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Commit add6a0cd authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

KVM: MMU: try to fix up page faults before giving up 



The vGPU folks would like to trap the first access to a BAR by setting
vm_ops on the VMAs produced by mmap-ing a VFIO device.  The fault handler
then can use remap_pfn_range to place some non-reserved pages in the VMA.

This kind of VM_PFNMAP mapping is not handled by KVM, but follow_pfn
and fixup_user_fault together help supporting it.  The patch also supports
VM_MIXEDMAP vmas where the pfns are not reserved and thus subject to
reference counting.

Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Tested-by: default avatarNeo Jia <cjia@nvidia.com>
Reported-by: default avatarKirti Wankhede <kwankhede@nvidia.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 92176a8e

mm/gup.c

+1 −0
Original line number Diff line number Diff line
@@ -720,6 +720,7 @@ int fixup_user_fault(struct task_struct *tsk, struct mm_struct *mm, 
	}

	return 0;

}

EXPORT_SYMBOL_GPL(fixup_user_fault);



static __always_inline long __get_user_pages_locked(struct task_struct *tsk,

						struct mm_struct *mm,

virt/kvm/kvm_main.c

+42 −3
Original line number Diff line number Diff line
@@ -1446,9 +1446,45 @@ static int hva_to_pfn_remapped(struct vm_area_struct *vma, 
			       unsigned long addr, bool *async,

			       bool write_fault, kvm_pfn_t *p_pfn)

{

	*p_pfn = ((addr - vma->vm_start) >> PAGE_SHIFT) +

		vma->vm_pgoff;

	BUG_ON(!kvm_is_reserved_pfn(*p_pfn));

	unsigned long pfn;

	int r;



	r = follow_pfn(vma, addr, &pfn);

	if (r) {

		/*

		 * get_user_pages fails for VM_IO and VM_PFNMAP vmas and does

		 * not call the fault handler, so do it here.

		 */

		bool unlocked = false;

		r = fixup_user_fault(current, current->mm, addr,

				     (write_fault ? FAULT_FLAG_WRITE : 0),

				     &unlocked);

		if (unlocked)

			return -EAGAIN;

		if (r)

			return r;



		r = follow_pfn(vma, addr, &pfn);

		if (r)

			return r;



	}





	/*

	 * Get a reference here because callers of *hva_to_pfn* and

	 * *gfn_to_pfn* ultimately call kvm_release_pfn_clean on the

	 * returned pfn.  This is only needed if the VMA has VM_MIXEDMAP

	 * set, but the kvm_get_pfn/kvm_release_pfn_clean pair will

	 * simply do nothing for reserved pfns.

	 *

	 * Whoever called remap_pfn_range is also going to call e.g.

	 * unmap_mapping_range before the underlying pages are freed,

	 * causing a call to our MMU notifier.

	 */ 

	kvm_get_pfn(pfn);



	*p_pfn = pfn;

	return 0;

}
@@ -1493,12 +1529,15 @@ static kvm_pfn_t hva_to_pfn(unsigned long addr, bool atomic, bool *async, 
		goto exit;

	}



retry:

	vma = find_vma_intersection(current->mm, addr, addr + 1);



	if (vma == NULL)

		pfn = KVM_PFN_ERR_FAULT;

	else if (vma->vm_flags & (VM_IO | VM_PFNMAP)) {

		r = hva_to_pfn_remapped(vma, addr, async, write_fault, &pfn);

		if (r == -EAGAIN)

			goto retry;

		if (r < 0)

			pfn = KVM_PFN_ERR_FAULT;

	} else {