Re: [dpdk-dev] [PATCH v3] vhost: enable IOMMU for async vhost

["Hu, Jiayu" <jiayu.hu@intel.com>]

Hi Maxime,

> -----Original Message-----
> From: Maxime Coquelin <maxime.coquelin@redhat.com>
> Sent: Saturday, June 19, 2021 12:18 AM
> To: Ding, Xuan <xuan.ding@intel.com>; Xia, Chenbo <chenbo.xia@intel.com>
> Cc: dev@dpdk.org; Hu, Jiayu <jiayu.hu@intel.com>; Pai G, Sunil
> <sunil.pai.g@intel.com>; Richardson, Bruce <bruce.richardson@intel.com>;
> Van Haaren, Harry <harry.van.haaren@intel.com>; Liu, Yong
> <yong.liu@intel.com>
> Subject: Re: [PATCH v3] vhost: enable IOMMU for async vhost
> 
> Hi Xuan,
> 
> On 6/3/21 7:30 PM, xuan.ding@intel.com wrote:
> > From: Xuan Ding <xuan.ding@intel.com>
> >
> > For async copy, it is unsafe to directly use the physical address.
> > And current address translation from GPA to HPA via SW also takes CPU
> > cycles, these can all benefit from IOMMU.
> >
> > Since the existing DMA engine supports to use platform IOMMU, this
> > patch enables IOMMU for async vhost, which defines IOAT devices to use
> > virtual address instead of physical address.
> 
> We have to keep in mind a generic DMA api is coming, and maybe we want a
> SW implementation of a dmadev based on memcpy at least for
> testing/debugging purpose.

Agree, we need to support SW fallback, and I think this is also what this
patch wants to do. Originally, vhost passes IOVA to DMA callbacks; if
DPDK in PA mode, we cannot fallback to SW copy. In this patch, vhost
passes both VA for pktmbuf and guest's buffer to DMA callbacks, which
makes SW fallback possible.

In terms of generic DMA api, no matter it uses VA or IOVA as buffer addresses,
I think this design can work, as DMA callback implementations can do address
translation anyway.

> 
> > When set memory table, the frontend's memory will be mapped to the
> > default container of DPDK where IOAT devices have been added into.
> > When DMA copy fails, the virtual address provided to IOAT devices also
> > allow us fallback to SW copy or PA copy.
> >
> > With IOMMU enabled, to use IOAT devices:
> > 1. IOAT devices must be binded to vfio-pci, rather than igb_uio.
> > 2. DPDK must use "--iova-mode=va".
> 
> I think this is problematic, at least we need to check the right iova mode has
> been selected, but even with doing that it is limiting.
> 
> What prevent us to reuse add_guest_pages() alogrithm to implement
> IOVA_AS_PA?

In the original design, vfio doesn't work, as vhost doesn't programs iommu
table with guest's memory. Specifically, if DPDK is in VA mode, IOVA passed
to DMA callback is VA, but IOMMU cannot find corresponding PA for guest
buffers; if DPDK is in PA mode, IOVA passed to DMA callback is PA. In this case,
there are random errors for guest buffers when VT-d is enabled, as IOMMU
behavior is uncertain. I think supporting vfio is one of reasons of this patch.

One concern about this patch is how to handle when IOVA is PA. If IOVA is PA,
IOMMU cannot find correct PA for pktmbuf via VA passed by vhost. But can
DMA callback translate VA to PA before calling ioat/dmadev API? IMHO, IOVA
as PA with vfio is not a recommended configuration. Do you think it's a must
for vhost to support this case?

Thanks,
Jiayu

> 
> >
> > Signed-off-by: Xuan Ding <xuan.ding@intel.com>
> > ---
> >
> > v3:
> > * Fixed some typos.
> >
> > v2:
> > * Fixed a format issue.
> > * Added the dma unmap logic when device is closed.
> > ---
> >  doc/guides/prog_guide/vhost_lib.rst |  20 +++++
> >  lib/vhost/vhost_user.c              | 125 +++++++++-------------------
> >  lib/vhost/virtio_net.c              |  30 +++----
> >  3 files changed, 69 insertions(+), 106 deletions(-)
> >
> > diff --git a/doc/guides/prog_guide/vhost_lib.rst
> > b/doc/guides/prog_guide/vhost_lib.rst
> > index d18fb98910..5777f0da96 100644
> > --- a/doc/guides/prog_guide/vhost_lib.rst
> > +++ b/doc/guides/prog_guide/vhost_lib.rst
> > @@ -420,3 +420,23 @@ Finally, a set of device ops is defined for device
> specific operations:
> >  * ``get_notify_area``
> >
> >    Called to get the notify area info of the queue.
> > +
> > +Vhost async data path
> > +---------------------
> > +
> > +* Address mode
> > +
> > +  Modern IOAT devices support to use the IOMMU, which can avoid using
> > + the unsafe HPA. Besides, the CPU cycles took by SW to translate from
> > + GPA to HPA can also be saved. So IOAT devices are defined to use
> > + virtual address instead of physical address.
> > +
> > +  With IOMMU enabled, to use IOAT devices:
> > +  1. IOAT devices must be binded to vfio-pci, rather than igb_uio.
> > +  2. DPDK must use ``--iova-mode=va``.
> > +
> > +* Fallback
> > +
> > +  When the DMA copy fails, the user who implements the transfer_data
> > + callback can fallback to SW copy or fallback to PA copy through
> > + rte_mem_virt2iova().
> > diff --git a/lib/vhost/vhost_user.c b/lib/vhost/vhost_user.c index
> > 8f0eba6412..c33fa784ff 100644
> > --- a/lib/vhost/vhost_user.c
> > +++ b/lib/vhost/vhost_user.c
> > @@ -45,6 +45,7 @@
> >  #include <rte_common.h>
> >  #include <rte_malloc.h>
> >  #include <rte_log.h>
> > +#include <rte_vfio.h>
> >
> >  #include "iotlb.h"
> >  #include "vhost.h"
> > @@ -141,6 +142,34 @@ get_blk_size(int fd)
> >  	return ret == -1 ? (uint64_t)-1 : (uint64_t)stat.st_blksize;  }
> >
> > +static int
> > +async_dma_map(struct rte_vhost_mem_region *region, bool do_map) {
> > +	int ret = 0;
> > +	if (do_map) {
> > +		/* Add mapped region into the default container of DPDK. */
> > +		ret =
> rte_vfio_container_dma_map(RTE_VFIO_DEFAULT_CONTAINER_FD,
> > +						 region->host_user_addr,
> > +						 region->host_user_addr,
> > +						 region->size);
> > +		if (ret) {
> > +			VHOST_LOG_CONFIG(ERR, "DMA engine map
> failed\n");
> > +			return ret;
> > +		}
> > +	} else {
> > +		/* Remove mapped region from the default container of
> DPDK. */
> > +		ret =
> rte_vfio_container_dma_unmap(RTE_VFIO_DEFAULT_CONTAINER_FD,
> > +						   region->host_user_addr,
> > +						   region->host_user_addr,
> > +						   region->size);
> > +		if (ret) {
> > +			VHOST_LOG_CONFIG(ERR, "DMA engine unmap
> failed\n");
> > +			return ret;
> > +		}
> > +	}
> > +	return ret;
> > +}
> > +
> >  static void
> >  free_mem_region(struct virtio_net *dev)  { @@ -155,6 +184,9 @@
> > free_mem_region(struct virtio_net *dev)
> >  		if (reg->host_user_addr) {
> >  			munmap(reg->mmap_addr, reg->mmap_size);
> >  			close(reg->fd);
> > +
> > +			if (dev->async_copy)
> > +				async_dma_map(reg, false);
> >  		}
> >  	}
> >  }
> > @@ -866,87 +898,6 @@ vhost_user_set_vring_base(struct virtio_net
> **pdev,
> >  	return RTE_VHOST_MSG_RESULT_OK;
> >  }
> >
> > -static int
> > -add_one_guest_page(struct virtio_net *dev, uint64_t guest_phys_addr,
> > -		   uint64_t host_phys_addr, uint64_t size)
> > -{
> > -	struct guest_page *page, *last_page;
> > -	struct guest_page *old_pages;
> > -
> > -	if (dev->nr_guest_pages == dev->max_guest_pages) {
> > -		dev->max_guest_pages *= 2;
> > -		old_pages = dev->guest_pages;
> > -		dev->guest_pages = rte_realloc(dev->guest_pages,
> > -					dev->max_guest_pages *
> sizeof(*page),
> > -					RTE_CACHE_LINE_SIZE);
> > -		if (dev->guest_pages == NULL) {
> > -			VHOST_LOG_CONFIG(ERR, "cannot realloc
> guest_pages\n");
> > -			rte_free(old_pages);
> > -			return -1;
> > -		}
> > -	}
> > -
> > -	if (dev->nr_guest_pages > 0) {
> > -		last_page = &dev->guest_pages[dev->nr_guest_pages - 1];
> > -		/* merge if the two pages are continuous */
> > -		if (host_phys_addr == last_page->host_phys_addr +
> > -				      last_page->size) {
> > -			last_page->size += size;
> > -			return 0;
> > -		}
> > -	}
> > -
> > -	page = &dev->guest_pages[dev->nr_guest_pages++];
> > -	page->guest_phys_addr = guest_phys_addr;
> > -	page->host_phys_addr  = host_phys_addr;
> > -	page->size = size;
> > -
> > -	return 0;
> > -}
> > -
> > -static int
> > -add_guest_pages(struct virtio_net *dev, struct rte_vhost_mem_region
> *reg,
> > -		uint64_t page_size)
> > -{
> > -	uint64_t reg_size = reg->size;
> > -	uint64_t host_user_addr  = reg->host_user_addr;
> > -	uint64_t guest_phys_addr = reg->guest_phys_addr;
> > -	uint64_t host_phys_addr;
> > -	uint64_t size;
> > -
> > -	host_phys_addr = rte_mem_virt2iova((void
> *)(uintptr_t)host_user_addr);
> > -	size = page_size - (guest_phys_addr & (page_size - 1));
> > -	size = RTE_MIN(size, reg_size);
> > -
> > -	if (add_one_guest_page(dev, guest_phys_addr, host_phys_addr, size)
> < 0)
> > -		return -1;
> > -
> > -	host_user_addr  += size;
> > -	guest_phys_addr += size;
> > -	reg_size -= size;
> > -
> > -	while (reg_size > 0) {
> > -		size = RTE_MIN(reg_size, page_size);
> > -		host_phys_addr = rte_mem_virt2iova((void *)(uintptr_t)
> > -						  host_user_addr);
> > -		if (add_one_guest_page(dev, guest_phys_addr,
> host_phys_addr,
> > -				size) < 0)
> > -			return -1;
> > -
> > -		host_user_addr  += size;
> > -		guest_phys_addr += size;
> > -		reg_size -= size;
> > -	}
> > -
> > -	/* sort guest page array if over binary search threshold */
> > -	if (dev->nr_guest_pages >= VHOST_BINARY_SEARCH_THRESH) {
> > -		qsort((void *)dev->guest_pages, dev->nr_guest_pages,
> > -			sizeof(struct guest_page), guest_page_addrcmp);
> > -	}
> > -
> > -	return 0;
> > -}
> > -
> >  #ifdef RTE_LIBRTE_VHOST_DEBUG
> >  /* TODO: enable it only in debug mode? */  static void @@ -1105,6
> > +1056,7 @@ vhost_user_mmap_region(struct virtio_net *dev,
> >  	uint64_t mmap_size;
> >  	uint64_t alignment;
> >  	int populate;
> > +	int ret;
> >
> >  	/* Check for memory_size + mmap_offset overflow */
> >  	if (mmap_offset >= -region->size) {
> > @@ -1158,12 +1110,13 @@ vhost_user_mmap_region(struct virtio_net
> *dev,
> >  	region->mmap_size = mmap_size;
> >  	region->host_user_addr = (uint64_t)(uintptr_t)mmap_addr +
> > mmap_offset;
> >
> > -	if (dev->async_copy)
> > -		if (add_guest_pages(dev, region, alignment) < 0) {
> > -			VHOST_LOG_CONFIG(ERR,
> > -					"adding guest pages to region
> failed.\n");
> > +	if (dev->async_copy) {
> > +		ret = async_dma_map(region, true);
> > +		if (ret) {
> > +			VHOST_LOG_CONFIG(ERR, "Configure IOMMU for
> DMA engine failed\n");
> >  			return -1;
> 
> Maybe we're too late in the init already, but I would think we may want to
> fallback to SW implementation insea
> 
> > -		}
> > +			}
> > +	}
> >
> >  	VHOST_LOG_CONFIG(INFO,
> >  			"guest memory region size: 0x%" PRIx64 "\n"
> > diff --git a/lib/vhost/virtio_net.c b/lib/vhost/virtio_net.c index
> > 8da8a86a10..88110d2cb3 100644
> > --- a/lib/vhost/virtio_net.c
> > +++ b/lib/vhost/virtio_net.c
> > @@ -980,11 +980,9 @@ async_mbuf_to_desc(struct virtio_net *dev, struct
> vhost_virtqueue *vq,
> >  	struct batch_copy_elem *batch_copy = vq->batch_copy_elems;
> >  	struct virtio_net_hdr_mrg_rxbuf tmp_hdr, *hdr = NULL;
> >  	int error = 0;
> > -	uint64_t mapped_len;
> >
> >  	uint32_t tlen = 0;
> >  	int tvec_idx = 0;
> > -	void *hpa;
> >
> >  	if (unlikely(m == NULL)) {
> >  		error = -1;
> > @@ -1074,27 +1072,19 @@ async_mbuf_to_desc(struct virtio_net *dev,
> > struct vhost_virtqueue *vq,
> >
> >  		cpy_len = RTE_MIN(buf_avail, mbuf_avail);
> >
> > -		while (unlikely(cpy_len && cpy_len >= cpy_threshold)) {
> > -			hpa = (void *)(uintptr_t)gpa_to_first_hpa(dev,
> > -					buf_iova + buf_offset,
> > -					cpy_len, &mapped_len);
> > -
> > -			if (unlikely(!hpa || mapped_len < cpy_threshold))
> > -				break;
> > -
> > +		if (unlikely(cpy_len >= cpy_threshold)) {
> >  			async_fill_vec(src_iovec + tvec_idx,
> > -				(void *)(uintptr_t)rte_pktmbuf_iova_offset(m,
> > -				mbuf_offset), (size_t)mapped_len);
> > +				rte_pktmbuf_mtod_offset(m, void *,
> mbuf_offset),
> > +(size_t)cpy_len);
> >
> >  			async_fill_vec(dst_iovec + tvec_idx,
> > -					hpa, (size_t)mapped_len);
> > -
> > -			tlen += (uint32_t)mapped_len;
> > -			cpy_len -= (uint32_t)mapped_len;
> > -			mbuf_avail  -= (uint32_t)mapped_len;
> > -			mbuf_offset += (uint32_t)mapped_len;
> > -			buf_avail  -= (uint32_t)mapped_len;
> > -			buf_offset += (uint32_t)mapped_len;
> > +				(void *)((uintptr_t)(buf_addr + buf_offset)),
> (size_t)cpy_len);
> > +
> > +			tlen += cpy_len;
> > +			mbuf_avail  -= cpy_len;
> > +			mbuf_offset += cpy_len;
> > +			buf_avail  -= cpy_len;
> > +			buf_offset += cpy_len;
> > +			cpy_len = 0;
> >  			tvec_idx++;
> >  		}
> >
> >