lkml.org 
[lkml]   [2021]   [Mar]   [9]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant
    On Tue, Mar 09, 2021 at 12:26:07PM -0700, Alex Williamson wrote:
    > On Tue, 9 Mar 2021 13:47:39 -0500
    > Peter Xu <peterx@redhat.com> wrote:
    >
    > > On Tue, Mar 09, 2021 at 12:40:04PM -0400, Jason Gunthorpe wrote:
    > > > On Tue, Mar 09, 2021 at 08:29:51AM -0700, Alex Williamson wrote:
    > > > > On Tue, 9 Mar 2021 08:46:09 -0400
    > > > > Jason Gunthorpe <jgg@nvidia.com> wrote:
    > > > >
    > > > > > On Tue, Mar 09, 2021 at 03:49:09AM +0000, Zengtao (B) wrote:
    > > > > > > Hi guys:
    > > > > > >
    > > > > > > Thanks for the helpful comments, after rethinking the issue, I have proposed
    > > > > > > the following change:
    > > > > > > 1. follow_pte instead of follow_pfn.
    > > > > >
    > > > > > Still no on follow_pfn, you don't need it once you use vmf_insert_pfn
    > > > >
    > > > > vmf_insert_pfn() only solves the BUG_ON, follow_pte() is being used
    > > > > here to determine whether the translation is already present to avoid
    > > > > both duplicate work in inserting the translation and allocating a
    > > > > duplicate vma tracking structure.
    > > >
    > > > Oh.. Doing something stateful in fault is not nice at all
    > > >
    > > > I would rather see __vfio_pci_add_vma() search the vma_list for dups
    > > > than call follow_pfn/pte..
    > >
    > > It seems to me that searching vma list is still the simplest way to fix the
    > > problem for the current code base. I see io_remap_pfn_range() is also used in
    > > the new series - maybe that'll need to be moved to where PCI_COMMAND_MEMORY got
    > > turned on/off in the new series (I just noticed remap_pfn_range modifies vma
    > > flags..), as you suggested in the other email.
    >
    >
    > In the new series, I think the fault handler becomes (untested):
    >
    > static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
    > {
    > struct vm_area_struct *vma = vmf->vma;
    > struct vfio_pci_device *vdev = vma->vm_private_data;
    > unsigned long base_pfn, pgoff;
    > vm_fault_t ret = VM_FAULT_SIGBUS;
    >
    > if (vfio_pci_bar_vma_to_pfn(vma, &base_pfn))
    > return ret;
    >
    > pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;
    >
    > down_read(&vdev->memory_lock);
    >
    > if (__vfio_pci_memory_enabled(vdev))
    > ret = vmf_insert_pfn(vma, vmf->address, pgoff + base_pfn);
    >
    > up_read(&vdev->memory_lock);
    >
    > return ret;
    > }

    It's just that the initial MMIO access delay would be spread to the 1st access
    of each mmio page access rather than using the previous pre-fault scheme. I
    think an userspace cares the delay enough should pre-fault all pages anyway,
    but just raise this up. Otherwise looks sane.

    Thanks,

    --
    Peter Xu

    \
     
     \ /
      Last update: 2021-03-09 20:50    [W:5.548 / U:0.316 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site