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Venkatesh Srinivas

2015-Nov-11 00:21 UTC

[PATCH] virtio_ring: Shadow available ring flags & index

Improves cacheline transfer flow of available ring header.

Virtqueues are implemented as a pair of rings, one producer->consumer
avail ring and one consumer->producer used ring; preceding the
avail ring in memory are two contiguous u16 fields -- avail->flags
and avail->idx. A producer posts work by writing to avail->idx and
a consumer reads avail->idx.

The flags and idx fields only need to be written by a producer CPU
and only read by a consumer CPU; when the producer and consumer are
running on different CPUs and the virtio_ring code is structured to
only have source writes/sink reads, we can continuously transfer the
avail header cacheline between 'M' states between cores. This flow
optimizes core -> core bandwidth on certain CPUs.

(see: "Software Optimization Guide for AMD Family 15h Processors",
Section 11.6; similar language appears in the 10h guide and should
apply to CPUs w/ exclusive caches, using LLC as a transfer cache)

Unfortunately the existing virtio_ring code issued reads to the
avail->idx and read-modify-writes to avail->flags on the producer.

This change shadows the flags and index fields in producer memory;
the vring code now reads from the shadows and only ever writes to
avail->flags and avail->idx, allowing the cacheline to transfer
core -> core optimally.

In a concurrent version of vring_bench, the time required for
10,000,000 buffer checkout/returns was reduced by ~2% (average
across many runs) on an AMD Piledriver (15h) CPU:

(w/o shadowing):
 Performance counter stats for './vring_bench':
     5,451,082,016      L1-dcache-loads
     ...
       2.221477739 seconds time elapsed

(w/ shadowing):
 Performance counter stats for './vring_bench':
     5,405,701,361      L1-dcache-loads
     ...
       2.168405376 seconds time elapsed

The further away (in a NUMA sense) virtio producers and consumers are
from each other, the more we expect to benefit. Physical implementations
of virtio devices and implementations of virtio where the consumer polls
vring avail indexes (vhost) should also benefit.

Signed-off-by: Venkatesh Srinivas <venkateshs at google.com>
---
 drivers/virtio/virtio_ring.c | 46 ++++++++++++++++++++++++++++++++------------
 1 file changed, 34 insertions(+), 12 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 096b857..6262015 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -80,6 +80,12 @@ struct vring_virtqueue {
 	/* Last used index we've seen. */
 	u16 last_used_idx;
 
+	/* Last written value to avail->flags */
+	u16 avail_flags_shadow;
+
+	/* Last written value to avail->idx in guest byte order */
+	u16 avail_idx_shadow;
+
 	/* How to notify other side. FIXME: commonalize hcalls! */
 	bool (*notify)(struct virtqueue *vq);
 
@@ -235,13 +241,14 @@ static inline int virtqueue_add(struct virtqueue *_vq,
 
 	/* Put entry in available array (but don't update avail->idx until they
 	 * do sync). */
-	avail = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) &
(vq->vring.num - 1);
+	avail = vq->avail_idx_shadow & (vq->vring.num - 1);
 	vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
 
 	/* Descriptors and available array need to be set before we expose the
 	 * new available array entries. */
 	virtio_wmb(vq->weak_barriers);
-	vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) + 1);
+	vq->avail_idx_shadow++;
+	vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
 	vq->num_added++;
 
 	pr_debug("Added buffer head %i to %p\n", head, vq);
@@ -354,8 +361,8 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
 	 * event. */
 	virtio_mb(vq->weak_barriers);
 
-	old = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
vq->num_added;
-	new = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx);
+	old = vq->avail_idx_shadow - vq->num_added;
+	new = vq->avail_idx_shadow;
 	vq->num_added = 0;
 
 #ifdef DEBUG
@@ -510,7 +517,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int
*len)
 	/* If we expect an interrupt for the next entry, tell host
 	 * by writing event index and flush out the write before
 	 * the read in the next get_buf call. */
-	if (!(vq->vring.avail->flags & cpu_to_virtio16(_vq->vdev,
VRING_AVAIL_F_NO_INTERRUPT))) {
+	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
 		vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
vq->last_used_idx);
 		virtio_mb(vq->weak_barriers);
 	}
@@ -537,7 +544,11 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	vq->vring.avail->flags |= cpu_to_virtio16(_vq->vdev,
VRING_AVAIL_F_NO_INTERRUPT);
+	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
+		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
+		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
+
 }
 EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
 
@@ -565,7 +576,10 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
 	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
 	 * either clear the flags bit or point the event index at the next
 	 * entry. Always do both to keep code simple. */
-	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev,
~VRING_AVAIL_F_NO_INTERRUPT);
+	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
+		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
+		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
 	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
 	END_USE(vq);
 	return last_used_idx;
@@ -633,9 +647,12 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
 	 * either clear the flags bit or point the event index at the next
 	 * entry. Always do both to keep code simple. */
-	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev,
~VRING_AVAIL_F_NO_INTERRUPT);
+	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
+		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
+		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
+	}
 	/* TODO: tune this threshold */
-	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
vq->last_used_idx) * 3 / 4;
+	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
 	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
vq->last_used_idx + bufs);
 	virtio_mb(vq->weak_barriers);
 	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev, vq->vring.used->idx) -
vq->last_used_idx) > bufs)) {
@@ -670,7 +687,8 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
 		/* detach_buf clears data, so grab it now. */
 		buf = vq->data[i];
 		detach_buf(vq, i);
-		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) - 1);
+		vq->avail_idx_shadow--;
+		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
 		END_USE(vq);
 		return buf;
 	}
@@ -735,6 +753,8 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 	vq->weak_barriers = weak_barriers;
 	vq->broken = false;
 	vq->last_used_idx = 0;
+	vq->avail_flags_shadow = 0;
+	vq->avail_idx_shadow = 0;
 	vq->num_added = 0;
 	list_add_tail(&vq->vq.list, &vdev->vqs);
 #ifdef DEBUG
@@ -746,8 +766,10 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
 	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
 
 	/* No callback?  Tell other side not to bother us. */
-	if (!callback)
-		vq->vring.avail->flags |= cpu_to_virtio16(vdev,
VRING_AVAIL_F_NO_INTERRUPT);
+	if (!callback) {
+		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
+		vq->vring.avail->flags = cpu_to_virtio16(vdev,
vq->avail_flags_shadow);
+	}
 
 	/* Put everything in free lists. */
 	vq->free_head = 0;
-- 
2.6.0.rc2.230.g3dd15c0

Michael S. Tsirkin

2015-Nov-11 12:34 UTC

head link

[PATCH] virtio_ring: Shadow available ring flags & index

On Tue, Nov 10, 2015 at 04:21:07PM -0800, Venkatesh Srinivas
wrote:> Improves cacheline transfer flow of available ring header.
> 
> Virtqueues are implemented as a pair of rings, one producer->consumer
> avail ring and one consumer->producer used ring; preceding the
> avail ring in memory are two contiguous u16 fields -- avail->flags
> and avail->idx. A producer posts work by writing to avail->idx and
> a consumer reads avail->idx.
> 
> The flags and idx fields only need to be written by a producer CPU
> and only read by a consumer CPU; when the producer and consumer are
> running on different CPUs and the virtio_ring code is structured to
> only have source writes/sink reads, we can continuously transfer the
> avail header cacheline between 'M' states between cores. This flow
> optimizes core -> core bandwidth on certain CPUs.
> 
> (see: "Software Optimization Guide for AMD Family 15h
Processors",
> Section 11.6; similar language appears in the 10h guide and should
> apply to CPUs w/ exclusive caches, using LLC as a transfer cache)
> 
> Unfortunately the existing virtio_ring code issued reads to the
> avail->idx and read-modify-writes to avail->flags on the producer.
> 
> This change shadows the flags and index fields in producer memory;
> the vring code now reads from the shadows and only ever writes to
> avail->flags and avail->idx, allowing the cacheline to transfer
> core -> core optimally.
Sounds logical, I'll apply this after a  bit of testing
of my own, thanks!
> In a concurrent version of vring_bench, the time required for
> 10,000,000 buffer checkout/returns was reduced by ~2% (average
> across many runs) on an AMD Piledriver (15h) CPU:
> 
> (w/o shadowing):
>  Performance counter stats for './vring_bench':
>      5,451,082,016      L1-dcache-loads
>      ...
>        2.221477739 seconds time elapsed
> 
> (w/ shadowing):
>  Performance counter stats for './vring_bench':
>      5,405,701,361      L1-dcache-loads
>      ...
>        2.168405376 seconds time elapsed
Could you supply the full command line you used
to test this?
> The further away (in a NUMA sense) virtio producers and consumers are
> from each other, the more we expect to benefit. Physical implementations
> of virtio devices and implementations of virtio where the consumer polls
> vring avail indexes (vhost) should also benefit.
> 
> Signed-off-by: Venkatesh Srinivas <venkateshs at google.com>
Here's a similar patch for the ring itself:
https://lkml.org/lkml/2015/9/10/111

Does it help you as well?

> ---
>  drivers/virtio/virtio_ring.c | 46
++++++++++++++++++++++++++++++++------------
>  1 file changed, 34 insertions(+), 12 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 096b857..6262015 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -80,6 +80,12 @@ struct vring_virtqueue {
>  	/* Last used index we've seen. */
>  	u16 last_used_idx;
>  
> +	/* Last written value to avail->flags */
> +	u16 avail_flags_shadow;
> +
> +	/* Last written value to avail->idx in guest byte order */
> +	u16 avail_idx_shadow;
> +
>  	/* How to notify other side. FIXME: commonalize hcalls! */
>  	bool (*notify)(struct virtqueue *vq);
>  
> @@ -235,13 +241,14 @@ static inline int virtqueue_add(struct virtqueue
*_vq,
>  
>  	/* Put entry in available array (but don't update avail->idx until
they
>  	 * do sync). */
> -	avail = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) &
(vq->vring.num - 1);
> +	avail = vq->avail_idx_shadow & (vq->vring.num - 1);
>  	vq->vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head);
>  
>  	/* Descriptors and available array need to be set before we expose the
>  	 * new available array entries. */
>  	virtio_wmb(vq->weak_barriers);
> -	vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) + 1);
> +	vq->avail_idx_shadow++;
> +	vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
>  	vq->num_added++;
>  
>  	pr_debug("Added buffer head %i to %p\n", head, vq);
> @@ -354,8 +361,8 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
>  	 * event. */
>  	virtio_mb(vq->weak_barriers);
>  
> -	old = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
vq->num_added;
> -	new = virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx);
> +	old = vq->avail_idx_shadow - vq->num_added;
> +	new = vq->avail_idx_shadow;
>  	vq->num_added = 0;
>  
>  #ifdef DEBUG
> @@ -510,7 +517,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned
int *len)
>  	/* If we expect an interrupt for the next entry, tell host
>  	 * by writing event index and flush out the write before
>  	 * the read in the next get_buf call. */
> -	if (!(vq->vring.avail->flags & cpu_to_virtio16(_vq->vdev,
VRING_AVAIL_F_NO_INTERRUPT))) {
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>  		vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
vq->last_used_idx);
>  		virtio_mb(vq->weak_barriers);
>  	}
> @@ -537,7 +544,11 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
>  {
>  	struct vring_virtqueue *vq = to_vvq(_vq);
>  
> -	vq->vring.avail->flags |= cpu_to_virtio16(_vq->vdev,
VRING_AVAIL_F_NO_INTERRUPT);
> +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> +		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
> +
>  }
>  EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
>  
> @@ -565,7 +576,10 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue
*_vq)
>  	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
>  	 * either clear the flags bit or point the event index at the next
>  	 * entry. Always do both to keep code simple. */
> -	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev,
~VRING_AVAIL_F_NO_INTERRUPT);
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
>  	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
last_used_idx = vq->last_used_idx);
>  	END_USE(vq);
>  	return last_used_idx;
> @@ -633,9 +647,12 @@ bool virtqueue_enable_cb_delayed(struct virtqueue
*_vq)
>  	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
>  	 * either clear the flags bit or point the event index at the next
>  	 * entry. Always do both to keep code simple. */
> -	vq->vring.avail->flags &= cpu_to_virtio16(_vq->vdev,
~VRING_AVAIL_F_NO_INTERRUPT);
> +	if (vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT) {
> +		vq->avail_flags_shadow &= ~VRING_AVAIL_F_NO_INTERRUPT;
> +		vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
vq->avail_flags_shadow);
> +	}
>  	/* TODO: tune this threshold */
> -	bufs = (u16)(virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) -
vq->last_used_idx) * 3 / 4;
> +	bufs = (u16)(vq->avail_idx_shadow - vq->last_used_idx) * 3 / 4;
>  	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev,
vq->last_used_idx + bufs);
>  	virtio_mb(vq->weak_barriers);
>  	if (unlikely((u16)(virtio16_to_cpu(_vq->vdev,
vq->vring.used->idx) - vq->last_used_idx) > bufs)) {
> @@ -670,7 +687,8 @@ void *virtqueue_detach_unused_buf(struct virtqueue
*_vq)
>  		/* detach_buf clears data, so grab it now. */
>  		buf = vq->data[i];
>  		detach_buf(vq, i);
> -		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
virtio16_to_cpu(_vq->vdev, vq->vring.avail->idx) - 1);
> +		vq->avail_idx_shadow--;
> +		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
vq->avail_idx_shadow);
>  		END_USE(vq);
>  		return buf;
>  	}
> @@ -735,6 +753,8 @@ struct virtqueue *vring_new_virtqueue(unsigned int
index,
>  	vq->weak_barriers = weak_barriers;
>  	vq->broken = false;
>  	vq->last_used_idx = 0;
> +	vq->avail_flags_shadow = 0;
> +	vq->avail_idx_shadow = 0;
>  	vq->num_added = 0;
>  	list_add_tail(&vq->vq.list, &vdev->vqs);
>  #ifdef DEBUG
> @@ -746,8 +766,10 @@ struct virtqueue *vring_new_virtqueue(unsigned int
index,
>  	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
>  
>  	/* No callback?  Tell other side not to bother us. */
> -	if (!callback)
> -		vq->vring.avail->flags |= cpu_to_virtio16(vdev,
VRING_AVAIL_F_NO_INTERRUPT);
> +	if (!callback) {
> +		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> +		vq->vring.avail->flags = cpu_to_virtio16(vdev,
vq->avail_flags_shadow);
> +	}
>  
>  	/* Put everything in free lists. */
>  	vq->free_head = 0;
> -- 
> 2.6.0.rc2.230.g3dd15c0

Venkatesh Srinivas

2015-Nov-13 23:41 UTC

head link

[PATCH] virtio_ring: Shadow available ring flags & index

On Wed, Nov 11, 2015 at 02:34:33PM +0200, Michael S. Tsirkin
wrote:> On Tue, Nov 10, 2015 at 04:21:07PM -0800, Venkatesh Srinivas wrote:
> > Improves cacheline transfer flow of available ring header.
> >
> > Virtqueues are implemented as a pair of rings, one
producer->consumer
> > avail ring and one consumer->producer used ring; preceding the
> > avail ring in memory are two contiguous u16 fields -- avail->flags
> > and avail->idx. A producer posts work by writing to avail->idx
and
> > a consumer reads avail->idx.
> >
> > The flags and idx fields only need to be written by a producer CPU
> > and only read by a consumer CPU; when the producer and consumer are
> > running on different CPUs and the virtio_ring code is structured to
> > only have source writes/sink reads, we can continuously transfer the
> > avail header cacheline between 'M' states between cores. This
flow
> > optimizes core -> core bandwidth on certain CPUs.
> >
> > (see: "Software Optimization Guide for AMD Family 15h
Processors",
> > Section 11.6; similar language appears in the 10h guide and should
> > apply to CPUs w/ exclusive caches, using LLC as a transfer cache)
> >
> > Unfortunately the existing virtio_ring code issued reads to the
> > avail->idx and read-modify-writes to avail->flags on the
producer.
> >
> > This change shadows the flags and index fields in producer memory;
> > the vring code now reads from the shadows and only ever writes to
> > avail->flags and avail->idx, allowing the cacheline to transfer
> > core -> core optimally.
>
> Sounds logical, I'll apply this after a  bit of testing
> of my own, thanks!
Thanks!
> > In a concurrent version of vring_bench, the time required for
> > 10,000,000 buffer checkout/returns was reduced by ~2% (average
> > across many runs) on an AMD Piledriver (15h) CPU:
> >
> > (w/o shadowing):
> >  Performance counter stats for './vring_bench':
> >      5,451,082,016      L1-dcache-loads
> >      ...
> >        2.221477739 seconds time elapsed
> >
> > (w/ shadowing):
> >  Performance counter stats for './vring_bench':
> >      5,405,701,361      L1-dcache-loads
> >      ...
> >        2.168405376 seconds time elapsed
>
> Could you supply the full command line you used
> to test this?
Yes --

perf stat -e L1-dcache-loads,L1-dcache-load-misses,L1-dcache-stores \
	./vring_bench

The standard version of vring_bench is single-threaded (posted on this list
but never submitted); my tests were with a version that has a worker thread
polling the VQ. How should I share it? Should I just attach it to an email
here?
> > The further away (in a NUMA sense) virtio producers and consumers are
> > from each other, the more we expect to benefit. Physical
implementations
> > of virtio devices and implementations of virtio where the consumer
polls
> > vring avail indexes (vhost) should also benefit.
> >
> > Signed-off-by: Venkatesh Srinivas <venkateshs at google.com>
>
> Here's a similar patch for the ring itself:
> https://lkml.org/lkml/2015/9/10/111
>
> Does it help you as well?
I tested your patch in our environment; our virtqueues do not support
Indirect entries and your patch does not manage to elide many writes, so I
do not see a performance difference. In an environment with Indirect, your
patch will likely be a win.

(My patch gets most of its win by eliminating reads on the producer; when
the producer reads avail fields at the same time the consumer is polling,
we see cacheline transfers that hurt performance. Your patch eliminates
writes, which is nice, but our tests w/ polling are not as sensitive to
writes from the producer.)

I have two quick comments on your patch --
1) I think you need to kfree vq->avail when deleting the virtqueue.

2) Should we avoid allocating a cache for virtqueues that are not
   performance critical? (ex: virtio-scsi eventq/controlq, virtio-net
   controlq)

Should I post comments in reply to the original patch email (given that it
is ~2 months old)?

Thanks!
-- vs;

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