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 14:17:19 +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 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.

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>
>> ---
>>   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