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

[Gavin Shan]

Hi Marc,

On 4/22/21 4:50 PM, Marc Zyngier wrote:
> On Thu, 22 Apr 2021 03:02:00 +0100,
> Gavin Shan <gshan@xxxxxxxxxx> wrote:
> > On 4/21/21 9:59 PM, Marc Zyngier wrote:
> > > On Wed, 21 Apr 2021 07:17:44 +0100,
> > > Keqian Zhu <zhukeqian1@xxxxxxxxxx> 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.
> >
> > 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.

hmm, ok. I was thinking it's related to VFIO mdev driver when Santosh was
talking about "mdev driver". Anyway, it's always nice to support the case :)

Thanks,
Gavin