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Re: [PATCH] virtio-blk: Convert QEMUBH callback to "bitops.h" API
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH] virtio-blk: Convert QEMUBH callback to "bitops.h" API |
Date: |
Thu, 6 May 2021 21:42:46 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 |
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 :)