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

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Date:   Thu, 7 May 2020 17:24:43 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        cohuck@...hat.com, jgg@...pe.ca
Subject: Re: [PATCH v2 1/3] vfio/type1: Support faulting PFNMAP vmas

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@...hat.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