linux-kernel - Re: [PATCH v2 2/4] virtio_test: move magic number in code as defined constant

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Date:   Thu, 7 Jul 2022 20:19:19 +0800
From:   Guo Zhi <qtxuning1999@...u.edu.cn>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     jasowang@...hat.com, eperezma@...hat.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, sgarzare@...hat.com
Subject: Re: [PATCH v2 2/4] virtio_test: move magic number in code as defined
 constant

On 2022/7/7 15:36, Michael S. Tsirkin wrote:
> On Thu, Jul 07, 2022 at 02:17:19PM +0800, Guo Zhi wrote:
>> On 2022/7/7 13:09, Michael S. Tsirkin wrote:
>>> On Thu, Jul 07, 2022 at 10:44:07AM +0800, Guo Zhi wrote:
>>>> We should avoid using magic numbers directly.
>>> Not necessarily. For repeated values - I guess.
>>> But this kind of thing:
>>>
>>> 	#define BUF_SIZE 1024
>>> 	int buf_size = BUF_SIZE;
>>>
>>> brings no benefit IMHO.
>>>
>>> And this has cost - values are now removed from code.
>> Firstly, as a test, user will have to change the config to test virtio and
>> vhost frequently. If these magic number are put together like these, change
>> will be more easier.
> If tweaking these is useful for users we should add a command line flags
> as opposed to asking users to tweak code.

I think you suggestions are correct, I should not change that magic 
numbers which doesn't have relationship with my work. So in next version 
patch, I will stop changing these magic number to defined values.

>
>> Secondly, many configs will be repeated, such as MAX_SG_FRAGS, even if some
>> varibale only appear once, subsequent upgrade of virtio_test will use these
>> variable in other place. For example, in this series of patch, patch 4/4
>> will use RINGSIZE to set INDIRECTS_BUF_SIZE.
>>
>>>> Signed-off-by: Guo Zhi <qtxuning1999@...u.edu.cn>
> Isn't there some way to use the size we get as 1st parameter of kmalloc?
>
>>>> ---
>>>>    tools/virtio/virtio_test.c | 18 +++++++++++-------
>>>>    1 file changed, 11 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/tools/virtio/virtio_test.c b/tools/virtio/virtio_test.c
>>>> index 95f78b311..1ecd64271 100644
>>>> --- a/tools/virtio/virtio_test.c
>>>> +++ b/tools/virtio/virtio_test.c
>>>> @@ -20,7 +20,10 @@
>>>>    #include "../../drivers/vhost/test.h"
>>>>    #define RANDOM_BATCH -1
>>>> -
>>>> +#define ALIGN 4096
>>>> +#define RINGSIZE   256
>>>> +#define TEST_BUF_NUM 0x100000
>>>> +#define BUF_SIZE   1024
>>>>    /* Unused */
>>>>    void *__kmalloc_fake, *__kfree_ignore_start, *__kfree_ignore_end;
>>>> @@ -100,8 +103,8 @@ static void vq_reset(struct vq_info *info, int num, struct virtio_device *vdev)
>>>>    	if (info->vq)
>>>>    		vring_del_virtqueue(info->vq);
>>>> -	memset(info->ring, 0, vring_size(num, 4096));
>>>> -	vring_init(&info->vring, num, info->ring, 4096);
>>>> +	memset(info->ring, 0, vring_size(num, ALIGN));
>>>> +	vring_init(&info->vring, num, info->ring, ALIGN);
>>>>    	info->vq = __vring_new_virtqueue(info->idx, info->vring, vdev, true,
>>>>    					 false, vq_notify, vq_callback, "test");
>>>>    	assert(info->vq);
>>>> @@ -115,7 +118,7 @@ static void vq_info_add(struct vdev_info *dev, int num)
>>>>    	info->idx = dev->nvqs;
>>>>    	info->kick = eventfd(0, EFD_NONBLOCK);
>>>>    	info->call = eventfd(0, EFD_NONBLOCK);
>>>> -	r = posix_memalign(&info->ring, 4096, vring_size(num, 4096));
>>>> +	r = posix_memalign(&info->ring, PAGE_SIZE, vring_size(num, ALIGN));
>>>>    	assert(r >= 0);
>>>>    	vq_reset(info, num, &dev->vdev);
>>>>    	vhost_vq_setup(dev, info);
>>> This is actually doing more than what commit log says.
>>>
>>>> @@ -131,7 +134,7 @@ static void vdev_info_init(struct vdev_info* dev, unsigned long long features)
>>>>    	dev->vdev.features = features;
>>>>    	INIT_LIST_HEAD(&dev->vdev.vqs);
>>>>    	spin_lock_init(&dev->vdev.vqs_list_lock);
>>>> -	dev->buf_size = 1024;
>>>> +	dev->buf_size = BUF_SIZE;
>>>>
>>>>
>>>> This seems to have zero added value.
>>>>
>> Since the reason above and other magic number have been set as a defined
>> constant, change here for simplicity and consistency.
>>>>    	dev->buf = malloc(dev->buf_size);
>>>>    	assert(dev->buf);
>>>>            dev->control = open("/dev/vhost-test", O_RDWR);
>>>> @@ -396,7 +399,8 @@ int main(int argc, char **argv)
>>>>    done:
>>>>    	vdev_info_init(&dev, features);
>>>> -	vq_info_add(&dev, 256);
>>>> -	run_test(&dev, &dev.vqs[0], delayed, batch, reset, 0x100000);
>>>> +	vq_info_add(&dev, RINGSIZE);
>>>> +
>>>> +	run_test(&dev, &dev.vqs[0], delayed, batch, reset, TEST_BUF_NUM);
>>>>    	return 0;
>>> This replacement does not buy us anything either.
>> RINGSIZE will be used in other place in subsequent modification.
>>>>    }
>>>> -- 
>>>> 2.17.1