On 5/6/21 8:22 PM, Richard Henderson wrote: > On 5/6/21 8:54 AM, Philippe Mathieu-Daudé wrote: >> 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 (j = 0; j < nvqs; j += BITS_PER_LONG) { >> - unsigned long bits = bitmap[j / BITS_PER_LONG]; >> - >> - while (bits != 0) { >> - unsigned i = j + ctzl(bits); >> + for (unsigned i = 0; i < s->conf->num_queues; i++) { > > Is this bitmap dense enough that you want to iterate by index,
By 'iterate by index' do you mean the actual iteration with 'j'? > or is it > sparse enough to iterate via find_first_bit/find_next_bit? I looked at find_first_bit/find_next_bit() but they seemed to do a lot more than test_and_clear_bit(). As Stefan said this is hot path, I thought this would be cheaper, but haven't profiled the performance. > In either case, leave the copy of s->conf->num_queues to a local variable. That is sensible to do :)