RE: [PATCH 2/2] vfio/pci: Use unmap_mapping_range()

Tian, Kevin
2024-05-24T08:40:32Z

> From: Peter Xu <peterx@redhat.com>
> Sent: Friday, May 24, 2024 8:49 AM
>
> Hi, Yan,
>
> On Fri, May 24, 2024 at 08:39:37AM +0800, Yan Zhao wrote:
> > On Thu, May 23, 2024 at 01:56:27PM -0600, Alex Williamson wrote:
> > > With the vfio device fd tied to the address space of the pseudo fs
> > > inode, we can use the mm to track all vmas that might be mmap'ing
> > > device BARs, which removes our vma_list and all the complicated lock
> > > ordering necessary to manually zap each related vma.
> > >
> > > Note that we can no longer store the pfn in vm_pgoff if we want to use
> > > unmap_mapping_range() to zap a selective portion of the device fd
> > > corresponding to BAR mappings.
> > >
> > > This also converts our mmap fault handler to use vmf_insert_pfn()
> > Looks vmf_insert_pfn() does not call memtype_reserve() to reserve
> memory type
> > for the PFN on x86 as what's done in io_remap_pfn_range().
> >
> > Instead, it just calls lookup_memtype() and determine the final prot based
> on
> > the result from this lookup, which might not prevent others from reserving
> the
> > PFN to other memory types.
>
> I didn't worry too much on others reserving the same pfn range, as that
> should be the mmio region for this device, and this device should be owned
> by vfio driver.

and the earliest point doing memtype_reserve() is here:

vfio_pci_core_mmap()
vdev->barmap[index] = pci_iomap(pdev, index, 0);

>
> However I share the same question, see:
>
> https://lore.kernel.org/r/20240523223745.395337-2-peterx@redhat.com
>
> So far I think it's not a major issue as VFIO always use UC- mem type, and
> that's also the default. But I do also feel like there's something we can
> do better, and I'll keep you copied too if I'll resend the series.
>

vfio-nvgrace uses WC. But it directly does remap_pfn_range() in its
nvgrace_gpu_mmap() so not suffering from the issue here.