linux-kernel - Re: [PATCH v1 10/16] iommufd: Add mmap interface

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Message-ID: <aAaENzZDO9/HQSKX@Asurada-Nvidia>
Date: Mon, 21 Apr 2025 10:45:27 -0700
From: Nicolin Chen <nicolinc@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: "jgg@...dia.com" <jgg@...dia.com>, "corbet@....net" <corbet@....net>,
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Subject: Re: [PATCH v1 10/16] iommufd: Add mmap interface

On Mon, Apr 21, 2025 at 08:16:54AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Friday, April 11, 2025 2:38 PM
> > + * previously given to user space via a prior ioctl output.
> > + */
> > +static int iommufd_fops_mmap(struct file *filp, struct vm_area_struct *vma)
> > +{
> > +	struct iommufd_ctx *ictx = filp->private_data;
> > +	size_t size = vma->vm_end - vma->vm_start;
> > +	struct iommufd_mmap *immap;
> > +
> > +	if (size & ~PAGE_MASK)
> > +		return -EINVAL;
> > +	if (!(vma->vm_flags & VM_SHARED))
> > +		return -EINVAL;
> > +	if (vma->vm_flags & VM_EXEC)
> > +		return -EPERM;
> > +
> > +	/* vm_pgoff carries an index of an mtree entry/immap */
> > +	immap = mtree_load(&ictx->mt_mmap, vma->vm_pgoff);
> > +	if (!immap)
> > +		return -EINVAL;
> > +	if (size >> PAGE_SHIFT > immap->pfn_end - immap->pfn_start + 1)
> > +		return -EINVAL;
> 
> Do we want to document in uAPI that iommufd mmap allows to map
> a sub-region (starting from offset zero) of the reported size from earlier
> alloc ioctl, but not from random offset (of course impossible by forcing
> vm_pgoff to be a mtree index)?

I also did this:

diff --git a/Documentation/userspace-api/iommufd.rst b/Documentation/userspace-api/iommufd.rst
index ace0579432d57..f57a5bf2feea1 100644
--- a/Documentation/userspace-api/iommufd.rst
+++ b/Documentation/userspace-api/iommufd.rst
@@ -128,11 +128,13 @@ Following IOMMUFD objects are exposed to userspace:
   virtualization feature for a VM to directly execute guest-issued commands to
   invalidate HW cache entries holding the mappings or translations of a guest-
   owned stage-1 page table. Along with this queue object, iommufd provides the
-  user space a new mmap interface that the VMM can mmap a physical MMIO region
-  from the host physical address space to a guest physical address space. To use
-  this mmap interface, the VMM must define an IOMMU specific driver structure
-  to ask for a pair of VMA info (vm_pgoff/size) to do mmap after a vCMDQ gets
-  allocated.
+  user space an mmap interface for VMM to mmap a physical MMIO region from the
+  host physical address space to a guest physical address space. When allocating
+  a vCMDQ, the VMM must request a pair of VMA info (vm_pgoff/size) for a later
+  mmap call. The length argument of an mmap call can be smaller than the given
+  size for a paritial mmap, but the given vm_pgoff (as the addr argument of the
+  mmap call) should never be changed, which implies that the mmap always starts
+  from the beginning of the MMIO region.
 
 All user-visible objects are destroyed via the IOMMU_DESTROY uAPI.
 
Thanks
Nicolin