Re: [PATCH] KVM: arm64: Correctly handle the mmio faulting

[Santosh Shukla <santosh.shukla1982@gmail.com>]

On Thu, Apr 22, 2021 at 1:07 PM Tarun Gupta (SW-GPU)
<targupta@nvidia.com> wrote:
>
>
>
> On 4/22/2021 12:20 PM, Marc Zyngier wrote:
> > External email: Use caution opening links or attachments
> >
> >
> > On Thu, 22 Apr 2021 03:02:00 +0100,
> > Gavin Shan <gshan@redhat.com> wrote:
> >>
> >> Hi Marc,
> >>
> >> On 4/21/21 9:59 PM, Marc Zyngier wrote:
> >>> On Wed, 21 Apr 2021 07:17:44 +0100,
> >>> Keqian Zhu <zhukeqian1@huawei.com> wrote:
> >>>> On 2021/4/21 14:20, Gavin Shan wrote:
> >>>>> On 4/21/21 12:59 PM, Keqian Zhu wrote:
> >>>>>> On 2020/10/22 0:16, Santosh Shukla wrote:
> >>>>>>> The Commit:6d674e28 introduces a notion to detect and handle the
> >>>>>>> device mapping. The commit checks for the VM_PFNMAP flag is set
> >>>>>>> in vma->flags and if set then marks force_pte to true such that
> >>>>>>> if force_pte is true then ignore the THP function check
> >>>>>>> (/transparent_hugepage_adjust()).
> >>>>>>>
> >>>>>>> There could be an issue with the VM_PFNMAP flag setting and checking.
> >>>>>>> For example consider a case where the mdev vendor driver register's
> >>>>>>> the vma_fault handler named vma_mmio_fault(), which maps the
> >>>>>>> host MMIO region in-turn calls remap_pfn_range() and maps
> >>>>>>> the MMIO's vma space. Where, remap_pfn_range implicitly sets
> >>>>>>> the VM_PFNMAP flag into vma->flags.
> >>>>>> Could you give the name of the mdev vendor driver that triggers this issue?
> >>>>>> I failed to find one according to your description. Thanks.
> >>>>>>
> >>>>>
> >>>>> I think it would be fixed in driver side to set VM_PFNMAP in
> >>>>> its mmap() callback (call_mmap()), like vfio PCI driver does.
> >>>>> It means it won't be delayed until page fault is issued and
> >>>>> remap_pfn_range() is called. It's determined from the beginning
> >>>>> that the vma associated the mdev vendor driver is serving as
> >>>>> PFN remapping purpose. So the vma should be populated completely,
> >>>>> including the VM_PFNMAP flag before it becomes visible to user
> >>>>> space.
> >>>
> >>> Why should that be a requirement? Lazy populating of the VMA should be
> >>> perfectly acceptable if the fault can only happen on the CPU side.
> >>>

Right.
Hi keqian,
You can refer to case
http://lkml.iu.edu/hypermail/linux/kernel/2010.3/00952.html

(Sorry Guys, I am not with nvidia, but My quick input.)

> >>
> >> It isn't a requirement and the drivers needn't follow strictly. I checked
> >> several drivers before looking into the patch and found almost all the
> >> drivers have VM_PFNMAP set at mmap() time. In drivers/vfio/vfio-pci.c,
> >> there is a comment as below, but it doesn't reveal too much about why
> >> we can't set VM_PFNMAP at fault time.
> >>
> >> static int vfio_pci_mmap(void *device_data, struct vm_area_struct *vma)
> >> {
> >>        :
> >>          /*
> >>           * See remap_pfn_range(), called from vfio_pci_fault() but we can't
> >>           * change vm_flags within the fault handler.  Set them now.
> >>           */
> >>          vma->vm_flags |= VM_IO | VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
> >>          vma->vm_ops = &vfio_pci_mmap_ops;
> >>
> >>          return 0;
> >> }
> >>
> >> To set these flags in advance does have advantages. For example,
> >> VM_DONTEXPAND prevents the vma to be merged with another
> >> one. VM_DONTDUMP make this vma isn't eligible for
> >> coredump. Otherwise, the address space, which is associated with the
> >> vma is accessed and unnecessary page faults are triggered on
> >> coredump.  VM_IO and VM_PFNMAP avoids to walk the page frames
> >> associated with the vma since we don't have valid PFN in the
> >> mapping.
> >
> > But PCI clearly isn't the case we are dealing with here, and not
> > everything is VFIO either. I can *today* create a driver that
> > implements a mmap+fault handler, call mmap() on it, pass the result to
> > a memslot, and get to the exact same result Santosh describes.
> >
> > No PCI, no VFIO, just a random driver. We are *required* to handle
> > that.
>
> Agree with Marc here, that kernel should be able to handle it without
> VM_PFNMAP flag set in driver.
>
> For driver reference, you could check the V2 version of this patch that
> got accepted upstream and has details as-to how this can be reproduced
> using vfio-pci: https://www.spinics.net/lists/arm-kernel/msg848491.html
>
> >
> >          M.
> >
> > --
> > Without deviation from the norm, progress is not possible.
> >
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