linux-kernel - Re: [RFC v2 07/15] vfio/cxl: expose CXL region to the userspace via a new VFIO device region

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Message-ID: <20251222140009.0000298a@huawei.com>
Date: Mon, 22 Dec 2025 14:00:09 +0000
From: Jonathan Cameron <jonathan.cameron@...wei.com>
To: <mhonap@...dia.com>
CC: <aniketa@...dia.com>, <ankita@...dia.com>, <alwilliamson@...dia.com>,
	<vsethi@...dia.com>, <jgg@...dia.com>, <mochs@...dia.com>,
	<skolothumtho@...dia.com>, <alejandro.lucero-palau@....com>,
	<dave@...olabs.net>, <dave.jiang@...el.com>, <alison.schofield@...el.com>,
	<vishal.l.verma@...el.com>, <ira.weiny@...el.com>,
	<dan.j.williams@...el.com>, <jgg@...pe.ca>, <yishaih@...dia.com>,
	<kevin.tian@...el.com>, <cjia@...dia.com>, <kwankhede@...dia.com>,
	<targupta@...dia.com>, <zhiw@...dia.com>, <kjaju@...dia.com>,
	<linux-kernel@...r.kernel.org>, <linux-cxl@...r.kernel.org>,
	<kvm@...r.kernel.org>
Subject: Re: [RFC v2 07/15] vfio/cxl: expose CXL region to the userspace via
 a new VFIO device region

On Tue, 9 Dec 2025 22:20:11 +0530
mhonap@...dia.com wrote:

> From: Manish Honap <mhonap@...dia.com>
> 
> To directly access the device memory, a CXL region is required. Creating
> a CXL region requires to configure HDM decoders on the path to map the
> access of HPA level by level and evetually hit the DPA in the CXL
> topology.
> 
> For the userspace, e.g. QEMU, to access the CXL region, the region is
> required to be exposed via VFIO interfaces.
> 
> Introduce a new VFIO device region and region ops to expose the created
> CXL region when initialize the device in the vfio-cxl-core. Introduce a
> new sub region type for the userspace to identify a CXL region.
> 
> Co-developed-by: Zhi Wang <zhiw@...dia.com>
> Signed-off-by: Zhi Wang <zhiw@...dia.com>
> Signed-off-by: Manish Honap <mhonap@...dia.com>
A few really minor things inline.

> ---
>  drivers/vfio/pci/vfio_cxl_core.c | 122 +++++++++++++++++++++++++++++++
>  drivers/vfio/pci/vfio_pci_core.c |   3 +-
>  include/linux/vfio_pci_core.h    |   5 ++
>  include/uapi/linux/vfio.h        |   4 +
>  4 files changed, 133 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/vfio/pci/vfio_cxl_core.c b/drivers/vfio/pci/vfio_cxl_core.c
> index cf53720c0cb7..35d95de47fa8 100644
> --- a/drivers/vfio/pci/vfio_cxl_core.c
> +++ b/drivers/vfio/pci/vfio_cxl_core.c
> @@ -231,6 +231,128 @@ void vfio_cxl_core_destroy_cxl_region(struct vfio_cxl_core_device *cxl)
>  }
>  EXPORT_SYMBOL_GPL(vfio_cxl_core_destroy_cxl_region);
>  
> +static int vfio_cxl_region_mmap(struct vfio_pci_core_device *pci,
> +				struct vfio_pci_region *region,
> +				struct vm_area_struct *vma)
> +{
> +	struct vfio_cxl_region *cxl_region = region->data;
> +	u64 req_len, pgoff, req_start, end;
> +	int ret;
> +
> +	if (!(region->flags & VFIO_REGION_INFO_FLAG_MMAP))
> +		return -EINVAL;
> +
> +	if (!(region->flags & VFIO_REGION_INFO_FLAG_READ) &&
> +	    (vma->vm_flags & VM_READ))
> +		return -EPERM;
> +
> +	if (!(region->flags & VFIO_REGION_INFO_FLAG_WRITE) &&
> +	    (vma->vm_flags & VM_WRITE))
> +		return -EPERM;
> +
> +	pgoff = vma->vm_pgoff &
> +		((1U << (VFIO_PCI_OFFSET_SHIFT - PAGE_SHIFT)) - 1);

GENMASK() might be slightly easier to read and makes it really obvious
this is a simple masking operation.

> +
> +	if (check_sub_overflow(vma->vm_end, vma->vm_start, &req_len) ||
> +	    check_add_overflow(PHYS_PFN(cxl_region->addr), pgoff, &req_start) ||
> +	    check_add_overflow(PFN_PHYS(pgoff), req_len, &end))
> +		return -EOVERFLOW;
> +
> +	if (end > cxl_region->size)
> +		return -EINVAL;
> +
> +	if (cxl_region->noncached)
> +		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +	vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> +
> +	vm_flags_set(vma, VM_ALLOW_ANY_UNCACHED | VM_IO | VM_PFNMAP |
> +		     VM_DONTEXPAND | VM_DONTDUMP);
> +
> +	ret = remap_pfn_range(vma, vma->vm_start, req_start,
> +			      req_len, vma->vm_page_prot);
> +	if (ret)
> +		return ret;
> +
> +	vma->vm_pgoff = req_start;
> +
> +	return 0;
> +}
> +
> +static ssize_t vfio_cxl_region_rw(struct vfio_pci_core_device *core_dev,
> +				  char __user *buf, size_t count, loff_t *ppos,
> +				  bool iswrite)
> +{
> +	unsigned int i = VFIO_PCI_OFFSET_TO_INDEX(*ppos) - VFIO_PCI_NUM_REGIONS;
> +	struct vfio_cxl_region *cxl_region = core_dev->region[i].data;
> +	loff_t pos = *ppos & VFIO_PCI_OFFSET_MASK;
> +
> +	if (!count)
> +		return 0;
> +
> +	return vfio_pci_core_do_io_rw(core_dev, false,
> +				      cxl_region->vaddr,
> +				      (char __user *)buf, pos, count,

buf is already a char __user * so not sure why you'd need a cast here.

> +				      0, 0, iswrite);
> +}