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

From: Richard Henderson <richard.henderson@linaro.org>
To: "Philippe Mathieu-Daudé" <philmd@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Li Hangjing <lihangjing@baidu.com>,
	Sergio Lopez <slp@redhat.com>,
	qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Chai Wen <chaiwen@baidu.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Xie Yongji <xieyongji@baidu.com>
Subject: Re: [PATCH] virtio-blk: Convert QEMUBH callback to "bitops.h" API
Date: Thu, 6 May 2021 11:22:14 -0700	[thread overview]
Message-ID: <fdf97a71-1359-e186-a895-3383c31ca20c@linaro.org> (raw)
In-Reply-To: <20210506155419.1796056-1-philmd@redhat.com>

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, or is it sparse 
enough to iterate via find_first_bit/find_next_bit?

In either case, leave the copy of  s->conf->num_queues to a local variable.

r~