Re: [PATCH 5/9] vfio/pci: Provide a user-facing name for BAR mappings

[Matt Evans <mattev@meta.com>]

Sent too soon, :|

On 07/05/2026 17:56, Matt Evans wrote:
> Hi Alex,
> 
> On 01/05/2026 23:44, Alex Williamson wrote:
>>
>> On Thu, 16 Apr 2026 06:17:48 -0700
>> Matt Evans <mattev@meta.com> wrote:
>>
>>> Since converting BAR mmap()s to using DMABUFs, we lose the original
>>> device path in /proc/<pid>/maps, lsof, etc.  Generate a debug-oriented
>>> synthetic 'filename' based on the cdev, plus BDF, plus resource index.
>>>
>>> This applies only to BAR mappings via the VFIO device fd, as
>>> explicitly-exported DMABUFs are named by userspace via the
>>> DMA_BUF_SET_NAME ioctl.
>>>
>>> Signed-off-by: Matt Evans <mattev@meta.com>
>>> ---
>>>   drivers/vfio/pci/vfio_pci_dmabuf.c | 27 +++++++++++++++++++++++++--
>>>   1 file changed, 25 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/vfio/pci/vfio_pci_dmabuf.c b/drivers/vfio/pci/ 
>>> vfio_pci_dmabuf.c
>>> index a12432825e5e..04c7733fe712 100644
>>> --- a/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> +++ b/drivers/vfio/pci/vfio_pci_dmabuf.c
>>> @@ -4,6 +4,7 @@
>>>   #include <linux/dma-buf-mapping.h>
>>>   #include <linux/pci-p2pdma.h>
>>>   #include <linux/dma-resv.h>
>>> +#include <uapi/linux/dma-buf.h>
>>>   #include "vfio_pci_priv.h"
>>> @@ -467,6 +468,7 @@ int vfio_pci_core_mmap_prep_dmabuf(struct 
>>> vfio_pci_core_device *vdev,
>>>   {
>>>       struct vfio_pci_dma_buf *priv;
>>>       const unsigned int nr_ranges = 1;
>>> +    char *bufname;
>>>       int ret;
>>>       priv = kzalloc_obj(*priv);
>>> @@ -479,6 +481,20 @@ int vfio_pci_core_mmap_prep_dmabuf(struct 
>>> vfio_pci_core_device *vdev,
>>>           goto err_free_priv;
>>>       }
>>> +    bufname = kzalloc(DMA_BUF_NAME_LEN, GFP_KERNEL);
>>> +    if (!bufname) {
>>> +        ret = -ENOMEM;
>>> +        goto err_free_phys;
>>> +    }
>>> +
>>> +    /*
>>> +     * Maximum size of the friendly debug name is
>>> +     * vfio1234567890:ffff:ff:3f.7-9 = 30, which fits within
>>> +     * DMA_BUF_NAME_LEN.
>>> +     */
>>> +    snprintf(bufname, DMA_BUF_NAME_LEN, "%s:%s/%x",
>>> +         dev_name(&vdev->vdev.device), pci_name(vdev->pdev), 
>>> res_index);
>>
>> Comment suggests 9 is the max res_index that can be printed, but mmap
>> only directly supports standard BARs 0-5.  Comment also uses a '-'
>> while the code uses a '/'.  Thanks,
> 
> Right you are.  Fixed, but, since...
> https://lore.kernel.org/kvm/52162da4-e1cc-4f90-a95a-218d6089cd71@meta.com/
> 
> ...I'm keeping the resource index encoded in the vm_pgoffs and as that's
> in /proc/<pid>/maps it doesn't need to be in the name. I.e., an example
> mapping of BAR 2 looks like:
> 
> ffffa9330000-ffffad300000 rw-s 20000030000 00:0b 12 / 
> dmabuf:vfio0:0000:00:03.0

BUT, the name's visible via paths other than just /proc/<pid>/maps, e.g.
/sys/kernel/debug/dma_buf/bufinfo or /proc/<pid>/map_files which don't 
have the vm_offs, and so back to plan A.  Just made the comment consistent.

Matt