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Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH v2 2/2] virtio-blk: Convert QEMUBH callback to "bitops.h" API |
Date: |
Mon, 10 May 2021 16:05:01 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 |
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).