Re: [dpdk-dev] [PATCH v2 2/6] vhost: introduce vhost_log_write

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Mon, Dec 21, 2015 at 03:06:43PM +0000, Xie, Huawei wrote:
> On 12/17/2015 11:11 AM, 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.
> >
> > Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
> > Signed-off-by: Victor Kaplansky <victork@redhat.com
> > ---
> >  lib/librte_vhost/rte_virtio_net.h | 29 +++++++++++++++++++++++++++++
> >  1 file changed, 29 insertions(+)
> >
> > diff --git a/lib/librte_vhost/rte_virtio_net.h b/lib/librte_vhost/rte_virtio_net.h
> > index 8acee02..5726683 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};
> > @@ -205,6 +208,32 @@ 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)
> > +{
> > +	log_base[page / 8] |= 1 << (page % 8);
> > +}
> > +
> Those logging functions are not supposed to be API. Could we move them
> into an internal header file?

Agreed. I should have put them into vhost_rxtx.c

> > +static inline void __attribute__((always_inline))
> > +vhost_log_write(struct virtio_net *dev, uint64_t addr, uint64_t len)
> > +{
> > +	uint64_t page;
> > +
> Before we log, we need memory barrier to make sure updates are in place.
> > +	if (likely(((dev->features & (1ULL << VHOST_F_LOG_ALL)) == 0) ||
> > +		   !dev->log_base || !len))
> > +		return;

Put a memory barrier inside set_features()?

I see no var dependence here, why putting a barrier then? We are
accessing and modifying same var, doesn't the cache MESI protocol
will get rid of your concerns?

> > +
> > +	if (unlikely(dev->log_size < ((addr + len - 1) / VHOST_LOG_PAGE / 8)))
> > +		return;
> > +
> > +	page = addr / VHOST_LOG_PAGE;
> > +	while (page * VHOST_LOG_PAGE < addr + len) {
> Let us have a page_end var to make the code simpler?

Could do that.


> > +		vhost_log_page((uint8_t *)(uintptr_t)dev->log_base, page);
> > +		page += VHOST_LOG_PAGE;
> page += 1?

Oops, right.

	--yliu

> > +	}
> > +}
> > +
> > +
> >  /**
> >   *  Disable features in feature_mask. Returns 0 on success.
> >   */
>