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Date: Wed, 27 Sep 2017 08:38:24 +0800 From: Jason Wang <jasowang@...hat.com> To: "Michael S. Tsirkin" <mst@...hat.com> Cc: virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org Subject: Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx() On 2017年09月27日 03:13, Michael S. Tsirkin wrote: > On Fri, Sep 22, 2017 at 04:02:33PM +0800, Jason Wang wrote: >> This patch introduces a helper which just increase the used idx. This >> will be used in pair with vhost_prefetch_desc_indices() by batching >> code. >> >> Signed-off-by: Jason Wang <jasowang@...hat.com> >> --- >> drivers/vhost/vhost.c | 33 +++++++++++++++++++++++++++++++++ >> drivers/vhost/vhost.h | 1 + >> 2 files changed, 34 insertions(+) >> >> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >> index 8424166d..6532cda 100644 >> --- a/drivers/vhost/vhost.c >> +++ b/drivers/vhost/vhost.c >> @@ -2178,6 +2178,39 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len) >> } >> EXPORT_SYMBOL_GPL(vhost_add_used); >> >> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n) >> +{ >> + u16 old, new; >> + >> + old = vq->last_used_idx; >> + new = (vq->last_used_idx += n); >> + /* If the driver never bothers to signal in a very long while, >> + * used index might wrap around. If that happens, invalidate >> + * signalled_used index we stored. TODO: make sure driver >> + * signals at least once in 2^16 and remove this. >> + */ >> + if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old))) >> + vq->signalled_used_valid = false; >> + >> + /* Make sure buffer is written before we update index. */ >> + smp_wmb(); >> + if (vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx), >> + &vq->used->idx)) { >> + vq_err(vq, "Failed to increment used idx"); >> + return -EFAULT; >> + } >> + if (unlikely(vq->log_used)) { >> + /* Log used index update. */ >> + log_write(vq->log_base, >> + vq->log_addr + offsetof(struct vring_used, idx), >> + sizeof(vq->used->idx)); >> + if (vq->log_ctx) >> + eventfd_signal(vq->log_ctx, 1); >> + } >> + return 0; >> +} >> +EXPORT_SYMBOL_GPL(vhost_add_used_idx); >> + >> static int __vhost_add_used_n(struct vhost_virtqueue *vq, >> struct vring_used_elem *heads, >> unsigned count) >> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h >> index 16c2cb6..5dd6c05 100644 >> --- a/drivers/vhost/vhost.h >> +++ b/drivers/vhost/vhost.h >> @@ -199,6 +199,7 @@ int __vhost_get_vq_desc(struct vhost_virtqueue *vq, >> void vhost_discard_vq_desc(struct vhost_virtqueue *, int n); >> >> int vhost_vq_init_access(struct vhost_virtqueue *); >> +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n); >> int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len); >> int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads, >> unsigned count); > Please change the API to hide the fact that there's an index that needs > to be updated. In fact, an interesting optimization on top is just call vhost_add_used_idx(vq, n) instead of n vhost_add_used_idx(vq, 1). That's the reason I leave n in the API. Thanks > >> -- >> 2.7.4
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