[dpdk-dev] [PATCH 2/4] vhost: introduce vhost_log_write

[Victor Kaplansky]

On Wed, Dec 02, 2015 at 11:43:11AM +0800, Yuanhan Liu wrote:
> Introduce vhost_log_write() helper function to log the dirty pages we
> touched. Page size is harded code to 4096 (VHOST_LOG_PAGE), and each
> log is presented by 1 bit.
> 
> Therefore, vhost_log_write() simply finds the right bit for related
> page we are gonna change, and set it to 1. dev->log_base denotes the
> start of the dirty page bitmap.
> 
> The page address is biased by log_guest_addr, which is derived from
> SET_VRING_ADDR request as part of the vring related addresses.
> 
> Signed-off-by: Yuanhan Liu <yuanhan.liu at linux.intel.com>
> ---
>  lib/librte_vhost/rte_virtio_net.h | 34 ++++++++++++++++++++++++++++++++++
>  lib/librte_vhost/virtio-net.c     |  4 ++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/lib/librte_vhost/rte_virtio_net.h b/lib/librte_vhost/rte_virtio_net.h
> index 416dac2..191c1be 100644
> --- a/lib/librte_vhost/rte_virtio_net.h
> +++ b/lib/librte_vhost/rte_virtio_net.h
> @@ -40,6 +40,7 @@
>   */
>  
>  #include <stdint.h>
> +#include <linux/vhost.h>
>  #include <linux/virtio_ring.h>
>  #include <linux/virtio_net.h>
>  #include <sys/eventfd.h>
> @@ -59,6 +60,8 @@ struct rte_mbuf;
>  /* Backend value set by guest. */
>  #define VIRTIO_DEV_STOPPED -1
>  
> +#define VHOST_LOG_PAGE	4096
> +
>  
>  /* Enum for virtqueue management. */
>  enum {VIRTIO_RXQ, VIRTIO_TXQ, VIRTIO_QNUM};
> @@ -82,6 +85,7 @@ struct vhost_virtqueue {
>  	struct vring_desc	*desc;			/**< Virtqueue descriptor ring. */
>  	struct vring_avail	*avail;			/**< Virtqueue available ring. */
>  	struct vring_used	*used;			/**< Virtqueue used ring. */
> +	uint64_t		log_guest_addr;		/**< Physical address of used ring, for logging */
>  	uint32_t		size;			/**< Size of descriptor ring. */
>  	uint32_t		backend;		/**< Backend value to determine if device should started/stopped. */
>  	uint16_t		vhost_hlen;		/**< Vhost header length (varies depending on RX merge buffers. */
> @@ -203,6 +207,36 @@ gpa_to_vva(struct virtio_net *dev, uint64_t guest_pa)
>  	return vhost_va;
>  }
>  
> +static inline void __attribute__((always_inline))
> +vhost_log_page(uint8_t *log_base, uint64_t page)
> +{
> +	/* TODO: to make it atomic? */
> +	log_base[page / 8] |= 1 << (page % 8);

I think the atomic OR operation is necessary only if there can be
more than one vhost-user back-end updating the guest's memory
simultaneously. However probably it is pretty safe to perform
regular OR operation, since rings are not shared between
back-end. What about buffers pointed by descriptors?  To be on
the safe side, I would use a GCC built-in function
__sync_fetch_and_or(). 

> +}
> +
> +static inline void __attribute__((always_inline))
> +vhost_log_write(struct virtio_net *dev, struct vhost_virtqueue *vq,
> +		uint64_t offset, uint64_t len)
> +{
> +	uint64_t addr = vq->log_guest_addr;
> +	uint64_t page;
> +
> +	if (unlikely(((dev->features & (1ULL << VHOST_F_LOG_ALL)) == 0) ||
> +		     !dev->log_base || !len))
> +		return;

Isn't "likely" more appropriate in above, since the whole
expression is expected to be true most of the time?

> +
> +	addr += offset;
> +	if (dev->log_size < ((addr + len - 1) / VHOST_LOG_PAGE / 8))
> +		return;
> +
> +	page = addr / VHOST_LOG_PAGE;
> +	while (page * VHOST_LOG_PAGE < addr + len) {
> +		vhost_log_page(dev->log_base, page);
> +		page += VHOST_LOG_PAGE;
> +	}
> +}
> +
> +
>  /**
>   *  Disable features in feature_mask. Returns 0 on success.
>   */
> diff --git a/lib/librte_vhost/virtio-net.c b/lib/librte_vhost/virtio-net.c
> index 8364938..4481827 100644
> --- a/lib/librte_vhost/virtio-net.c
> +++ b/lib/librte_vhost/virtio-net.c
> @@ -666,12 +666,16 @@ set_vring_addr(struct vhost_device_ctx ctx, struct vhost_vring_addr *addr)
>  		return -1;
>  	}
>  
> +	vq->log_guest_addr = addr->log_guest_addr;
> +
>  	LOG_DEBUG(VHOST_CONFIG, "(%"PRIu64") mapped address desc: %p\n",
>  			dev->device_fh, vq->desc);
>  	LOG_DEBUG(VHOST_CONFIG, "(%"PRIu64") mapped address avail: %p\n",
>  			dev->device_fh, vq->avail);
>  	LOG_DEBUG(VHOST_CONFIG, "(%"PRIu64") mapped address used: %p\n",
>  			dev->device_fh, vq->used);
> +	LOG_DEBUG(VHOST_CONFIG, "(%"PRIu64") log_guest_addr: %p\n",
> +			dev->device_fh, (void *)(uintptr_t)vq->log_guest_addr);
>  
>  	return 0;
>  }
> -- 
> 1.9.0