linux-kernel - [PATCH] vfio iommu type1: improve memory pinning process for raw PFN mapping

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Date:   Thu, 22 Mar 2018 12:52:16 +0800
From:   "Jason Cai (Xiang Feng)" <jason.cai@...ux.alibaba.com>
To:     alex.williamson@...hat.com, pbonzini@...hat.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     gnehzuil@...ux.alibaba.com, jason.cai@...ux.alibaba.com
Subject: [PATCH] vfio iommu type1: improve memory pinning process for raw PFN mapping

When using vfio to pass through a PCIe device (e.g. a GPU card) that
has a huge BAR (e.g. 16GB), a lot of cycles are wasted on memory
pinning because PFNs of PCI BAR are not backed by struct page, and
the corresponding VMA has flag VM_PFNMAP.

With this change, when pinning a region which is a raw PFN mapping,
it can skip unnecessary user memory pinning process, and thus, can
significantly improve VM's boot up time when passing through devices
via VFIO. In my test on a Xeon E5 2.6GHz, the time mapping a 16GB
BAR was reduced from about 0.4s to 1.5us.

Signed-off-by: Jason Cai (Xiang Feng) <jason.cai@...ux.alibaba.com>
---
 drivers/vfio/vfio_iommu_type1.c | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
index 45657e2b1ff7..0658f35318b8 100644
--- a/drivers/vfio/vfio_iommu_type1.c
+++ b/drivers/vfio/vfio_iommu_type1.c
@@ -397,7 +397,6 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 {
 	unsigned long pfn = 0;
 	long ret, pinned = 0, lock_acct = 0;
-	bool rsvd;
 	dma_addr_t iova = vaddr - dma->vaddr + dma->iova;
 
 	/* This code path is only user initiated */
@@ -408,14 +407,22 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 	if (ret)
 		return ret;
 
+	if (is_invalid_reserved_pfn(*pfn_base)) {
+		struct vm_area_struct *vma;
+		down_read(&current->mm->mmap_sem);
+		vma = find_vma_intersection(current->mm, vaddr, vaddr + 1);
+		pinned = min(npage, (long)vma_pages(vma));
+		up_read(&current->mm->mmap_sem);
+		return pinned;
+	}
+
 	pinned++;
-	rsvd = is_invalid_reserved_pfn(*pfn_base);
 
 	/*
 	 * Reserved pages aren't counted against the user, externally pinned
 	 * pages are already counted against the user.
 	 */
-	if (!rsvd && !vfio_find_vpfn(dma, iova)) {
+	if (!vfio_find_vpfn(dma, iova)) {
 		if (!lock_cap && current->mm->locked_vm + 1 > limit) {
 			put_pfn(*pfn_base, dma->prot);
 			pr_warn("%s: RLIMIT_MEMLOCK (%ld) exceeded\n", __func__,
@@ -435,13 +442,12 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 		if (ret)
 			break;
 
-		if (pfn != *pfn_base + pinned ||
-		    rsvd != is_invalid_reserved_pfn(pfn)) {
+		if (pfn != *pfn_base + pinned) {
 			put_pfn(pfn, dma->prot);
 			break;
 		}
 
-		if (!rsvd && !vfio_find_vpfn(dma, iova)) {
+		if (!vfio_find_vpfn(dma, iova)) {
 			if (!lock_cap &&
 			    current->mm->locked_vm + lock_acct + 1 > limit) {
 				put_pfn(pfn, dma->prot);
@@ -459,10 +465,8 @@ static long vfio_pin_pages_remote(struct vfio_dma *dma, unsigned long vaddr,
 
 unpin_out:
 	if (ret) {
-		if (!rsvd) {
-			for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
-				put_pfn(pfn, dma->prot);
-		}
+		for (pfn = *pfn_base ; pinned ; pfn++, pinned--)
+			put_pfn(pfn, dma->prot);
 
 		return ret;
 	}
-- 
2.13.6