Re: [PATCH 10/20] vfio/cxl: CXL region management

[Jonathan Cameron <jonathan.cameron@huawei.com>]

On Thu, 12 Mar 2026 15:55:32 -0700
Dave Jiang <dave.jiang@intel.com> wrote:

> On 3/11/26 1:34 PM, mhonap@nvidia.com wrote:
> > From: Manish Honap <mhonap@nvidia.com>
> > 
> > Add CXL region management for future guest access.
> > 
> > Region Management makes use of APIs provided by CXL_CORE as below:
> > 
> > CREATE_REGION flow:
> > 1. Validate request (size, decoder availability)
> > 2. Allocate HPA via cxl_get_hpa_freespace()
> > 3. Allocate DPA via cxl_request_dpa()
> > 4. Create region via cxl_create_region() - commits HDM decoder!
> > 5. Get HPA range via cxl_get_region_range()
> > 
> > DESTROY_REGION flow:
> > 1. Detach decoder via cxl_decoder_detach()
> > 2. Free DPA via cxl_dpa_free()
> > 3. Release root decoder via cxl_put_root_decoder()
> > 
> > Signed-off-by: Manish Honap <mhonap@nvidia.com>
A few additional comments from me.

> > ---
> >  drivers/vfio/pci/cxl/vfio_cxl_core.c | 118 ++++++++++++++++++++++++++-
> >  drivers/vfio/pci/cxl/vfio_cxl_priv.h |   5 ++
> >  drivers/vfio/pci/vfio_pci_priv.h     |   8 ++
> >  3 files changed, 130 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vfio/pci/cxl/vfio_cxl_core.c b/drivers/vfio/pci/cxl/vfio_cxl_core.c
> > index 2da6da1c0605..9c71f592e74e 100644
> > --- a/drivers/vfio/pci/cxl/vfio_cxl_core.c
> > +++ b/drivers/vfio/pci/cxl/vfio_cxl_core.c
> > @@ -126,6 +126,112 @@ static int vfio_cxl_setup_regs(struct vfio_pci_core_device *vdev)
> >  	return 0;
> >  }
> >  
> > +int vfio_cxl_create_cxl_region(struct vfio_pci_core_device *vdev, resource_size_t size)
> > +{
> > +	struct vfio_pci_cxl_state *cxl = vdev->cxl;
> > +	resource_size_t max_size;
> > +	int ret;
> > +
> > +	if (cxl->precommitted)
> > +		return 0;
> > +
> > +	cxl->cxlrd = cxl_get_hpa_freespace(cxl->cxlmd, 1,
> > +					   CXL_DECODER_F_RAM |
> > +					   CXL_DECODER_F_TYPE2,
> > +					   &max_size);  
> 
> Not sure what VFIO subsystem's policy is on scoped base resource cleanup, but a __free() here can get you out of managing put() of the root decoder.
> 
> > +	if (IS_ERR(cxl->cxlrd))
> > +		return PTR_ERR(cxl->cxlrd);
> > +
> > +	/* Insufficient HPA space */
> > +	if (max_size < size) {
> > +		cxl_put_root_decoder(cxl->cxlrd);
> > +		cxl->cxlrd = NULL;
Similar to other cases, I'd keep assigning stuff in cxl to the point
where there are no more error paths.  Use local variables until then.
(that would fit with using __free() as well which I'd also favor if
 accepted in VFIO).

> > +		return -ENOSPC;
> > +	}
> > +
> > +	cxl->cxled = cxl_request_dpa(cxl->cxlmd, CXL_PARTMODE_RAM, size);  
> 
> Same comment here about __free().
> 
> > +	if (IS_ERR(cxl->cxled)) {
> > +		ret = PTR_ERR(cxl->cxled);
> > +		goto err_free_hpa;
> > +	}
> > +
> > +	cxl->region = cxl_create_region(cxl->cxlrd, &cxl->cxled, 1);
> > +	if (IS_ERR(cxl->region)) {
> > +		ret = PTR_ERR(cxl->region);

You carefully NULL this in vfio_cxl_destroy_region, but if you fail
here you end up with it containing an ERR_PTR(). I'd avoid that by
using a local variable and only assigning cxl->region after this
suceeds.

> > +		goto err_free_dpa;
> > +	}
> > +
> > +	return 0;
> > +
> > +err_free_dpa:
> > +	cxl_dpa_free(cxl->cxled);
> > +err_free_hpa:
> > +	if (cxl->cxlrd)
> > +		cxl_put_root_decoder(cxl->cxlrd);
> > +
> > +	return ret;
> > +}
> > +
> > +void vfio_cxl_destroy_cxl_region(struct vfio_pci_core_device *vdev)
> > +{
> > +	struct vfio_pci_cxl_state *cxl = vdev->cxl;
> > +
> > +	if (!cxl->region)
> > +		return;
> > +
> > +	cxl_unregister_region(cxl->region);
> > +	cxl->region = NULL;
> > +
> > +	if (cxl->precommitted)
> > +		return;
> > +
> > +	cxl_dpa_free(cxl->cxled);
> > +	cxl_put_root_decoder(cxl->cxlrd);
> > +}
> > +
> > +static int vfio_cxl_create_region_helper(struct vfio_pci_core_device *vdev,
> > +					 resource_size_t capacity)
> > +{
> > +	struct vfio_pci_cxl_state *cxl = vdev->cxl;
> > +	struct pci_dev *pdev = vdev->pdev;
> > +	int ret;
> > +
> > +	if (cxl->precommitted) {
> > +		cxl->cxled = cxl_get_committed_decoder(cxl->cxlmd,
> > +						       &cxl->region);
> > +		if (IS_ERR(cxl->cxled))
> > +			return PTR_ERR(cxl->cxled);
> > +	} else {
> > +		ret = vfio_cxl_create_cxl_region(vdev, capacity);
> > +		if (ret)
> > +			return ret;
> > +	}
> > +
> > +	if (cxl->region) {  
> 
> Maybe if you do 'if (!cxl->region)' first and just exit, then you don't need to indent the normal code path.
> 
> > +		struct range range;
> > +
> > +		ret = cxl_get_region_range(cxl->region, &range);
> > +		if (ret)
> > +			goto failed;
> > +
> > +		cxl->region_hpa = range.start;
> > +		cxl->region_size = range_len(&range);
> > +
> > +		pci_dbg(pdev, "Precommitted decoder: HPA 0x%llx size %lu MB\n",
> > +			cxl->region_hpa, cxl->region_size >> 20);
> > +	} else {
> > +		pci_err(pdev, "Failed to create CXL region\n");
> > +		ret = -ENODEV;
> > +		goto failed;
> > +	}
> > +
> > +	return 0;
> > +
> > +failed:
> > +	vfio_cxl_destroy_cxl_region(vdev);

Little bit of refactoring and this could be replaced with __free() magic.

> > +	return ret;
> > +}
> > +
> >  /**
> >   * vfio_pci_cxl_detect_and_init - Detect and initialize CXL Type-2 device
> >   * @vdev: VFIO PCI device
> > @@ -172,6 +278,12 @@ void vfio_pci_cxl_detect_and_init(struct vfio_pci_core_device *vdev)
> >  
> >  	pci_disable_device(pdev);
> >  
> > +	ret = vfio_cxl_create_region_helper(vdev, SZ_256M);  
> 
> Maybe a comment on why this size?
:)  I wondered that as well.  I'm guessing your bios isn't always providing
the decoder and this lets you test.


> 
> DJ
> 
> > +	if (ret)
> > +		goto failed;
> > +
> > +	cxl->precommitted = true;
> > +
> >  	return;
> >  
> >  failed:
> > @@ -181,6 +293,10 @@ void vfio_pci_cxl_detect_and_init(struct vfio_pci_core_device *vdev)
> >  
> >  void vfio_pci_cxl_cleanup(struct vfio_pci_core_device *vdev)
> >  {
> > -	if (!vdev->cxl)
> > +	struct vfio_pci_cxl_state *cxl = vdev->cxl;

Do that in the earlier patch to reduce churn a tiny bit.

> > +
> > +	if (!cxl || !cxl->region)
> >  		return;
> > +
> > +	vfio_cxl_destroy_cxl_region(vdev);
> >  }