On Fri, Mar 5, 2021 at 11:36 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
>
>
> On 2021/3/4 1:12 下午, Yongji Xie wrote:
> > On Thu, Mar 4, 2021 at 12:21 PM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> >>
> >> On 2021/2/23 7:50 下午, Xie Yongji wrote:
> >>> This implements a MMU-based IOMMU driver to support mapping
> >>> kernel dma buffer into userspace. The basic idea behind it is
> >>> treating MMU (VA->PA) as IOMMU (IOVA->PA). The driver will set
> >>> up MMU mapping instead of IOMMU mapping for the DMA transfer so
> >>> that the userspace process is able to use its virtual address to
> >>> access the dma buffer in kernel.
> >>>
> >>> And to avoid security issue, a bounce-buffering mechanism is
> >>> introduced to prevent userspace accessing the original buffer
> >>> directly.
> >>>
> >>> Signed-off-by: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
> >>> ---
> >>> drivers/vdpa/vdpa_user/iova_domain.c | 486 +++++++++++++++++++++++++++++++++++
> >>> drivers/vdpa/vdpa_user/iova_domain.h | 61 +++++
> >>> 2 files changed, 547 insertions(+)
> >>> create mode 100644 drivers/vdpa/vdpa_user/iova_domain.c
> >>> create mode 100644 drivers/vdpa/vdpa_user/iova_domain.h
> >>>
> >>> diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> >>> new file mode 100644
> >>> index 000000000000..9285d430d486
> >>> --- /dev/null
> >>> +++ b/drivers/vdpa/vdpa_user/iova_domain.c
> >>> @@ -0,0 +1,486 @@
> >>> +// SPDX-License-Identifier: GPL-2.0-only
> >>> +/*
> >>> + * MMU-based IOMMU implementation
> >>> + *
> >>> + * Copyright (C) 2020 Bytedance Inc. and/or its affiliates. All rights reserved.
> >>> + *
> >>> + * Author: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
> >>> + *
> >>> + */
> >>> +
> >>> +#include <linux/slab.h>
> >>> +#include <linux/file.h>
> >>> +#include <linux/anon_inodes.h>
> >>> +#include <linux/highmem.h>
> >>> +
> >>> +#include "iova_domain.h"
> >>> +
> >>> +#define IOVA_START_PFN 1
> >>> +#define IOVA_ALLOC_ORDER 12
> >>> +#define IOVA_ALLOC_SIZE (1 << IOVA_ALLOC_ORDER)
> >>> +
> >>> +static inline struct page *
> >>> +vduse_domain_get_bounce_page(struct vduse_iova_domain *domain, u64 iova)
> >>> +{
> >>> + u64 index = iova >> PAGE_SHIFT;
> >>> +
> >>> + return domain->bounce_pages[index];
> >>> +}
> >>> +
> >>> +static inline void
> >>> +vduse_domain_set_bounce_page(struct vduse_iova_domain *domain,
> >>> + u64 iova, struct page *page)
> >>> +{
> >>> + u64 index = iova >> PAGE_SHIFT;
> >>> +
> >>> + domain->bounce_pages[index] = page;
> >>> +}
> >>> +
> >>> +static enum dma_data_direction perm_to_dir(int perm)
> >>> +{
> >>> + enum dma_data_direction dir;
> >>> +
> >>> + switch (perm) {
> >>> + case VHOST_MAP_WO:
> >>> + dir = DMA_FROM_DEVICE;
> >>> + break;
> >>> + case VHOST_MAP_RO:
> >>> + dir = DMA_TO_DEVICE;
> >>> + break;
> >>> + case VHOST_MAP_RW:
> >>> + dir = DMA_BIDIRECTIONAL;
> >>> + break;
> >>> + default:
> >>> + break;
> >>> + }
> >>> +
> >>> + return dir;
> >>> +}
> >>> +
> >>> +static int dir_to_perm(enum dma_data_direction dir)
> >>> +{
> >>> + int perm = -EFAULT;
> >>> +
> >>> + switch (dir) {
> >>> + case DMA_FROM_DEVICE:
> >>> + perm = VHOST_MAP_WO;
> >>> + break;
> >>> + case DMA_TO_DEVICE:
> >>> + perm = VHOST_MAP_RO;
> >>> + break;
> >>> + case DMA_BIDIRECTIONAL:
> >>> + perm = VHOST_MAP_RW;
> >>> + break;
> >>> + default:
> >>> + break;
> >>> + }
> >>> +
> >>> + return perm;
> >>> +}
> >>
> >> Let's move the above two helpers to vhost_iotlb.h so they could be used
> >> by other driver e.g (vpda_sim)
> >>
> > Sure.
> >
> >>> +
> >>> +static void do_bounce(phys_addr_t orig, void *addr, size_t size,
> >>> + enum dma_data_direction dir)
> >>> +{
> >>> + unsigned long pfn = PFN_DOWN(orig);
> >>> +
> >>> + if (PageHighMem(pfn_to_page(pfn))) {
> >>> + unsigned int offset = offset_in_page(orig);
> >>> + char *buffer;
> >>> + unsigned int sz = 0;
> >>> + unsigned long flags;
> >>> +
> >>> + while (size) {
> >>> + sz = min_t(size_t, PAGE_SIZE - offset, size);
> >>> +
> >>> + local_irq_save(flags);
> >>> + buffer = kmap_atomic(pfn_to_page(pfn));
> >>> + if (dir == DMA_TO_DEVICE)
> >>> + memcpy(addr, buffer + offset, sz);
> >>> + else
> >>> + memcpy(buffer + offset, addr, sz);
> >>> + kunmap_atomic(buffer);
> >>> + local_irq_restore(flags);
> >>
> >> I wonder why we need to deal with highmem and irq flags explicitly like
> >> this. Doesn't kmap_atomic() will take care all of those?
> >>
> > Yes, irq flags is useless here. Will remove it.
> >
> >>> +
> >>> + size -= sz;
> >>> + pfn++;
> >>> + addr += sz;
> >>> + offset = 0;
> >>> + }
> >>> + } else if (dir == DMA_TO_DEVICE) {
> >>> + memcpy(addr, phys_to_virt(orig), size);
> >>> + } else {
> >>> + memcpy(phys_to_virt(orig), addr, size);
> >>> + }
> >>> +}
> >>> +
> >>> +static struct page *
> >>> +vduse_domain_get_mapping_page(struct vduse_iova_domain *domain, u64 iova)
> >>> +{
> >>> + u64 start = iova & PAGE_MASK;
> >>> + u64 last = start + PAGE_SIZE - 1;
> >>> + struct vhost_iotlb_map *map;
> >>> + struct page *page = NULL;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + map = vhost_iotlb_itree_first(domain->iotlb, start, last);
> >>> + if (!map)
> >>> + goto out;
> >>> +
> >>> + page = pfn_to_page((map->addr + iova - map->start) >> PAGE_SHIFT);
> >>> + get_page(page);
> >>> +out:
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> +
> >>> + return page;
> >>> +}
> >>> +
> >>> +static struct page *
> >>> +vduse_domain_alloc_bounce_page(struct vduse_iova_domain *domain, u64 iova)
> >>> +{
> >>> + u64 start = iova & PAGE_MASK;
> >>> + u64 last = start + PAGE_SIZE - 1;
> >>> + struct vhost_iotlb_map *map;
> >>> + struct page *page = NULL, *new_page = alloc_page(GFP_KERNEL);
> >>> +
> >>> + if (!new_page)
> >>> + return NULL;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + if (!vhost_iotlb_itree_first(domain->iotlb, start, last)) {
> >>> + __free_page(new_page);
> >>> + goto out;
> >>> + }
> >>> + page = vduse_domain_get_bounce_page(domain, iova);
> >>> + if (page) {
> >>> + get_page(page);
> >>> + __free_page(new_page);
> >>> + goto out;
> >>> + }
> >>> + vduse_domain_set_bounce_page(domain, iova, new_page);
> >>> + get_page(new_page);
> >>> + page = new_page;
> >>> +
> >>> + for (map = vhost_iotlb_itree_first(domain->iotlb, start, last); map;
> >>> + map = vhost_iotlb_itree_next(map, start, last)) {
> >>> + unsigned int src_offset = 0, dst_offset = 0;
> >>> + phys_addr_t src;
> >>> + void *dst;
> >>> + size_t sz;
> >>> +
> >>> + if (perm_to_dir(map->perm) == DMA_FROM_DEVICE)
> >>> + continue;
> >>> +
> >>> + if (start > map->start)
> >>> + src_offset = start - map->start;
> >>> + else
> >>> + dst_offset = map->start - start;
> >>> +
> >>> + src = map->addr + src_offset;
> >>> + dst = page_address(page) + dst_offset;
> >>> + sz = min_t(size_t, map->size - src_offset,
> >>> + PAGE_SIZE - dst_offset);
> >>> + do_bounce(src, dst, sz, DMA_TO_DEVICE);
> >>> + }
> >>> +out:
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> +
> >>> + return page;
> >>> +}
> >>> +
> >>> +static void
> >>> +vduse_domain_free_bounce_pages(struct vduse_iova_domain *domain,
> >>> + u64 iova, size_t size)
> >>> +{
> >>> + struct page *page;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + if (WARN_ON(vhost_iotlb_itree_first(domain->iotlb, iova,
> >>> + iova + size - 1)))
> >>> + goto out;
> >>> +
> >>> + while (size > 0) {
> >>> + page = vduse_domain_get_bounce_page(domain, iova);
> >>> + if (page) {
> >>> + vduse_domain_set_bounce_page(domain, iova, NULL);
> >>> + __free_page(page);
> >>> + }
> >>> + size -= PAGE_SIZE;
> >>> + iova += PAGE_SIZE;
> >>> + }
> >>> +out:
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> +}
> >>> +
> >>> +static void vduse_domain_bounce(struct vduse_iova_domain *domain,
> >>> + dma_addr_t iova, phys_addr_t orig,
> >>> + size_t size, enum dma_data_direction dir)
> >>> +{
> >>> + unsigned int offset = offset_in_page(iova);
> >>> +
> >>> + while (size) {
> >>> + struct page *p = vduse_domain_get_bounce_page(domain, iova);
> >>> + size_t sz = min_t(size_t, PAGE_SIZE - offset, size);
> >>> +
> >>> + WARN_ON(!p && dir == DMA_FROM_DEVICE);
> >>> +
> >>> + if (p)
> >>> + do_bounce(orig, page_address(p) + offset, sz, dir);
> >>> +
> >>> + size -= sz;
> >>> + orig += sz;
> >>> + iova += sz;
> >>> + offset = 0;
> >>> + }
> >>> +}
> >>> +
> >>> +static dma_addr_t vduse_domain_alloc_iova(struct iova_domain *iovad,
> >>> + unsigned long size, unsigned long limit)
> >>> +{
> >>> + unsigned long shift = iova_shift(iovad);
> >>> + unsigned long iova_len = iova_align(iovad, size) >> shift;
> >>> + unsigned long iova_pfn;
> >>> +
> >>> + if (iova_len < (1 << (IOVA_RANGE_CACHE_MAX_SIZE - 1)))
> >>> + iova_len = roundup_pow_of_two(iova_len);
> >>> + iova_pfn = alloc_iova_fast(iovad, iova_len, limit >> shift, true);
> >>> +
> >>> + return iova_pfn << shift;
> >>> +}
> >>> +
> >>> +static void vduse_domain_free_iova(struct iova_domain *iovad,
> >>> + dma_addr_t iova, size_t size)
> >>> +{
> >>> + unsigned long shift = iova_shift(iovad);
> >>> + unsigned long iova_len = iova_align(iovad, size) >> shift;
> >>> +
> >>> + free_iova_fast(iovad, iova >> shift, iova_len);
> >>> +}
> >>> +
> >>> +dma_addr_t vduse_domain_map_page(struct vduse_iova_domain *domain,
> >>> + struct page *page, unsigned long offset,
> >>> + size_t size, enum dma_data_direction dir,
> >>> + unsigned long attrs)
> >>> +{
> >>> + struct iova_domain *iovad = &domain->stream_iovad;
> >>> + unsigned long limit = domain->bounce_size - 1;
> >>> + phys_addr_t pa = page_to_phys(page) + offset;
> >>> + dma_addr_t iova = vduse_domain_alloc_iova(iovad, size, limit);
> >>> + int ret;
> >>> +
> >>> + if (!iova)
> >>> + return DMA_MAPPING_ERROR;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + ret = vhost_iotlb_add_range(domain->iotlb, (u64)iova,
> >>> + (u64)iova + size - 1,
> >>> + pa, dir_to_perm(dir));
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> + if (ret) {
> >>> + vduse_domain_free_iova(iovad, iova, size);
> >>> + return DMA_MAPPING_ERROR;
> >>> + }
> >>> + if (dir == DMA_TO_DEVICE || dir == DMA_BIDIRECTIONAL)
> >>> + vduse_domain_bounce(domain, iova, pa, size, DMA_TO_DEVICE);
> >>> +
> >>> + return iova;
> >>> +}
> >>> +
> >>> +void vduse_domain_unmap_page(struct vduse_iova_domain *domain,
> >>> + dma_addr_t dma_addr, size_t size,
> >>> + enum dma_data_direction dir, unsigned long attrs)
> >>> +{
> >>> + struct iova_domain *iovad = &domain->stream_iovad;
> >>> + struct vhost_iotlb_map *map;
> >>> + phys_addr_t pa;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + map = vhost_iotlb_itree_first(domain->iotlb, (u64)dma_addr,
> >>> + (u64)dma_addr + size - 1);
> >>> + if (WARN_ON(!map)) {
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> + return;
> >>> + }
> >>> + pa = map->addr;
> >>> + vhost_iotlb_map_free(domain->iotlb, map);
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> +
> >>> + if (dir == DMA_FROM_DEVICE || dir == DMA_BIDIRECTIONAL)
> >>> + vduse_domain_bounce(domain, dma_addr, pa,
> >>> + size, DMA_FROM_DEVICE);
> >>> +
> >>> + vduse_domain_free_iova(iovad, dma_addr, size);
> >>> +}
> >>> +
> >>> +void *vduse_domain_alloc_coherent(struct vduse_iova_domain *domain,
> >>> + size_t size, dma_addr_t *dma_addr,
> >>> + gfp_t flag, unsigned long attrs)
> >>> +{
> >>> + struct iova_domain *iovad = &domain->consistent_iovad;
> >>> + unsigned long limit = domain->iova_limit;
> >>> + dma_addr_t iova = vduse_domain_alloc_iova(iovad, size, limit);
> >>> + void *orig = alloc_pages_exact(size, flag);
> >>> + int ret;
> >>> +
> >>> + if (!iova || !orig)
> >>> + goto err;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + ret = vhost_iotlb_add_range(domain->iotlb, (u64)iova,
> >>> + (u64)iova + size - 1,
> >>> + virt_to_phys(orig), VHOST_MAP_RW);
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> + if (ret)
> >>> + goto err;
> >>> +
> >>> + *dma_addr = iova;
> >>> +
> >>> + return orig;
> >>> +err:
> >>> + *dma_addr = DMA_MAPPING_ERROR;
> >>> + if (orig)
> >>> + free_pages_exact(orig, size);
> >>> + if (iova)
> >>> + vduse_domain_free_iova(iovad, iova, size);
> >>> +
> >>> + return NULL;
> >>> +}
> >>> +
> >>> +void vduse_domain_free_coherent(struct vduse_iova_domain *domain, size_t size,
> >>> + void *vaddr, dma_addr_t dma_addr,
> >>> + unsigned long attrs)
> >>> +{
> >>> + struct iova_domain *iovad = &domain->consistent_iovad;
> >>> + struct vhost_iotlb_map *map;
> >>> + phys_addr_t pa;
> >>> +
> >>> + spin_lock(&domain->iotlb_lock);
> >>> + map = vhost_iotlb_itree_first(domain->iotlb, (u64)dma_addr,
> >>> + (u64)dma_addr + size - 1);
> >>> + if (WARN_ON(!map)) {
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> + return;
> >>> + }
> >>> + pa = map->addr;
> >>> + vhost_iotlb_map_free(domain->iotlb, map);
> >>> + spin_unlock(&domain->iotlb_lock);
> >>> +
> >>> + vduse_domain_free_iova(iovad, dma_addr, size);
> >>> + free_pages_exact(phys_to_virt(pa), size);
> >>> +}
> >>> +
> >>> +static vm_fault_t vduse_domain_mmap_fault(struct vm_fault *vmf)
> >>> +{
> >>> + struct vduse_iova_domain *domain = vmf->vma->vm_private_data;
> >>> + unsigned long iova = vmf->pgoff << PAGE_SHIFT;
> >>> + struct page *page;
> >>> +
> >>> + if (!domain)
> >>> + return VM_FAULT_SIGBUS;
> >>> +
> >>> + if (iova < domain->bounce_size)
> >>> + page = vduse_domain_alloc_bounce_page(domain, iova);
> >>> + else
> >>> + page = vduse_domain_get_mapping_page(domain, iova);
> >>> +
> >>> + if (!page)
> >>> + return VM_FAULT_SIGBUS;
> >>> +
> >>> + vmf->page = page;
> >>> +
> >>> + return 0;
> >>> +}
> >>> +
> >>> +static const struct vm_operations_struct vduse_domain_mmap_ops = {
> >>> + .fault = vduse_domain_mmap_fault,
> >>> +};
> >>> +
> >>> +static int vduse_domain_mmap(struct file *file, struct vm_area_struct *vma)
> >>> +{
> >>> + struct vduse_iova_domain *domain = file->private_data;
> >>> +
> >>> + vma->vm_flags |= VM_DONTDUMP | VM_DONTEXPAND;
> >>> + vma->vm_private_data = domain;
> >>> + vma->vm_ops = &vduse_domain_mmap_ops;
> >>> +
> >>> + return 0;
> >>> +}
> >>> +
> >>> +static int vduse_domain_release(struct inode *inode, struct file *file)
> >>> +{
> >>> + struct vduse_iova_domain *domain = file->private_data;
> >>> +
> >>> + vduse_domain_free_bounce_pages(domain, 0, domain->bounce_size);
> >>> + put_iova_domain(&domain->stream_iovad);
> >>> + put_iova_domain(&domain->consistent_iovad);
> >>> + vhost_iotlb_free(domain->iotlb);
> >>> + vfree(domain->bounce_pages);
> >>> + kfree(domain);
> >>> +
> >>> + return 0;
> >>> +}
> >>> +
> >>> +static const struct file_operations vduse_domain_fops = {
> >>> + .mmap = vduse_domain_mmap,
> >>> + .release = vduse_domain_release,
> >>> +};
> >>> +
> >>> +void vduse_domain_destroy(struct vduse_iova_domain *domain)
> >>> +{
> >>> + fput(domain->file);
> >>> +}
> >>> +
> >>> +struct vduse_iova_domain *
> >>> +vduse_domain_create(unsigned long iova_limit, size_t bounce_size)
> >>> +{
> >>> + struct vduse_iova_domain *domain;
> >>> + struct file *file;
> >>> + unsigned long bounce_pfns = PAGE_ALIGN(bounce_size) >> PAGE_SHIFT;
> >>> +
> >>> + if (iova_limit <= bounce_size)
> >>> + return NULL;
> >>> +
> >>> + domain = kzalloc(sizeof(*domain), GFP_KERNEL);
> >>> + if (!domain)
> >>> + return NULL;
> >>> +
> >>> + domain->iotlb = vhost_iotlb_alloc(0, 0);
> >>> + if (!domain->iotlb)
> >>> + goto err_iotlb;
> >>> +
> >>> + domain->iova_limit = iova_limit;
> >>> + domain->bounce_size = PAGE_ALIGN(bounce_size);
> >>> + domain->bounce_pages = vzalloc(bounce_pfns * sizeof(struct page *));
> >>> + if (!domain->bounce_pages)
> >>> + goto err_page;
> >>> +
> >>> + file = anon_inode_getfile("[vduse-domain]", &vduse_domain_fops,
> >>> + domain, O_RDWR);
> >>> + if (IS_ERR(file))
> >>> + goto err_file;
> >>> +
> >>> + domain->file = file;
> >>> + spin_lock_init(&domain->iotlb_lock);
> >>> + init_iova_domain(&domain->stream_iovad,
> >>> + IOVA_ALLOC_SIZE, IOVA_START_PFN);
> >>> + init_iova_domain(&domain->consistent_iovad,
> >>> + PAGE_SIZE, bounce_pfns);
> >>> +
> >>> + return domain;
> >>> +err_file:
> >>> + vfree(domain->bounce_pages);
> >>> +err_page:
> >>> + vhost_iotlb_free(domain->iotlb);
> >>> +err_iotlb:
> >>> + kfree(domain);
> >>> + return NULL;
> >>> +}
> >>> +
> >>> +int vduse_domain_init(void)
> >>> +{
> >>> + return iova_cache_get();
> >>> +}
> >>> +
> >>> +void vduse_domain_exit(void)
> >>> +{
> >>> + iova_cache_put();
> >>> +}
> >>> diff --git a/drivers/vdpa/vdpa_user/iova_domain.h b/drivers/vdpa/vdpa_user/iova_domain.h
> >>> new file mode 100644
> >>> index 000000000000..9c85d8346626
> >>> --- /dev/null
> >>> +++ b/drivers/vdpa/vdpa_user/iova_domain.h
> >>> @@ -0,0 +1,61 @@
> >>> +/* SPDX-License-Identifier: GPL-2.0-only */
> >>> +/*
> >>> + * MMU-based IOMMU implementation
> >>> + *
> >>> + * Copyright (C) 2020 Bytedance Inc. and/or its affiliates. All rights reserved.
> >>> + *
> >>> + * Author: Xie Yongji <xieyongji@xxxxxxxxxxxxx>
> >>> + *
> >>> + */
> >>> +
> >>> +#ifndef _VDUSE_IOVA_DOMAIN_H
> >>> +#define _VDUSE_IOVA_DOMAIN_H
> >>> +
> >>> +#include <linux/iova.h>
> >>> +#include <linux/dma-mapping.h>
> >>> +#include <linux/vhost_iotlb.h>
> >>> +
> >>> +struct vduse_iova_domain {
> >>> + struct iova_domain stream_iovad;
> >>> + struct iova_domain consistent_iovad;
> >>> + struct page **bounce_pages;
> >>> + size_t bounce_size;
> >>> + unsigned long iova_limit;
> >>> + struct vhost_iotlb *iotlb;
> >>
> >> Sorry if I've asked this before.
> >>
> >> But what's the reason for maintaing a dedicated IOTLB here? I think we
> >> could reuse vduse_dev->iommu since the device can not be used by both
> >> virtio and vhost in the same time or use vduse_iova_domain->iotlb for
> >> set_map().
> >>
> > The main difference between domain->iotlb and dev->iotlb is the way to
> > deal with bounce buffer. In the domain->iotlb case, bounce buffer
> > needs to be mapped each DMA transfer because we need to get the bounce
> > pages by an IOVA during DMA unmapping. In the dev->iotlb case, bounce
> > buffer only needs to be mapped once during initialization, which will
> > be used to tell userspace how to do mmap().
> >
> >> Also, since vhost IOTLB support per mapping token (opauqe), can we use
> >> that instead of the bounce_pages *?
> >>
> > Sorry, I didn't get you here. Which value do you mean to store in the
> > opaque pointer？
>
>
> So I would like to have a way to use a single IOTLB for manage all kinds
> of mappings. Two possible ideas:
>
> 1) map bounce page one by one in vduse_dev_map_page(), in
> VDUSE_IOTLB_GET_FD, try to merge the result if we had the same fd. Then
> for bounce pages, userspace still only need to map it once and we can
> maintain the actual mapping by storing the page or pa in the opaque
> field of IOTLB entry.
Looks like userspace still needs to unmap the old region and map a new
region (size is changed) with the fd in each VDUSE_IOTLB_GET_FD ioctl.
> 2) map bounce page once in vduse_dev_map_page() and store struct page
> **bounce_pages in the opaque field of this single IOTLB entry.
>
We can get the struct page **bounce_pages from vduse_iova_domain. Why
do we need to store it in the opaque field? Should the opaque field
be used to store vdpa_map_file?
And I think it works. One problem is we need to find a place to store
the original DMA buffer's address and size. I think we can modify the
array of bounce_pages for this purpose.
Thanks,
Yongji
