Subject: Re: [PATCH net-next V4 5/5] vhost: access vq metadata through kernel virtual address

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 23 Jan 2019 23:51:59 -0500

On Thu, Jan 24, 2019 at 12:07:54PM +0800, Jason Wang wrote:
> 
> On 2019/1/23 下午10:08, Michael S. Tsirkin wrote:
> > On Wed, Jan 23, 2019 at 05:55:57PM +0800, Jason Wang wrote:
> > > It was noticed that the copy_user() friends that was used to access
> > > virtqueue metdata tends to be very expensive for dataplane
> > > implementation like vhost since it involves lots of software checks,
> > > speculation barrier, hardware feature toggling (e.g SMAP). The
> > > extra cost will be more obvious when transferring small packets since
> > > the time spent on metadata accessing become more significant.
> > > 
> > > This patch tries to eliminate those overheads by accessing them
> > > through kernel virtual address by vmap(). To make the pages can be
> > > migrated, instead of pinning them through GUP, we use MMU notifiers to
> > > invalidate vmaps and re-establish vmaps during each round of metadata
> > > prefetching if necessary. For devices that doesn't use metadata
> > > prefetching, the memory accessors fallback to normal copy_user()
> > > implementation gracefully. The invalidation was synchronized with
> > > datapath through vq mutex, and in order to avoid hold vq mutex during
> > > range checking, MMU notifier was teared down when trying to modify vq
> > > metadata.
> > > 
> > > Another thing is kernel lacks efficient solution for tracking dirty
> > > pages by vmap(), this will lead issues if vhost is using file backed
> > > memory which needs care of writeback. This patch solves this issue by
> > > just skipping the vma that is file backed and fallback to normal
> > > copy_user() friends. This might introduce some overheads for file
> > > backed users but consider this use case is rare we could do
> > > optimizations on top.
> > > 
> > > Note that this was only done when device IOTLB is not enabled. We
> > > could use similar method to optimize it in the future.
> > > 
> > > Tests shows at most about 22% improvement on TX PPS when using
> > > virtio-user + vhost_net + xdp1 + TAP on 2.6GHz Broadwell:
> > > 
> > >          SMAP on | SMAP off
> > > Before: 5.0Mpps | 6.6Mpps
> > > After:  6.1Mpps | 7.4Mpps
> > > 
> > > Signed-off-by: Jason Wang <[email-masked]>
> > 
> > So this is the bulk of the change.
> > Threee things that I need to look into
> > - Are there any security issues with bypassing the speculation barrier
> >    that is normally present after access_ok?
> 
> 
> If we can make sure the bypassing was only used in a kthread (vhost), it
> should be fine I think.
> 
> 
> > - How hard does the special handling for
> >    file backed storage make testing?
> 
> 
> It's as simple as un-commenting vhost_can_vmap()? Or I can try to hack qemu
> or dpdk to test this.
> 
> 
> >    On the one hand we could add a module parameter to
> >    force copy to/from user. on the other that's
> >    another configuration we need to support.
> 
> 
> That sounds sub-optimal since it leave the choice to users.
> 
> 
> >    But iotlb is not using vmap, so maybe that's enough
> >    for testing.
> > - How hard is it to figure out which mode uses which code.
> > 
> > 
> > 
> > Meanwhile, could you pls post data comparing this last patch with the
> > below?  This removes the speculation barrier replacing it with a
> > (useless but at least more lightweight) data dependency.
> 
> 
> SMAP off
> 
> Your patch: 7.2MPPs
> 
> vmap: 7.4Mpps
> 

Sounds more or less as expected. Up to 3% gain with vmap - I think
that's a bit higher than what we saw previously when we switched from
get_user to __get_user and that's probably because of all the
array_index_nospec trickery.

> I don't test SMAP on, since it will be much slow for sure.

Right. So bypassing SMAP remains the main reason to do vmap tricks.

> Thanks

> 
> > 
> > Thanks!
> > 
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index bac939af8dbb..352ee7e14476 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -739,7 +739,7 @@ static int vhost_copy_to_user(struct vhost_virtqueue *vq, void __user *to,
> >   	int ret;
> >   	if (!vq->iotlb)
> > -		return __copy_to_user(to, from, size);
> > +		return copy_to_user(to, from, size);
> >   	else {
> >   		/* This function should be called after iotlb
> >   		 * prefetch, which means we're sure that all vq
> > @@ -752,7 +752,7 @@ static int vhost_copy_to_user(struct vhost_virtqueue *vq, void __user *to,
> >   				     VHOST_ADDR_USED);
> >   		if (uaddr)
> > -			return __copy_to_user(uaddr, from, size);
> > +			return copy_to_user(uaddr, from, size);
> >   		ret = translate_desc(vq, (u64)(uintptr_t)to, size, vq->iotlb_iov,
> >   				     ARRAY_SIZE(vq->iotlb_iov),
> > @@ -774,7 +774,7 @@ static int vhost_copy_from_user(struct vhost_virtqueue *vq, void *to,
> >   	int ret;
> >   	if (!vq->iotlb)
> > -		return __copy_from_user(to, from, size);
> > +		return copy_from_user(to, from, size);
> >   	else {
> >   		/* This function should be called after iotlb
> >   		 * prefetch, which means we're sure that vq
> > @@ -787,7 +787,7 @@ static int vhost_copy_from_user(struct vhost_virtqueue *vq, void *to,
> >   		struct iov_iter f;
> >   		if (uaddr)
> > -			return __copy_from_user(to, uaddr, size);
> > +			return copy_from_user(to, uaddr, size);
> >   		ret = translate_desc(vq, (u64)(uintptr_t)from, size, vq->iotlb_iov,
> >   				     ARRAY_SIZE(vq->iotlb_iov),
> > @@ -855,13 +855,13 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
> >   ({ \
> >   	int ret = -EFAULT; \
> >   	if (!vq->iotlb) { \
> > -		ret = __put_user(x, ptr); \
> > +		ret = put_user(x, ptr); \
> >   	} else { \
> >   		__typeof__(ptr) to = \
> >   			(__typeof__(ptr)) __vhost_get_user(vq, ptr,	\
> >   					  sizeof(*ptr), VHOST_ADDR_USED); \
> >   		if (to != NULL) \
> > -			ret = __put_user(x, to); \
> > +			ret = put_user(x, to); \
> >   		else \
> >   			ret = -EFAULT;	\
> >   	} \
> > @@ -872,14 +872,14 @@ static inline void __user *__vhost_get_user(struct vhost_virtqueue *vq,
> >   ({ \
> >   	int ret; \
> >   	if (!vq->iotlb) { \
> > -		ret = __get_user(x, ptr); \
> > +		ret = get_user(x, ptr); \
> >   	} else { \
> >   		__typeof__(ptr) from = \
> >   			(__typeof__(ptr)) __vhost_get_user(vq, ptr, \
> >   							   sizeof(*ptr), \
> >   							   type); \
> >   		if (from != NULL) \
> > -			ret = __get_user(x, from); \
> > +			ret = get_user(x, from); \
> >   		else \
> >   			ret = -EFAULT; \
> >   	} \