[PATCH v2 0/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[PATCH v2 0/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Philippe Mathieu-Daudé]

This series follow a suggestion from Stefan to use the bitops
API in virtio-blk:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg805139.html

Since v1:
- improved "bitops.h" docstring
- addressed Richard's review comments

Philippe Mathieu-Daudé (2):
  bitops.h: Improve find_xxx_bit() documentation
  virtio-blk: Convert QEMUBH callback to "bitops.h" API

 include/qemu/bitops.h           | 15 ++++++++++++---
 hw/block/dataplane/virtio-blk.c | 19 ++++---------------
 2 files changed, 16 insertions(+), 18 deletions(-)

-- 
2.26.3

[PATCH v2 1/2] bitops.h: Improve find_xxx_bit() documentation

[Philippe Mathieu-Daudé]

Document the following functions return the bitmap size
if not matching bit is found:

- find_first_bit
- find_next_bit
- find_last_bit
- find_first_zero_bit
- find_next_zero_bit

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 include/qemu/bitops.h | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index 3acbf3384c6..eb0fc5416a9 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -140,7 +140,8 @@ static inline int test_bit(long nr, const unsigned long *addr)
  * @addr: The address to start the search at
  * @size: The maximum size to search
  *
- * Returns the bit number of the first set bit, or size.
+ * Returns the bit number of the last set bit,
+ * or @size if there is no set bit in the bitmap.
  */
 unsigned long find_last_bit(const unsigned long *addr,
                             unsigned long size);
@@ -150,6 +151,9 @@ unsigned long find_last_bit(const unsigned long *addr,
  * @addr: The address to base the search on
  * @offset: The bitnumber to start searching at
  * @size: The bitmap size in bits
+ *
+ * Returns the bit number of the next set bit,
+ * or @size if there is no set bit in the bitmap.
  */
 unsigned long find_next_bit(const unsigned long *addr,
                             unsigned long size,
@@ -160,6 +164,9 @@ unsigned long find_next_bit(const unsigned long *addr,
  * @addr: The address to base the search on
  * @offset: The bitnumber to start searching at
  * @size: The bitmap size in bits
+ *
+ * Returns the bit number of the next cleared bit,
+ * or @size if there is no clear bit in the bitmap.
  */
 
 unsigned long find_next_zero_bit(const unsigned long *addr,
@@ -171,7 +178,8 @@ unsigned long find_next_zero_bit(const unsigned long *addr,
  * @addr: The address to start the search at
  * @size: The maximum size to search
  *
- * Returns the bit number of the first set bit.
+ * Returns the bit number of the first set bit,
+ * or @size if there is no set bit in the bitmap.
  */
 static inline unsigned long find_first_bit(const unsigned long *addr,
                                            unsigned long size)
@@ -194,7 +202,8 @@ static inline unsigned long find_first_bit(const unsigned long *addr,
  * @addr: The address to start the search at
  * @size: The maximum size to search
  *
- * Returns the bit number of the first cleared bit.
+ * Returns the bit number of the first cleared bit,
+ * or @size if there is no clear bit in the bitmap.
  */
 static inline unsigned long find_first_zero_bit(const unsigned long *addr,
                                                 unsigned long size)
-- 
2.26.3

Re: [PATCH v2 1/2] bitops.h: Improve find_xxx_bit() documentation

[Richard Henderson]

On 5/7/21 10:06 AM, Philippe Mathieu-Daudé wrote:
> Document the following functions return the bitmap size
> if not matching bit is found:
> 
> - find_first_bit
> - find_next_bit
> - find_last_bit
> - find_first_zero_bit
> - find_next_zero_bit
> 
> Signed-off-by: Philippe Mathieu-Daudé<philmd@redhat.com>
> ---
>   include/qemu/bitops.h | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~

Re: [PATCH v2 1/2] bitops.h: Improve find_xxx_bit() documentation

[Eric Blake]

On 5/7/21 12:06 PM, Philippe Mathieu-Daudé wrote:
> Document the following functions return the bitmap size
> if not matching bit is found:

s/not/no/

> 
> - find_first_bit
> - find_next_bit
> - find_last_bit
> - find_first_zero_bit
> - find_next_zero_bit
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  include/qemu/bitops.h | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 

> @@ -150,6 +151,9 @@ unsigned long find_last_bit(const unsigned long *addr,
>   * @addr: The address to base the search on
>   * @offset: The bitnumber to start searching at
>   * @size: The bitmap size in bits
> + *
> + * Returns the bit number of the next set bit,
> + * or @size if there is no set bit in the bitmap.
>   */
>  unsigned long find_next_bit(const unsigned long *addr,

Misleading - there might be a set bit prior to @offset.  Better might be:

or @size if there are no further set bits in the bitmap.

>                              unsigned long size,
> @@ -160,6 +164,9 @@ unsigned long find_next_bit(const unsigned long *addr,
>   * @addr: The address to base the search on
>   * @offset: The bitnumber to start searching at
>   * @size: The bitmap size in bits
> + *
> + * Returns the bit number of the next cleared bit,
> + * or @size if there is no clear bit in the bitmap.
>   */
>  
>  unsigned long find_next_zero_bit(const unsigned long *addr,

similarly,

or @size if there are no further clear bits in the bitmap.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

[PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Philippe Mathieu-Daudé]

By directly using find_first_bit() and find_next_bit from the
"bitops.h" API to iterate over the bitmap, we can remove the
bitmap[] variable-length array copy on the stack and the complex
manual bit testing/clearing logic.

Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 hw/block/dataplane/virtio-blk.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index e9050c8987e..a7b5bda06fc 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -60,23 +60,12 @@ static void notify_guest_bh(void *opaque)
 {
     VirtIOBlockDataPlane *s = opaque;
     unsigned nvqs = s->conf->num_queues;
-    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
-    unsigned j;
 
-    memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
-    memset(s->batch_notify_vqs, 0, sizeof(bitmap));
+    for (unsigned long i = find_first_bit(s->batch_notify_vqs, nvqs);
+             i < nvqs; i = find_next_bit(s->batch_notify_vqs, nvqs, i)) {
+        VirtQueue *vq = virtio_get_queue(s->vdev, i);
 
-    for (j = 0; j < nvqs; j += BITS_PER_LONG) {
-        unsigned long bits = bitmap[j / BITS_PER_LONG];
-
-        while (bits != 0) {
-            unsigned i = j + ctzl(bits);
-            VirtQueue *vq = virtio_get_queue(s->vdev, i);
-
-            virtio_notify_irqfd(s->vdev, vq);
-
-            bits &= bits - 1; /* clear right-most bit */
-        }
+        virtio_notify_irqfd(s->vdev, vq);
     }
 }
 
-- 
2.26.3

Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Richard Henderson]

On 5/7/21 10:06 AM, Philippe Mathieu-Daudé wrote:
> By directly using find_first_bit() and find_next_bit from the
> "bitops.h" API to iterate over the bitmap, we can remove the
> bitmap[] variable-length array copy on the stack and the complex
> manual bit testing/clearing logic.
> 
> Suggested-by: Stefan Hajnoczi<stefanha@redhat.com>
> Suggested-by: Richard Henderson<richard.henderson@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé<philmd@redhat.com>
> ---
>   hw/block/dataplane/virtio-blk.c | 19 ++++---------------
>   1 file changed, 4 insertions(+), 15 deletions(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~

Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Stefan Hajnoczi]

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On Fri, May 07, 2021 at 07:06:34PM +0200, Philippe Mathieu-Daudé wrote:
> By directly using find_first_bit() and find_next_bit from the
> "bitops.h" API to iterate over the bitmap, we can remove the
> bitmap[] variable-length array copy on the stack and the complex
> manual bit testing/clearing logic.
> 
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 19 ++++---------------
>  1 file changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index e9050c8987e..a7b5bda06fc 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -60,23 +60,12 @@ static void notify_guest_bh(void *opaque)
>  {
>      VirtIOBlockDataPlane *s = opaque;
>      unsigned nvqs = s->conf->num_queues;
> -    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
> -    unsigned j;
>  
> -    memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
> -    memset(s->batch_notify_vqs, 0, sizeof(bitmap));
> +    for (unsigned long i = find_first_bit(s->batch_notify_vqs, nvqs);
> +             i < nvqs; i = find_next_bit(s->batch_notify_vqs, nvqs, i)) {
> +        VirtQueue *vq = virtio_get_queue(s->vdev, i);
>  
> -    for (j = 0; j < nvqs; j += BITS_PER_LONG) {
> -        unsigned long bits = bitmap[j / BITS_PER_LONG];
> -
> -        while (bits != 0) {
> -            unsigned i = j + ctzl(bits);
> -            VirtQueue *vq = virtio_get_queue(s->vdev, i);
> -
> -            virtio_notify_irqfd(s->vdev, vq);
> -
> -            bits &= bits - 1; /* clear right-most bit */
> -        }
> +        virtio_notify_irqfd(s->vdev, vq);
>      }
>  }

Bits in s->batch_notify_vqs[] must be cleared. Otherwise we may raise
spurious irqs next time this function is called. The memset() can be
moved after the loop.

Stefan

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Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API

[Philippe Mathieu-Daudé]

On 5/10/21 3:25 PM, Stefan Hajnoczi wrote:
> On Fri, May 07, 2021 at 07:06:34PM +0200, Philippe Mathieu-Daudé wrote:
>> By directly using find_first_bit() and find_next_bit from the
>> "bitops.h" API to iterate over the bitmap, we can remove the
>> bitmap[] variable-length array copy on the stack and the complex
>> manual bit testing/clearing logic.
>>
>> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
>> Suggested-by: Richard Henderson <richard.henderson@linaro.org>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  hw/block/dataplane/virtio-blk.c | 19 ++++---------------
>>  1 file changed, 4 insertions(+), 15 deletions(-)
>>
>> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
>> index e9050c8987e..a7b5bda06fc 100644
>> --- a/hw/block/dataplane/virtio-blk.c
>> +++ b/hw/block/dataplane/virtio-blk.c
>> @@ -60,23 +60,12 @@ static void notify_guest_bh(void *opaque)
>>  {
>>      VirtIOBlockDataPlane *s = opaque;
>>      unsigned nvqs = s->conf->num_queues;
>> -    unsigned long bitmap[BITS_TO_LONGS(nvqs)];
>> -    unsigned j;
>>  
>> -    memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
>> -    memset(s->batch_notify_vqs, 0, sizeof(bitmap));
>> +    for (unsigned long i = find_first_bit(s->batch_notify_vqs, nvqs);
>> +             i < nvqs; i = find_next_bit(s->batch_notify_vqs, nvqs, i)) {
>> +        VirtQueue *vq = virtio_get_queue(s->vdev, i);
>>  
>> -    for (j = 0; j < nvqs; j += BITS_PER_LONG) {
>> -        unsigned long bits = bitmap[j / BITS_PER_LONG];
>> -
>> -        while (bits != 0) {
>> -            unsigned i = j + ctzl(bits);
>> -            VirtQueue *vq = virtio_get_queue(s->vdev, i);
>> -
>> -            virtio_notify_irqfd(s->vdev, vq);
>> -
>> -            bits &= bits - 1; /* clear right-most bit */
>> -        }
>> +        virtio_notify_irqfd(s->vdev, vq);
>>      }
>>  }
> 
> Bits in s->batch_notify_vqs[] must be cleared. Otherwise we may raise
> spurious irqs next time this function is called. The memset() can be
> moved after the loop.

Doh... good catch! I missed it when removing the previous
test_and_clear_bit() call (from v1).