Re: [PATCH v2 1/3] vfio/type1: Support faulting PFNMAP vmas

Peter Xu
2020-05-07T21:24:51Z

On Tue, May 05, 2020 at 03:54:44PM -0600, Alex Williamson wrote:
> With conversion to follow_pfn(), DMA mapping a PFNMAP range depends on
> the range being faulted into the vma. Add support to manually provide
> that, in the same way as done on KVM with hva_to_pfn_remapped().
>
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
> drivers/vfio/vfio_iommu_type1.c | 36 +++++++++++++++++++++++++++++++++---
> 1 file changed, 33 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index cc1d64765ce7..4a4cb7cd86b2 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -317,6 +317,32 @@ static int put_pfn(unsigned long pfn, int prot)
> return 0;
> }
>
> +static int follow_fault_pfn(struct vm_area_struct *vma, struct mm_struct *mm,
> + unsigned long vaddr, unsigned long *pfn,
> + bool write_fault)
> +{
> + int ret;
> +
> + ret = follow_pfn(vma, vaddr, pfn);
> + if (ret) {
> + bool unlocked = false;
> +
> + ret = fixup_user_fault(NULL, mm, vaddr,
> + FAULT_FLAG_REMOTE |
> + (write_fault ? FAULT_FLAG_WRITE : 0),
> + &unlocked);
> + if (unlocked)
> + return -EAGAIN;

Hi, Alex,

IIUC this retry is not needed too because fixup_user_fault() will guarantee the
fault-in is done correctly with the valid PTE as long as ret==0, even if
unlocked==true.

Note: there's another patch just removed the similar retry in kvm:

https://lore.kernel.org/kvm/20200416155906.267462-1-peterx@redhat.com/

Thanks,

> +
> + if (ret)
> + return ret;
> +
> + ret = follow_pfn(vma, vaddr, pfn);
> + }
> +
> + return ret;
> +}

--
Peter Xu