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Date:   Thu, 7 Jul 2022 03:36:02 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Guo Zhi <qtxuning1999@sjtu.edu.cn>
Cc:     jasowang@redhat.com, eperezma@redhat.com,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, sgarzare@redhat.com
Subject: Re: [PATCH v2 2/4] virtio_test: move magic number in code as defined
 constant
Message-ID: <20220707033233-mutt-send-email-mst@kernel.org>
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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.

> 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@sjtu.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
>