From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: virtio: put last_used and last_avail index into ring itself. Date: Tue, 4 May 2010 21:22:36 +0300 Message-ID: <20100504182236.GA14141__36610.5056753897$1272997701$gmane$org@redhat.com> References: <200911061529.17500.rusty@rustcorp.com.au> <20091108113516.GA19016@redhat.com> <200911091647.29655.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <200911091647.29655.rusty@rustcorp.com.au> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Rusty Russell Cc: Eric Dumazet , kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-mm@kvack.org, s.hetze@linux-ag.com, hpa@zytor.com, Daniel Walker , mingo@elte.hu, akpm@linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org > virtio: put last_used and last_avail index into ring itself. > > Generally, the other end of the virtio ring doesn't need to see where > you're up to in consuming the ring. However, to completely understand > what's going on from the outside, this information must be exposed. > For example, if you want to save and restore a virtio_ring, but you're > not the consumer because the kernel is using it directly. > > Fortunately, we have room to expand: the ring is always a whole number > of pages and there's hundreds of bytes of padding after the avail ring > and the used ring, whatever the number of descriptors (which must be a > power of 2). > > We add a feature bit so the guest can tell the host that it's writing > out the current value there, if it wants to use that. > > Signed-off-by: Rusty Russell I've been looking at this patch some more (more on why later), and I wonder: would it be better to add some alignment to the last used index address, so that if we later add more stuff at the tail, it all fits in a single cache line? We use a new feature bit anyway, so layout change should not be a problem. Since I raised the question of caches: for used ring, the ring is not aligned to 64 bit, so on CPUs with 64 bit or larger cache lines, used entries will often cross cache line boundaries. Am I right and might it have been better to align ring entries to cache line boundaries? What do you think? > --- > drivers/virtio/virtio_ring.c | 23 +++++++++++++++-------- > include/linux/virtio_ring.h | 12 +++++++++++- > 2 files changed, 26 insertions(+), 9 deletions(-) > > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c > --- a/drivers/virtio/virtio_ring.c > +++ b/drivers/virtio/virtio_ring.c > @@ -71,9 +71,6 @@ struct vring_virtqueue > /* Number we've added since last sync. */ > unsigned int num_added; > > - /* Last used index we've seen. */ > - u16 last_used_idx; > - > /* How to notify other side. FIXME: commonalize hcalls! */ > void (*notify)(struct virtqueue *vq); > > @@ -278,12 +275,13 @@ static void detach_buf(struct vring_virt > > static inline bool more_used(const struct vring_virtqueue *vq) > { > - return vq->last_used_idx != vq->vring.used->idx; > + return vring_last_used(&vq->vring) != vq->vring.used->idx; > } > > static void *vring_get_buf(struct virtqueue *_vq, unsigned int *len) > { > struct vring_virtqueue *vq = to_vvq(_vq); > + struct vring_used_elem *u; > void *ret; > unsigned int i; > > @@ -300,8 +298,11 @@ static void *vring_get_buf(struct virtqu > return NULL; > } > > - i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id; > - *len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len; > + u = &vq->vring.used->ring[vring_last_used(&vq->vring) % vq->vring.num]; > + i = u->id; > + *len = u->len; > + /* Make sure we don't reload i after doing checks. */ > + rmb(); > > if (unlikely(i >= vq->vring.num)) { > BAD_RING(vq, "id %u out of range\n", i); > @@ -315,7 +316,8 @@ static void *vring_get_buf(struct virtqu > /* detach_buf clears data, so grab it now. */ > ret = vq->data[i]; > detach_buf(vq, i); > - vq->last_used_idx++; > + vring_last_used(&vq->vring)++; > + > END_USE(vq); > return ret; > } > @@ -402,7 +404,6 @@ struct virtqueue *vring_new_virtqueue(un > vq->vq.name = name; > vq->notify = notify; > vq->broken = false; > - vq->last_used_idx = 0; > vq->num_added = 0; > list_add_tail(&vq->vq.list, &vdev->vqs); > #ifdef DEBUG > @@ -413,6 +414,10 @@ struct virtqueue *vring_new_virtqueue(un > > vq->indirect = virtio_has_feature(vdev, VIRTIO_RING_F_INDIRECT_DESC); > > + /* We publish indices whether they offer it or not: if not, it's junk > + * space anyway. But calling this acknowledges the feature. */ > + virtio_has_feature(vdev, VIRTIO_RING_F_PUBLISH_INDICES); > + > /* No callback? Tell other side not to bother us. */ > if (!callback) > vq->vring.avail->flags |= VRING_AVAIL_F_NO_INTERRUPT; > @@ -443,6 +448,8 @@ void vring_transport_features(struct vir > switch (i) { > case VIRTIO_RING_F_INDIRECT_DESC: > break; > + case VIRTIO_RING_F_PUBLISH_INDICES: > + break; > default: > /* We don't understand this bit. */ > clear_bit(i, vdev->features); > diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h > --- a/include/linux/virtio_ring.h > +++ b/include/linux/virtio_ring.h > @@ -29,6 +29,9 @@ > /* We support indirect buffer descriptors */ > #define VIRTIO_RING_F_INDIRECT_DESC 28 > > +/* We publish our last-seen used index at the end of the avail ring. */ > +#define VIRTIO_RING_F_PUBLISH_INDICES 29 > + > /* Virtio ring descriptors: 16 bytes. These can chain together via "next". */ > struct vring_desc > { > @@ -87,6 +90,7 @@ struct vring { > * __u16 avail_flags; > * __u16 avail_idx; > * __u16 available[num]; > + * __u16 last_used_idx; > * > * // Padding to the next align boundary. > * char pad[]; > @@ -95,6 +99,7 @@ struct vring { > * __u16 used_flags; > * __u16 used_idx; > * struct vring_used_elem used[num]; > + * __u16 last_avail_idx; > * }; > */ > static inline void vring_init(struct vring *vr, unsigned int num, void *p, > @@ -111,9 +116,14 @@ static inline unsigned vring_size(unsign > { > return ((sizeof(struct vring_desc) * num + sizeof(__u16) * (2 + num) > + align - 1) & ~(align - 1)) > - + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num; > + + sizeof(__u16) * 2 + sizeof(struct vring_used_elem) * num + 2; > } > > +/* We publish the last-seen used index at the end of the available ring, and > + * vice-versa. These are at the end for backwards compatibility. */ > +#define vring_last_used(vr) ((vr)->avail->ring[(vr)->num]) > +#define vring_last_avail(vr) (*(__u16 *)&(vr)->used->ring[(vr)->num]) > + > #ifdef __KERNEL__ > #include > struct virtio_device;