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

Jason Wang
2019-01-24T04:07:54Z

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 <jasowang@redhat.com>
>
> 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

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

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; \
> } \
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