Re: [PATCH 3/3] virtio_ring: Use const to annotate read-only pointer params

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 07, 2023 at 09:17:55PM +0000, Feng Liu wrote:
> On 2023-03-07 04:14, David Edmondson wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > Feng Liu via Virtualization <virtualization@lists.linux-foundation.org>
> > writes:
> > 
> >> Add const to make the read-only pointer parameters clear, similar to
> >> many existing functions.
> > 
> > In many of the modified functions the local variable that is a cast of
> > the argument could also be const. Is there a reason not to do both at
> > the same time?
> > 
> 
> Hi,David
> 
> In order to prevent the content of a pointer parameter from being 
> modified and increase the readability of the function, it is recommended 
> to add the 'const' keyword to the parameter. This is not necessary for 
> local variables and non-pointer parameters, as they are only stored on 
> the stack and do not affect the original value or structure member 
> passed into the function. Therefore, in this case, the 'const' keyword 
> is only added to pointer parameters.

This makes no sense to me. If ytou cast away the const then it is
pointless.

> 
> >> Signed-off-by: Feng Liu <feliu@nvidia.com>
> >> Reviewed-by: Jiri Pirko <jiri@nvidia.com>
> >> Reviewed-by: Parav Pandit <parav@nvidia.com>
> >> Reviewed-by: Gavin Li <gavinl@nvidia.com>
> >> Reviewed-by: Bodong Wang <bodong@nvidia.com>
> >> ---
> >>   drivers/virtio/virtio_ring.c | 25 ++++++++++++-------------
> >>   include/linux/virtio.h       | 12 ++++++------
> >>   2 files changed, 18 insertions(+), 19 deletions(-)
> >>
> >> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> >> index 806cc44eae64..8010fd1d2047 100644
> >> --- a/drivers/virtio/virtio_ring.c
> >> +++ b/drivers/virtio/virtio_ring.c
> >> @@ -233,7 +233,7 @@ static void vring_free(struct virtqueue *_vq);
> >>
> >>   #define to_vvq(_vq) container_of(_vq, struct vring_virtqueue, vq)
> >>
> >> -static bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> >> +static bool virtqueue_use_indirect(const struct vring_virtqueue *vq,
> >>                                          unsigned int total_sg)
> >>   {
> >>        /*
> >> @@ -269,7 +269,7 @@ static bool virtqueue_use_indirect(struct vring_virtqueue *vq,
> >>    * unconditionally on data path.
> >>    */
> >>
> >> -static bool vring_use_dma_api(struct virtio_device *vdev)
> >> +static bool vring_use_dma_api(const struct virtio_device *vdev)
> >>   {
> >>        if (!virtio_has_dma_quirk(vdev))
> >>                return true;
> >> @@ -289,7 +289,7 @@ static bool vring_use_dma_api(struct virtio_device *vdev)
> >>        return false;
> >>   }
> >>
> >> -size_t virtio_max_dma_size(struct virtio_device *vdev)
> >> +size_t virtio_max_dma_size(const struct virtio_device *vdev)
> >>   {
> >>        size_t max_segment_size = SIZE_MAX;
> >>
> >> @@ -423,7 +423,7 @@ static void virtqueue_init(struct vring_virtqueue *vq, u32 num)
> >>    */
> >>
> >>   static void vring_unmap_one_split_indirect(const struct vring_virtqueue *vq,
> >> -                                        struct vring_desc *desc)
> >> +                                        const struct vring_desc *desc)
> >>   {
> >>        u16 flags;
> >>
> >> @@ -1183,7 +1183,7 @@ static u16 packed_last_used(u16 last_used_idx)
> >>   }
> >>
> >>   static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> >> -                                  struct vring_desc_extra *extra)
> >> +                                  const struct vring_desc_extra *extra)
> >>   {
> >>        u16 flags;
> >>
> >> @@ -1206,7 +1206,7 @@ static void vring_unmap_extra_packed(const struct vring_virtqueue *vq,
> >>   }
> >>
> >>   static void vring_unmap_desc_packed(const struct vring_virtqueue *vq,
> >> -                                struct vring_packed_desc *desc)
> >> +                                 const struct vring_packed_desc *desc)
> >>   {
> >>        u16 flags;
> >>
> >> @@ -2786,7 +2786,7 @@ EXPORT_SYMBOL_GPL(vring_transport_features);
> >>    * Returns the size of the vring.  This is mainly used for boasting to
> >>    * userspace.  Unlike other operations, this need not be serialized.
> >>    */
> >> -unsigned int virtqueue_get_vring_size(struct virtqueue *_vq)
> >> +unsigned int virtqueue_get_vring_size(const struct virtqueue *_vq)
> >>   {
> >>
> >>        struct vring_virtqueue *vq = to_vvq(_vq);
> >> @@ -2819,7 +2819,7 @@ void __virtqueue_unbreak(struct virtqueue *_vq)
> >>   }
> >>   EXPORT_SYMBOL_GPL(__virtqueue_unbreak);
> >>
> >> -bool virtqueue_is_broken(struct virtqueue *_vq)
> >> +bool virtqueue_is_broken(const struct virtqueue *_vq)
> >>   {
> >>        struct vring_virtqueue *vq = to_vvq(_vq);
> >>
> >> @@ -2827,8 +2827,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
> >>   }
> >>   EXPORT_SYMBOL_GPL(virtqueue_is_broken);
> >>
> >> -/*
> >> - * This should prevent the device from being used, allowing drivers to
> >> +/ This should prevent the device from being used, allowing drivers to
> >>    * recover.  You may need to grab appropriate locks to flush.
> >>    */
> >>   void virtio_break_device(struct virtio_device *dev)
> >> @@ -2881,7 +2880,7 @@ dma_addr_t virtqueue_get_desc_addr(struct virtqueue *_vq)
> >>   }
> >>   EXPORT_SYMBOL_GPL(virtqueue_get_desc_addr);
> >>
> >> -dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> >> +dma_addr_t virtqueue_get_avail_addr(const struct virtqueue *_vq)
> >>   {
> >>        struct vring_virtqueue *vq = to_vvq(_vq);
> >>
> >> @@ -2895,7 +2894,7 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> >>   }
> >>   EXPORT_SYMBOL_GPL(virtqueue_get_avail_addr);
> >>
> >> -dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> >> +dma_addr_t virtqueue_get_used_addr(const struct virtqueue *_vq)
> >>   {
> >>        struct vring_virtqueue *vq = to_vvq(_vq);
> >>
> >> @@ -2910,7 +2909,7 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> >>   EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
> >>
> >>   /* Only available for split ring */
> >> -const struct vring *virtqueue_get_vring(struct virtqueue *vq)
> >> +const struct vring *virtqueue_get_vring(const struct virtqueue *vq)
> >>   {
> >>        return &to_vvq(vq)->split.vring;
> >>   }
> >> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> >> index 2b472514c49b..36a79374e735 100644
> >> --- a/include/linux/virtio.h
> >> +++ b/include/linux/virtio.h
> >> @@ -84,14 +84,14 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
> >>
> >>   void *virtqueue_detach_unused_buf(struct virtqueue *vq);
> >>
> >> -unsigned int virtqueue_get_vring_size(struct virtqueue *vq);
> >> +unsigned int virtqueue_get_vring_size(const struct virtqueue *vq);
> >>
> >> -bool virtqueue_is_broken(struct virtqueue *vq);
> >> +bool virtqueue_is_broken(const struct virtqueue *vq);
> >>
> >> -const struct vring *virtqueue_get_vring(struct virtqueue *vq);
> >> -dma_addr_t virtqueue_get_desc_addr(struct virtqueue *vq);
> >> -dma_addr_t virtqueue_get_avail_addr(struct virtqueue *vq);
> >> -dma_addr_t virtqueue_get_used_addr(struct virtqueue *vq);
> >> +const struct vring *virtqueue_get_vring(const struct virtqueue *vq);
> >> +dma_addr_t virtqueue_get_desc_addr(const struct virtqueue *vq);
> >> +dma_addr_t virtqueue_get_avail_addr(const struct virtqueue *vq);
> >> +dma_addr_t virtqueue_get_used_addr(const struct virtqueue *vq);
> >>
> >>   int virtqueue_resize(struct virtqueue *vq, u32 num,
> >>                     void (*recycle)(struct virtqueue *vq, void *buf));
> >> --
> >> 2.34.1
> >>
> >> _______________________________________________
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> >> Virtualization@lists.linux-foundation.org
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> > --
> > Woke up in my clothes again this morning, don't know exactly where I am.
> 

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