid access
> > will halt the corresponding EUs. So, in order to activate the debugger,
> > kmd needs to setup the scratch pages to unhalt the EUs.
> >
> > This feature can only be enabled if scratch flag is set per VM. So, once
> > EU debugger is running, the debugger umd will set the scratch flag,
> > otherwise, this flag should not be set. So, in regular run, this feature
> > will not be activated.
> >
> > The idea is to bind a scratch vma if the page fault is from an
> > invalid access. This patch is taking advantage of null pte.
> > After the bind, the user app can continue to run without causing a
> > fatal failure or reset and stop.
> >
> > In case the app will bind this scratch vma to a valid address, GPUVA
> > handles all of this (e.g. it will create ops to unbind the old
> > VMA, bind the new one).
> >
> > This patch only kicks in when there is a failure for both page fault
> > and bind, so it should have no impact to regular code path. On
> > another hand, it uses actual page tables instead of special scratch
> > page tables, so it may not require to invalidate TLBs when doing
> > unbind if all upper layer page tables are still being used.
> >
> > tested on new scratch igt tests which will be sent out for review.
> >
> > v2: per Matt's suggestion, remove the scratch page unbind.
> >
> > Cc: Oak Zeng <oak.zeng at intel.com>
> > Cc: Brian Welty <brian.welty at intel.com>
> > Cc: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
> > Cc: Stuart Summers <stuart.summers at intel.com>
> > Cc: Matthew Brost <matthew.brost at intel.com>
> > ---
> >   drivers/gpu/drm/xe/xe_gt_pagefault.c |  9 +++++++--
> >   drivers/gpu/drm/xe/xe_vm.c           | 24 +++++++++++++++++++-----
> >   drivers/gpu/drm/xe/xe_vm.h           |  2 ++
> >   3 files changed, 28 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/xe/xe_gt_pagefault.c
> b/drivers/gpu/drm/xe/xe_gt_pagefault.c
> > index b6f781b3d9d7..adfd2206b942 100644
> > --- a/drivers/gpu/drm/xe/xe_gt_pagefault.c
> > +++ b/drivers/gpu/drm/xe/xe_gt_pagefault.c
> > @@ -137,8 +137,13 @@ static int handle_pagefault(struct xe_gt *gt, struct
> pagefault *pf)
> >   	write_locked = true;
> >   	vma = lookup_vma(vm, pf->page_addr);
> >   	if (!vma) {
> > -		ret = -EINVAL;
> > -		goto unlock_vm;
> > +		if (vm->flags & XE_VM_FLAG_SCRATCH_PAGE)
> > +			vma = xe_vm_create_scratch_vma(vm, pf-
> >page_addr);
> > +
> > +		if (!vma) {
> 
> Looks like vma could have an embedded error. So maybe need
> IS_ERR_OR_NULL() ?
> 

Good catch! Fixed now.

> > +			ret = -EINVAL;
> > +			goto unlock_vm;
> > +		}
> >   	}
> >
> >   	if (!xe_vma_is_userptr(vma) || !xe_vma_userptr_check_repin(vma)) {
> > diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> > index 389ac5ba8ddf..5190807089e8 100644
> > --- a/drivers/gpu/drm/xe/xe_vm.c
> > +++ b/drivers/gpu/drm/xe/xe_vm.c
> > @@ -1262,7 +1262,8 @@ struct xe_vm *xe_vm_create(struct xe_device
> *xe, u32 flags)
> >   		}
> >   	}
> >
> > -	if (flags & XE_VM_FLAG_SCRATCH_PAGE) {
> > +	if (flags & XE_VM_FLAG_SCRATCH_PAGE &&
> > +	    (!(flags & XE_VM_FLAG_FAULT_MODE))) {
> >   		for_each_tile(tile, xe, id) {
> >   			if (!vm->pt_root[id])
> >   				continue;
> > @@ -1998,10 +1999,6 @@ int xe_vm_create_ioctl(struct drm_device *dev,
> void *data,
> >   	if (XE_IOCTL_DBG(xe, args->flags &
> ~ALL_DRM_XE_VM_CREATE_FLAGS))
> >   		return -EINVAL;
> >
> > -	if (XE_IOCTL_DBG(xe, args->flags &
> DRM_XE_VM_CREATE_SCRATCH_PAGE &&
> > -			 args->flags & DRM_XE_VM_CREATE_FAULT_MODE))
> > -		return -EINVAL;
> > -
> >   	if (XE_IOCTL_DBG(xe, args->flags &
> DRM_XE_VM_CREATE_COMPUTE_MODE &&
> >   			 args->flags & DRM_XE_VM_CREATE_FAULT_MODE))
> >   		return -EINVAL;
> > @@ -2783,6 +2780,23 @@ static int __xe_vma_op_execute(struct xe_vm
> *vm, struct xe_vma *vma,
> >   	return err;
> >   }
> >
> > +struct xe_vma *xe_vm_create_scratch_vma(struct xe_vm *vm, u64 addr)
> > +{
> > +	struct xe_vma *vma = 0;
> > +
> > +	if (xe_vm_is_closed_or_banned(vm))
> > +		return ERR_PTR(-ENOENT);
> > +
> > +	vma = xe_vma_create(vm, NULL, 0, addr, addr + SZ_64K - 1, false,
> true, 0);
> > +	if (!vma)
> > +		return 0;
> > +	XE_WARN_ON(xe_vm_insert_vma(vm, vma));
> 
> I think might as well return the error to caller here?
> (let the caller decide to ignore it or not).
> And on error, seems best to call xe_vma_destroy_late() to free the
> memory allocated for vma structure.
> 

Good suggestions! Made the change and also removed the XE_WARN_ON
Since xe_vm_insert_vma already had it.

Thanks,
Bruce

> 
> > +
> > +	/* fault will handle the bind */
> > +
> > +	return vma;
> > +}
> > +
> >   static int xe_vma_op_execute(struct xe_vm *vm, struct xe_vma_op *op)
> >   {
> >   	int ret = 0;
> > diff --git a/drivers/gpu/drm/xe/xe_vm.h b/drivers/gpu/drm/xe/xe_vm.h
> > index 6de6e3edb24a..ddd387333cd2 100644
> > --- a/drivers/gpu/drm/xe/xe_vm.h
> > +++ b/drivers/gpu/drm/xe/xe_vm.h
> > @@ -212,6 +212,8 @@ int xe_vma_userptr_pin_pages(struct xe_vma
> *vma);
> >
> >   int xe_vma_userptr_check_repin(struct xe_vma *vma);
> >
> > +struct xe_vma *xe_vm_create_scratch_vma(struct xe_vm *vm, u64 addr);
> > +
> >   /*
> >    * XE_ONSTACK_TV is used to size the tv_onstack array that is input
> >    * to xe_vm_lock_dma_resv() and xe_vm_unlock_dma_resv().