linux-kernel - [PATCH AUTOSEL 6.9 20/21] vfio/pci: Insert full vma on mmap'd MMIO fault

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Date: Sun, 23 Jun 2024 09:43:53 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Alex Williamson <alex.williamson@...hat.com>,
	Yan Zhao <yan.y.zhao@...el.com>,
	Sasha Levin <sashal@...nel.org>,
	jgg@...pe.ca,
	yi.l.liu@...el.com,
	kevin.tian@...el.com,
	eric.auger@...hat.com,
	brauner@...nel.org,
	ankita@...dia.com,
	stefanha@...hat.com,
	kvm@...r.kernel.org
Subject: [PATCH AUTOSEL 6.9 20/21] vfio/pci: Insert full vma on mmap'd MMIO fault

From: Alex Williamson <alex.williamson@...hat.com>

[ Upstream commit d71a989cf5d961989c273093cdff2550acdde314 ]

In order to improve performance of typical scenarios we can try to insert
the entire vma on fault.  This accelerates typical cases, such as when
the MMIO region is DMA mapped by QEMU.  The vfio_iommu_type1 driver will
fault in the entire DMA mapped range through fixup_user_fault().

In synthetic testing, this improves the time required to walk a PCI BAR
mapping from userspace by roughly 1/3rd.

This is likely an interim solution until vmf_insert_pfn_{pmd,pud}() gain
support for pfnmaps.

Suggested-by: Yan Zhao <yan.y.zhao@...el.com>
Link: https://lore.kernel.org/all/Zl6XdUkt%2FzMMGOLF@yzhao56-desk.sh.intel.com/
Reviewed-by: Yan Zhao <yan.y.zhao@...el.com>
Link: https://lore.kernel.org/r/20240607035213.2054226-1-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/vfio/pci/vfio_pci_core.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
index 2baf4dfac3f43..9eaf10a8f134b 100644
--- a/drivers/vfio/pci/vfio_pci_core.c
+++ b/drivers/vfio/pci/vfio_pci_core.c
@@ -1639,6 +1639,7 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
 	struct vm_area_struct *vma = vmf->vma;
 	struct vfio_pci_core_device *vdev = vma->vm_private_data;
 	unsigned long pfn, pgoff = vmf->pgoff - vma->vm_pgoff;
+	unsigned long addr = vma->vm_start;
 	vm_fault_t ret = VM_FAULT_SIGBUS;
 
 	pfn = vma_to_pfn(vma);
@@ -1646,11 +1647,25 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
 	down_read(&vdev->memory_lock);
 
 	if (vdev->pm_runtime_engaged || !__vfio_pci_memory_enabled(vdev))
-		goto out_disabled;
+		goto out_unlock;
 
 	ret = vmf_insert_pfn(vma, vmf->address, pfn + pgoff);
+	if (ret & VM_FAULT_ERROR)
+		goto out_unlock;
 
-out_disabled:
+	/*
+	 * Pre-fault the remainder of the vma, abort further insertions and
+	 * supress error if fault is encountered during pre-fault.
+	 */
+	for (; addr < vma->vm_end; addr += PAGE_SIZE, pfn++) {
+		if (addr == vmf->address)
+			continue;
+
+		if (vmf_insert_pfn(vma, addr, pfn) & VM_FAULT_ERROR)
+			break;
+	}
+
+out_unlock:
 	up_read(&vdev->memory_lock);
 
 	return ret;
-- 
2.43.0